a. When there is a mixing valve in the system, the tempered water return (TWR) must split and be routed to the cold-water side of the mixing valve and to the cold-water inlet of the water heater. A balancing valve should be placed in the line going to the water heater and the mixing valve for flow adjustments if needed.

Ocean acidification affects marine organisms in two ways. First, carbonic acid reacts with carbonate ions in the water to form bicarbonate. However, those same carbonate ions are what shell-building animals like coral need to create calcium carbonate shells. With less carbonate available, the animals need to expend more energy to build their shells. As a result, the shells end up being thinner and more fragile.

Some of the excess CO2 emitted by human activity dissolves in the ocean, becoming carbonic acid. Increases in carbon dioxide are not only leading to warmer oceans, but also to more acidic oceans. (Photograph ©2010 Way Out West News.)

Since the beginning of the Industrial Revolution, when people first started burning fossil fuels, carbon dioxide concentrations in the atmosphere have risen from about 280 parts per million to 387 parts per million, a 39 percent increase. This means that for every million molecules in the atmosphere, 387 of them are now carbon dioxide—the highest concentration in two million years. Methane concentrations have risen from 715 parts per billion in 1750 to 1,774 parts per billion in 2005, the highest concentration in at least 650,000 years.

Current research estimates that permafrost in the Northern Hemisphere holds 1,672 billion tons (Petagrams) of organic carbon. If just 10 percent of this permafrost were to thaw, it could release enough extra carbon dioxide to the atmosphere to raise temperatures an additional 0.7 degrees Celsius (1.3 degrees Fahrenheit) by 2100.

The ASPE Plumbing Engineering Design Handbook, available to ASPE members, has a precise way of sizing the circulating pump based on a 20-degree temperature differential from the water heater out to the farthest fixture and back to the circulator near the water heater. If the system has 140-degree water in the water heater, then the sizing method maintains 130-degree hot water at the end of the system and then back at the cold water inlet to the water heater the temperature would be approximately 120 degrees. The calculation is based on heat loss in the hot water piping circuit. It lists the British thermal unit loss per hour (BTU/Hr) losses for insulated and bare piping based on a 70-degree ambient temperature. A quick and simple way to estimate insulated pipe is to assume 25 to 30 BTUs/Hr per linear foot ignoring the hot water supply and return pipe size. This may simply result in a system where the temperature differential in most cases will be slightly less than 20 F.

On very long time scales (millions to tens of millions of years), the movement of tectonic plates and changes in the rate at which carbon seeps from the Earth’s interior may change the temperature on the thermostat. Earth has undergone such a change over the last 50 million years, from the extremely warm climates of the Cretaceous (roughly 145 to 65 million years ago) to the glacial climates of the Pleistocene (roughly 1.8 million to 11,500 years ago). [See Divisions of Geologic Time—Major Chronostratigraphic and Geochronologic Units for more information about geological eras.]

Plants also need water, sunlight, and nutrients, especially nitrogen. If a plant doesn’t have one of these things, it won’t grow regardless of how abundant the other necessities are. There is a limit to how much carbon plants can take out of the atmosphere, and that limit varies from region to region. So far, it appears that carbon dioxide fertilization increases plant growth until the plant reaches a limit in the amount of water or nitrogen available.

This cycle peaks in August, with about 2 parts per million of carbon dioxide drawn out of the atmosphere. In the fall and winter, as vegetation dies back in the northern hemisphere, decomposition and respiration returns carbon dioxide to the atmosphere.

Left unperturbed, the fast and slow carbon cycles maintain a relatively steady concentration of carbon in the atmosphere, land, plants, and ocean. But when anything changes the amount of carbon in one reservoir, the effect ripples through the others.

Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On average, 1013 to 1014 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, human emissions of carbon to the atmosphere are on the order of 1015 grams, whereas the fast carbon cycle moves 1016 to 1017 grams of carbon per year.

Other circulator pump control methods are constant pressure control methods; the pump will adjust its speed to maintain a constant pressure. Another control method is constant temperature controls where the pump senses the return temperature. As the return temperature rises to the set point, it slows the pump down to prevent overheating when peak usage periods draw hot water out to the end of the system, and then the pump can slow down and save energy. Another option is a constant pump curve mode, which is used when there is a demand for constant flow and constant head. The pump can be adjusted to speed up or slow down to maintain a desired duty point on the pump curve. This setting can allow the elimination of a pressure-reducing valve on the pump discharge. For more information on the new circulator pump technologies contact the following manufacturers:

Water vapor concentrations in the air are controlled by Earth’s temperature. Warmer temperatures evaporate more water from the oceans, expand air masses, and lead to higher humidity. Cooling causes water vapor to condense and fall out as rain, sleet, or snow.

The uplift of the Himalaya, beginning 50 million years ago, reset Earth’s thermostat by providing a large source of fresh rock to pull more carbon into the slow carbon cycle through chemical weathering. The resulting drop in temperatures and the formation of ice sheets changed the ratio between heavy and light oxygen in the deep ocean, as shown in this graph. (Graph based on data from Zachos at al., 2001.)

Recently, criteria for temperature maintenance for hot water systems in the models codes were changed from 100-foot distance criteria to a 50-foot criteria. I wrote about this many years ago. I proposed code changes showing a maximum distance of about 25 feet from a circulated main or hot water source would be the ideal maximum distance to allow hot water within a reasonable time, but knowing that would have many industry groups upset with a requirement for recirculation systems in most residences and small buildings, I compromised and proposed a reduction to a maximum of 50 feet. This would allow most residences and smaller buildings to not be required to have temperature maintenance systems. The code change did not pass the first time, but eventually it prevailed.

So while carbon dioxide contributes less to the overall greenhouse effect than water vapor, scientists have found that carbon dioxide is the gas that sets the temperature. Carbon dioxide controls the amount of water vapor in the atmosphere and thus the size of the greenhouse effect.

Plants and phytoplankton are the main components of the fast carbon cycle. Phytoplankton (microscopic organisms in the ocean) and plants take carbon dioxide from the atmosphere by absorbing it into their cells. Using energy from the Sun, both plants and plankton combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen. The chemical reaction looks like this:

Forged in the heart of aging stars, carbon is the fourth most abundant element in the Universe. Most of Earth’s carbon—about 65,500 billion metric tons—is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels.

Shifts in Earth’s orbit are happening constantly, in predictable cycles. In about 30,000 years, Earth’s orbit will have changed enough to reduce sunlight in the Northern Hemisphere to the levels that led to the last ice age.

How does ahot water recirculating systemwork

This coal seam in Scotland was originally a layer of sediment, rich in organic carbon. The sedimentary layer was eventually buried deep underground, and the heat and pressure transformed it into coal. Coal and other fossil fuels are a convenient source of energy, but when they are burned, the stored carbon is released into the atmosphere. This alters the balance of the carbon cycle, and is changing Earth’s climate. (Photograph ©2010 Sandchem.)

Surveys of how water users showed wait times between 10 and 30 seconds were marginally acceptable and wait times in excess of 30 seconds were considered unacceptable.

With the seasonal cycle removed, the atmospheric carbon dioxide concentration measured at Mauna Loa Volcano, Hawaii, shows a steady increase since 1957. At the same time global average temperatures are rising as a result of heat trapped by the additional CO2 and increased water vapor concentration. (Graphs by Robert Simmon, using CO2 data from the NOAA Earth System Research Laboratory and temperature data from the Goddard Institute for Space Studies.)

During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars—for building plant structures. This process forms the foundation of the fast (biological) carbon cycle. (Illustration adapted from P.J. Sellers et al., 1992.)

I see this as a ticking time bomb and a lawsuit waiting to happen. I prefer minimizing liability and designing hot water systems the correct way with a dedicated hot water return piping system in the original design. The hot water return piping system should be properly sized and balanced. I will not design a building with a demand circulator connecting the domestic hot water to the cold water pipes. Demand circulators are retrofit products for improperly designed systems, that should only be used in single-family homes where the homeowner will live with the consequences of using such a product. Demand circulators should not be designed or installed in commercial or multi-family buildings because of the obvious crossconnection and water quality issues it brings with it.

Many of the questions scientists still need to answer about the carbon cycle revolve around how it is changing. The atmosphere now contains more carbon than at any time in at least two million years. Each reservoir of the cycle will change as this carbon makes its way through the cycle.

When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.

The controls on pumps that circulate water between a water heater and a storage tank for heated water shall limit operation of the pump from heating cycle start-up to no greater than five minutes after the end of the cycle.

Domestic hot water above 180 F is not recommended because of the potential for scalding, and as temperatures get higher, the corrosion accelerates. In some unique cases, domestic hot water temperatures can go higher than 180 F booster heaters and steam heat exchangers, or with some types of heat recovery systems or other industrial or institutional piping systems. In these cases, consider sizing the piping to keep velocities lower than two feet per second.

Hot water recirculatingpump

The slow cycle returns carbon to the atmosphere through volcanoes. Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide.

Future NASA satellites will continue these observations, and also measure carbon dioxide and methane in the atmosphere and vegetation height and structure.

In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids and the chalky white substance that dries on your faucet if you live in an area with hard water. In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera). After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone—limestone and its derivatives.

Hot water recirculatingpump installation diagram

In the early years, there were many explosions associated with uncontrolled heat to the water heater in closed piping systems. Eventually, controls were installed to relieve the pressure and temperature, and to control the fuel and combustion air. Coal and wood as a heating source were phased out because of the difficulty of controlling the heat input. Heating oil, natural gas, electricity, solar and geothermal were phased in over many years as heating sources for domestic hot water. The early plumbing fixtures had hot and cold spigots and drain connections to vented drainpipes. As buildings grew in size and complexity, and as the distance from the water heater to the most remote fixture increased, getting hot water from the fixture would take longer because the previously heated water in the pipes had to be drained first.

The burning of fossil fuels is the primary source of increased carbon dioxide in the atmosphere today. (Photograph ©2009 stevendepolo.)

In all four processes, the carbon dioxide released in the reaction usually ends up in the atmosphere. The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the Earth is breathing.

Carbon plays an essential role in biology because of its ability to form many bonds—up to four per atom—in a seemingly endless variety of complex organic molecules. Many organic molecules contain carbon atoms that have formed strong bonds to other carbon atoms, combining into long chains and rings. Such carbon chains and rings are the basis of living cells. For instance, DNA is made of two intertwined molecules built around a carbon chain.

Rivers carry calcium ions—the result of chemical weathering of rocks—into the ocean, where they react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes limestone. (Photograph ©2009 Greg Carley.)

Emissions of carbon dioxide by humanity (primarily from the burning of fossil fuels, with a contribution from cement production) have been growing steadily since the onset of the industrial revolution. About half of these emissions are removed by the fast carbon cycle each year, the rest remain in the atmosphere. (Graph by Robert Simmon, using data from the Carbon Dioxide Information Analysis Center and Global Carbon Project.)

All of these measurements will help us see how the global carbon cycle is changing through time. They will help us gauge the impact we are having on the carbon cycle by releasing carbon into the atmosphere or finding ways to store it elsewhere. They will show us how our changing climate is altering the carbon cycle, and how the changing carbon cycle is altering our climate.

Rising concentrations of carbon dioxide are warming the atmosphere. The increased temperature results in higher evaporation rates and a wetter atmosphere, which leads to a vicious cycle of further warming. (Photograph ©2011 Patrick Wilken.)

Carbon is both the foundation of all life on Earth, and the source of the majority of energy consumed by human civilization. [Photographs ©2007 MorBCN (top) and ©2009 sarahluv (lower).]

Warmer oceans—a product of the greenhouse effect—could also decrease the abundance of phytoplankton, which grow better in cool, nutrient-rich waters. This could limit the ocean’s ability to take carbon from the atmosphere through the fast carbon cycle.

Carbon is the backbone of life on Earth. We are made of carbon, we eat carbon, and our civilizations—our economies, our homes, our means of transport—are built on carbon. We need carbon, but that need is also entwined with one of the most serious problems facing us today: global climate change.

Time series of satellite data, like the imagery available from the Landsat satellites, allow scientists to monitor changes in forest cover. Deforestation can release carbon dioxide into the atmosphere, while forest regrowth removes CO2. This pair of false-color images shows clear cutting and forest regrowth between 1984 and 2010 in Washington State, northeast of Mount Rainier. Dark green corresponds to mature forests, red indicates bare ground or dead plant material (freshly cut areas), and light green indicates relatively new growth. (NASA image by Robert Simmon, using Landsat data from the USGS Global Visualization Viewer.)

However, the slow carbon cycle also contains a slightly faster component: the ocean. At the surface, where air meets water, carbon dioxide gas dissolves in and ventilates out of the ocean in a steady exchange with the atmosphere. Once in the ocean, carbon dioxide gas reacts with water molecules to release hydrogen, making the ocean more acidic. The hydrogen reacts with carbonate from rock weathering to produce bicarbonate ions.

607.2.1.2 Demand recirculation controls for distribution systems. A water distribution system having one or more recirculation pumps that pump water from a heated water supply pipe back to the heated water source through a cold water supply pipe shall be a demand recirculation water system. Pumps shall have controls that comply with both of the following:

In Earth’s past, the carbon cycle has changed in response to climate change. Variations in Earth’s orbit alter the amount of energy Earth receives from the Sun and leads to a cycle of ice ages and warm periods like Earth’s current climate. (See Milutin Milankovitch.) Ice ages developed when Northern Hemisphere summers cooled and ice built up on land, which in turn slowed the carbon cycle. Meanwhile, a number of factors including cooler temperatures and increased phytoplankton growth may have increased the amount of carbon the ocean took out of the atmosphere. The drop in atmospheric carbon caused additional cooling. Similarly, at the end of the last Ice Age, 10,000 years ago, carbon dioxide in the atmosphere rose dramatically as temperatures warmed.

This rise in temperature isn’t all the warming we will see based on current carbon dioxide concentrations. Greenhouse warming doesn’t happen right away because the ocean soaks up heat. This means that Earth’s temperature will increase at least another 0.6 degrees Celsius (1 degree Fahrenheit) because of carbon dioxide already in the atmosphere. The degree to which temperatures go up beyond that depends in part on how much more carbon humans release into the atmosphere in the future.

What will those changes look like? What will happen to plants as temperatures increase and climate changes? Will they remove more carbon from the atmosphere than they put back? Will they become less productive? How much extra carbon will melting permafrost put into the atmosphere, and how much will that amplify warming? Will ocean circulation or warming change the rate at which the ocean takes up carbon? Will ocean life become less productive? How much will the ocean acidify, and what effects will that have?

On the other hand, carbon dioxide is essential for plant and phytoplankton growth. An increase in carbon dioxide could increase growth by fertilizing those few species of phytoplankton and ocean plants (like sea grasses) that take carbon dioxide directly from the water. However, most species are not helped by the increased availability of carbon dioxide.

In the meantime, winds, currents, and temperature control the rate at which the ocean takes carbon dioxide from the atmosphere. (See The Ocean’s Carbon Balance on the Earth Observatory.) It is likely that changes in ocean temperatures and currents helped remove carbon from and then restore carbon to the atmosphere over the few thousand years in which the ice ages began and ended.

Domestic hot water systems have been installed in buildings for many years dating back to ancient times. Recirculating hot water systems are not quite that old. Gravity hot water circulation began in the U.S. in the late 1870s, right after plumbing moved indoors. During the early years, water and space heating was done in cabins by burning wood in a fireplace or cast iron stove, and water was heated in pots or kettles for bathing or cooking purposes. Eventually, coal replaced wood as a fuel source, but there was still no electricity for heating, lights or electrical circulating pumps during these early years. As domestic hot water systems became more sophisticated, cold water was piped to buildings and closed vessels were installed with burners or fire chambers below them for heating the domestic hot water.

Today, changes in the carbon cycle are happening because of people. We perturb the carbon cycle by burning fossil fuels and clearing land.

It is significant that so much carbon dioxide stays in the atmosphere because CO2 is the most important gas for controlling Earth’s temperature. Carbon dioxide, methane, and halocarbons are greenhouse gases that absorb a wide range of energy—including infrared energy (heat) emitted by the Earth—and then re-emit it. The re-emitted energy travels out in all directions, but some returns to Earth, where it heats the surface. Without greenhouse gases, Earth would be a frozen -18 degrees Celsius (0 degrees Fahrenheit). With too many greenhouse gases, Earth would be like Venus, where the greenhouse atmosphere keeps temperatures around 400 degrees Celsius (750 Fahrenheit).

Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale.

Chemistry regulates this dance between ocean, land, and atmosphere. If carbon dioxide rises in the atmosphere because of an increase in volcanic activity, for example, temperatures rise, leading to more rain, which dissolves more rock, creating more ions that will eventually deposit more carbon on the ocean floor. It takes a few hundred thousand years to rebalance the slow carbon cycle through chemical weathering.

Ideally, hot water should arrive at the fixture between zero and ten seconds from the time a faucet or fixture valve is opened. There are a couple of manufacturers that offer fittings and designs to allow the hot water to circulate right up to the fixture, and some manufacturers allow circulation right up to the faucet spout, such as Kemper hygiene systems (bit.do/Kemper) and Viega drinking water systems - Hygiene (bit.do/Viega).

Before the industrial age, the ocean vented carbon dioxide to the atmosphere in balance with the carbon the ocean received during rock weathering. However, since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Over millennia, the ocean will absorb up to 85 percent of the extra carbon people have put into the atmosphere by burning fossil fuels, but the process is slow because it is tied to the movement of water from the ocean’s surface to its depths.

The Copper Development Association recommends a maximum flow velocity of eight feet per second for cold water flowing in copper pipes and five feet per second for hot water. It also recommends a maximum velocity of two to three feet per second for hot water over 140 F. These recommendations are sufficiently vague enough to lead you in the right direction, however, I have come up with a more accurate table for pipe sizing and a chart you should refer to in order to assure the flow velocities do not erode the pipe walls. This table has worked well for me and should provide a system that will work without velocity erosion issues.

b. If the TWR is only piped back to the water heater, when there is no usage in the system and the tempered water circulating pump is running, hot water will leak through the mixing valve manufacturing tolerances, and the temperature of the tempered water system will rise above the mixing valve set point to reach the highest temperature flowing from the water heater.

Carbon stored in rocks is naturally returned to the atmosphere by volcanoes. In this photograph, Russia’s Kizimen Volcano vents ash and volcanic gases in January 2011. Kizimen is located on the Kamchatka Peninsula, where the Pacific Plate is subducting beneath Asia. (Photograph ©2011 Artyom Bezotechestvo/Photo Kamchatka.)

Hot water recirculating systemwith dedicated return line

Second, the more acidic water is, the better it dissolves calcium carbonate. In the long run, this reaction will allow the ocean to soak up excess carbon dioxide because more acidic water will dissolve more rock, release more carbonate ions, and increase the ocean’s capacity to absorb carbon dioxide. In the meantime, though, more acidic water will dissolve the carbonate shells of marine organisms, making them pitted and weak.

The time it takes carbon to move through the fast carbon cycle is measured in a lifespan. The fast carbon cycle is largely the movement of carbon through life forms on Earth, or the biosphere. Between 1015 and 1017 grams (1,000 to 100,000 million metric tons) of carbon move through the fast carbon cycle every year.

This thermostat works over a few hundred thousand years, as part of the slow carbon cycle. This means that for shorter time periods—tens to a hundred thousand years—the temperature of Earth can vary. And, in fact, Earth swings between ice ages and warmer interglacial periods on these time scales. Parts of the carbon cycle may even amplify these short-term temperature changes.

This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon. (Diagram adapted from U.S. DOE, Biological and Environmental Research Information System.)

With more atmospheric carbon dioxide available to convert to plant matter in photosynthesis, plants were able to grow more. This increased growth is referred to as carbon fertilization. Models predict that plants might grow anywhere from 12 to 76 percent more if atmospheric carbon dioxide is doubled, as long as nothing else, like water shortages, limits their growth. However, scientists don’t know how much carbon dioxide is increasing plant growth in the real world, because plants need more than carbon dioxide to grow.

All of this extra carbon needs to go somewhere. So far, land plants and the ocean have taken up about 55 percent of the extra carbon people have put into the atmosphere while about 45 percent has stayed in the atmosphere. Eventually, the land and oceans will take up most of the extra carbon dioxide, but as much as 20 percent may remain in the atmosphere for many thousands of years.

Without human interference, the carbon in fossil fuels would leak slowly into the atmosphere through volcanic activity over millions of years in the slow carbon cycle. By burning coal, oil, and natural gas, we accelerate the process, releasing vast amounts of carbon (carbon that took millions of years to accumulate) into the atmosphere every year. By doing so, we move the carbon from the slow cycle to the fast cycle. In 2009, humans released about 8.4 billion tons of carbon into the atmosphere by burning fossil fuel.

In the 2015 code change cycle, a change was presented to the model codes, and it was touted as saving water and energy along with reducing the time it takes to get hot water at a fixture. The code change was the technology, demand recirculation. I testified against this technology because health and safety should trump water and energy conservation. Many other in the backflow prevention industry have voiced concerns about this technology, but it fell on deaf ears at the code hearings. The code committee voted on this change based on the thought of having instant hot water in their homes, and saving a little water was more important to them than cross-connection. Many of the code committee members voted for this and commented that it would be nice to have for their own home. This code change will allow contaminated hot water to flow into the domestic cold water supply pipes. I have always said that circulating domestic hot water through the cold water pipes is a bad idea, and here’s why:

Water flow velocity is very important in domestic hot water pipes with copper piping and brass or copper alloy valves. High water velocities, combined with hot water, can cause velocity erosion issues for the pipe and valve walls. The minimum pipe size I use for the hot water return system piping is ¾-inch pipe. I often see half-inch pipe installed. Smaller diameter pipes create a condition where the velocity increases at the same flow rate, and it also causes system temperature differentials from the supply to the return temperature that exceed the design criteria of 5 F, 10 F or 20 F. In the old days, we would design the return system for a 20-degree temperature differential using the ASPE/ASHRAE sizing method because hot water recirculation systems with older technology like the bi-metallic coil type temperature actuated mixing valves used on master mixing valve installations required at least a 20-degree temperature differential for the bi-metallic coil technology to react properly. Digitally-controlled mixing valves use digital probes with products like the Armstrong “Brain,” which offers accuracies capable of mixing hot water return temperatures with less than 5 F temperature differential and still maintaining a mixing valve outlet temperature setting within 1 F to 2 F of the set point.

Four things can happen to move carbon from a plant and return it to the atmosphere, but all involve the same chemical reaction. Plants break down the sugar to get the energy they need to grow. Animals (including people) eat the plants or plankton, and break down the plant sugar to get energy. Plants and plankton die and decay (are eaten by bacteria) at the end of the growing season. Or fire consumes plants. In each case, oxygen combines with sugar to release water, carbon dioxide, and energy. The basic chemical reaction looks like this:

The warming caused by rising greenhouse gases may also “bake” the soil, accelerating the rate at which carbon seeps out in some places. This is of particular concern in the far north, where frozen soil—permafrost—is thawing. Permafrost contains rich deposits of carbon from plant matter that has accumulated for thousands of years because the cold slows decay. When the soil warms, the organic matter decays and carbon—in the form of methane and carbon dioxide—seeps into the atmosphere.

Plants on land have taken up approximately 25 percent of the carbon dioxide that humans have put into the atmosphere. The amount of carbon that plants take up varies greatly from year to year, but in general, the world’s plants have increased the amount of carbon dioxide they absorb since 1960. Only some of this increase occurred as a direct result of fossil fuel emissions.

The biggest problem to overcome was air trapped in the high point of the system. They addressed this by connecting a regularly used fixture or an automatic air vent at the top of the gravity hot water circulation loop to allow any air to be vented. If air was trapped, a large bubble would resist gravity circulation. A commonly used fixture to the top of the hot water riser vented air and allowed the gravity circulation to continue. Gravity domestic hot water systems were commonly installed before the introduction of electricity and circulating pumps, and some have been installed in newer homes with moderate success. Newer code requirements for water heaters require flappers or a device in the top of the water heater to prevent gravity circulation. This makes the water heater more efficient during efficiency testing, but makes many older buildings that install new water heaters experience problems with respect to gravity circulation. That is when it is time to install a circulating pump.

Dry, water-stressed plants are also more susceptible to fire and insects when growing seasons become longer. In the far north, where an increase in temperature has the greatest impact, the forests have already started to burn more, releasing carbon from the plants and the soil into the atmosphere. Tropical forests may also be extremely susceptible to drying. With less water, tropical trees slow their growth and take up less carbon, or die and release their stored carbon to the atmosphere.

In the tropics, however, forests are being removed, often through fire, and this releases carbon dioxide. As of 2008, deforestation accounted for about 12 percent of all human carbon dioxide emissions.

Over the long term, the carbon cycle seems to maintain a balance that prevents all of Earth’s carbon from entering the atmosphere (as is the case on Venus) or from being stored entirely in rocks. This balance helps keep Earth’s temperature relatively stable, like a thermostat.

For other than Group R2, R3 and R4 occupancies that are three stories or less in height above grade plane, the installation of heated water circulation and heat trace systems shall be in accordance with Section C404.6 of the International Energy Conservation Code.

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Levels of carbon dioxide in the atmosphere have corresponded closely with temperature over the past 800,000 years. Although the temperature changes were touched off by variations in Earth’s orbit, the increased global temperatures released CO2 into the atmosphere, which in turn warmed the Earth. Antarctic ice-core data show the long-term correlation until about 1900. (Graphs by Robert Simmon, using data from Lüthi et al., 2008, and Jouzel et al., 2007.)

Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth.

Some of the changes in carbon absorption are the result of land use decisions. Agriculture has become much more intensive, so we can grow more food on less land. In high and mid-latitudes, abandoned farmland is reverting to forest, and these forests store much more carbon, both in wood and soil, than crops would. In many places, we prevent plant carbon from entering the atmosphere by extinguishing wildfires. This allows woody material (which stores carbon) to build up. All of these land use decisions are helping plants absorb human-released carbon in the Northern Hemisphere.

Most of us, however, will observe changes in the carbon cycle in a more personal way. For us, the carbon cycle is the food we eat, the electricity in our homes, the gas in our cars, and the weather over our heads. We are a part of the carbon cycle, and so our decisions about how we live ripple across the cycle. Likewise, changes in the carbon cycle will impact the way we live. As each of us come to understand our role in the carbon cycle, the knowledge empowers us to control our personal impact and to understand the changes we are seeing in the world around us.

(Graph by Marit Jentoft-Nilsen and Robert Simmon, using data from the NOAA Earth System Research Laboratory. Maps by Robert Simmon and Reto Stöckli, using MODIS data.)

Recirculating hot water systemdiagram

Since the advent of the circulator pump, many improvements have been made. Early pumps were the same ones used on hydronic systems. The pumps were made of ferrous metals with cast iron and steel parts, and most of them suffered corrosion problems or had rusty water shortly after being installed. Hydronic systems were closed systems with air eliminators to keep air and oxygen out of the piping circuit. Some hydronic systems use corrosion inhibiting chemicals to help prevent corrosion of the ferrous metals. Oxygen contributes to the corrosion process and domestic water systems are open systems with air and oxygen entrained in the water flow. It is for this reason that hydronic pumps and piping can be black steel and cast iron ferrous metals, and domestic hot water systems should be non-ferrous bronze or stainless steel parts with copper piping. Pump manufacturers have continually improved the materials, bearings, seals and efficiency of the circulator pumps.

NASA’s role in answering these questions is to provide global satellite observations and related field observations. As of early 2011, two types of satellite instruments were collecting information relevant to the carbon cycle.

Changes in land cover—forests converted to fields and fields converted to forests—have a corresponding effect on the carbon cycle. In some Northern Hemisphere countries, many farms were abandoned in the early 20th century and the land reverted to forest. As a result, carbon was drawn out of the atmosphere and stored in trees on land. (Photograph ©2007 Husein Kadribegic.)

Because scientists know which wavelengths of energy each greenhouse gas absorbs, and the concentration of the gases in the atmosphere, they can calculate how much each gas contributes to warming the planet. Carbon dioxide causes about 20 percent of Earth’s greenhouse effect; water vapor accounts for about 50 percent; and clouds account for 25 percent. The rest is caused by small particles (aerosols) and minor greenhouse gases like methane.

The changes in the carbon cycle impact each reservoir. Excess carbon in the atmosphere warms the planet and helps plants on land grow more. Excess carbon in the ocean makes the water more acidic, putting marine life in danger.

Circulator pump manufacturers are coming out with smart pumps with intelligent, built-in controls that can adjust the speed with variable speed motor technology. Features offered on newer circulator pumps include proportional pressure controls. Some options adapt to the changing pressures and flows in the system and adjusts or reduces the speed/pump head to change the efficiency in order to operate at a better efficiency point when water is flowing in the system and the pump does not need to pump as hard. There are flow adapt limits that limit the maximum flow. This is good for minimizing flow velocities in the piping system and can eliminate the need for a balancing valve on the discharge of a circulating pump.

I was all for allowing this technology in residential applications only, but the code allows it anywhere. So, there will be a condominium or apartment building where someone decides to install one of these demand circulator pumps under their lavatory to circulate hot water. Now everyone in the building will be drinking water with high magnesium or aluminum content and possibly high bacteria content associated with new breeding grounds in the cold water pipes, which will be in the ideal temperature range for Legionella and other bacteria growth. In addition, most of the people in the building will not get clean cold water to cook or brush their teeth with.

The movement of carbon from the atmosphere to the lithosphere (rocks) begins with rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—that falls to the surface in rain. The acid dissolves rocks—a process called chemical weathering—and releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean.

The biggest changes in the land carbon cycle are likely to come because of climate change. Carbon dioxide increases temperatures, extending the growing season and increasing humidity. Both factors have led to some additional plant growth. However, warmer temperatures also stress plants. With a longer, warmer growing season, plants need more water to survive. Scientists are already seeing evidence that plants in the Northern Hemisphere slow their growth in the summer because of warm temperatures and water shortages.

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments, flying on NASA’s Terra and Aqua satellites, measure the amount of carbon plants and phytoplankton turn into matter as they grow, a measurement called net primary productivity. The MODIS sensors also measure how many fires occur and where they burn.

Two Landsat satellites provide a detailed view of ocean reefs, what is growing on land, and how land cover is changing. It is possible to see the growth of a city or a transformation from forest to farm. This information is crucial because land use accounts for one-third of all human carbon emissions.

Limestone, or its metamorphic cousin, marble, is rock made primarily of calcium carbonate. These rock types are often formed from the bodies of marine plants and animals, and their shells and skeletons can be preserved as fossils. Carbon locked up in limestone can be stored for millions—or even hundreds of millions—of years. (Photograph ©2008 Rookuzz (Hmm).)

About 30 percent of the carbon dioxide that people have put into the atmosphere has diffused into the ocean through the direct chemical exchange. Dissolving carbon dioxide in the ocean creates carbonic acid, which increases the acidity of the water. Or rather, a slightly alkaline ocean becomes a little less alkaline. Since 1750, the pH of the ocean’s surface has dropped by 0.1, a 30 percent change in acidity.

If the building has multiple hot water mains and branches, each branch should have a balancing valve and check valve before connecting to the hot water return main. Simply installing the valves is not enough; after the system is started up, it must be balanced to assure each branch has the calculated flow rate to maintain the desired temperature in that branch. This prevents short-cycling of the hot water through the path of least resistance (closest branch circuit). I have investigated numerous systems with problems and the problems began because the system was never balanced when it was installed. Untrained maintenance personnel find that there is no flow in the farthest portion of the piping system, so they install a bigger pump. This typically does not solve the problem, but soon after the larger pump is installed, the piping system starts to spring leaks near elbows and valves. Balancing the hot water system is a relatively simple process, but calculations must be performed and flows in gallons per minute must be determined for each balancing valve prior to setting.

The bonds in the long carbon chains contain a lot of energy. When the chains break apart, the stored energy is released. This energy makes carbon molecules an excellent source of fuel for all living things.

The ebb and flow of the fast carbon cycle is visible in the changing seasons. As the large land masses of Northern Hemisphere green in the spring and summer, they draw carbon out of the atmosphere. This graph shows the difference in carbon dioxide levels from the previous month, with the long-term trend removed.

Carbon dioxide, on the other hand, remains a gas at a wider range of atmospheric temperatures than water. Carbon dioxide molecules provide the initial greenhouse heating needed to maintain water vapor concentrations. When carbon dioxide concentrations drop, Earth cools, some water vapor falls out of the atmosphere, and the greenhouse warming caused by water vapor drops. Likewise, when carbon dioxide concentrations rise, air temperatures go up, and more water vapor evaporates into the atmosphere—which then amplifies greenhouse heating.

Recirculating hot water systemretrofit

When we clear forests, we remove a dense growth of plants that had stored carbon in wood, stems, and leaves—biomass. By removing a forest, we eliminate plants that would otherwise take carbon out of the atmosphere as they grow. We tend to replace the dense growth with crops or pasture, which store less carbon. We also expose soil that vents carbon from decayed plant matter into the atmosphere. Humans are currently emitting just under a billion tons of carbon into the atmosphere per year through land use changes.

Rising carbon dioxide concentrations are already causing the planet to heat up. At the same time that greenhouse gases have been increasing, average global temperatures have risen 0.8 degrees Celsius (1.4 degrees Fahrenheit) since 1880.

In the late 1870s, tradesmen were using looped hydronic heating systems to replace steam systems with limited safety controls. Tradesmen learned that hot water rises in the piping system because it was lighter than cold water. They also applied this gravity circulation to domestic hot water systems. Hot water leaving the water heater went up in a pipe vertically through the building and looped back down un-insulated and ran parallel to the hot water riser to the bottom of the water heater. The return riser was not insulated to encourage heat loss, and the cooler water caused gravity circulation. As people on the upper floors of the building used hot water, they only had to drain water from the branch piping until hot water from the riser arrived at the fixture.

If you want to take the time to calculate the system exactly, you can use the Table in the Plumbing Engineering Design Handbook and the BTU/Hr losses can be summed up for the various lengths of different pipe sizes and a total BTU/Hr loss can be calculated. For a 20-degree temperature differential, you would then divide by 10,000 to get the required gallons per minute (GPM) for the branch or pump. This is how the GPM is determined for the pump sizing. For the pump head requirement, the appropriate GPM is assigned to each section of pipe based on the BTU/Hr loss requirements above and from pipe friction loss charts, a total feet of head or pressure drop in pounds per square inch (PSI) can be determined. Remember when selecting pumps to convert from PSI to feet of head, most manufacturers list their pumps on curves listing feet of head on vertically and GPM horizontally. Just remember 1 PSI = 2.31 feet of head and 1 foot of head = .433 PSI.

The more vertical the system was, the better it worked up to a point. As buildings were built to be about three or four stories in height, depending on the insulation type and thickness, the systems would get too big, and the water would cool down and lose its buoyancy. There were also some other things that were problematic with gravity circulation systems: Horizontal swing check valves resisted flow. Large dips in the piping would allow water to cool off, and the cool water in trapped areas would resist flow. Long horizontal runs with minimal vertical rise had difficulty getting gravity circulation.