Solar, hydronics versus carbon emissions

The industrial revolution has been fueled by combustion since its very beginning, and combustion produces carbon dioxide gas (CO²). Whether from a power plant, a factory, a boiler or the tailpipe of a motor vehicle, it seems the biggest legacy of the “Combustion Age” is CO². The levels of CO² are higher now than at any other time in human history, and increasing steadily every year. The graph in figure 80-1 shows this trend. The concentration of CO² (carbon) has increased over the past few hundred years, with an alarming uptick in the past 20 years or so. 

CO² is a greenhouse gas that is linked to global climate change and violent weather events. By definition, a greenhouse gas tends to trap heat at the earth’s surface in the same way solar heat is trapped inside a transparent greenhouse. Clearly, ever-increasing CO² levels cannot be a good thing, and in excessive concentrations, have become a serious type of pollution. That is why there is a concerted effort to reduce the "human carbon footprint," and the heating and cooling industries are no exception.

The combustion multiplier

Fuel combustion produces CO² emissions at a surprising rate. To the casual observer, it may seem impossible that a gallon of liquid propane (which weighs about 4.2 pounds) could produce almost 13 pounds of CO² when burned. However, most of the weight of the CO² does not come from the propane itself, but from combining with the oxygen in the air. This multiplier effect is true for every combustion fuel such as natural gas, oil products and coal. So, every source of combustion is, in effect, a highly productive CO² factory, cranking out several times its own weight in CO² as it consumes oxygen. Figure 80-2 shows a table of different fuel types and the amount of CO² they produce when burned. It is interesting to note that coal produces almost twice as much CO² as natural gas (per million Btu) when burned, which is one of the reasons natural gas is considered a “cleaner” fuel.

Corrective action

When faced with excessive and even life-threating industrial pollution in the past, the rational response has always been for people to act out of self-preservation and reduce the threat. So, in the same way that we have overcome chemical pollution in our rivers and streams, learned to recycle our solid waste and reduced the ozone-depleting chlorofluorocarbons into our atmosphere, we can now turn our attention to the reduction of CO² emissions. It makes sense to use every proven technology at our disposal today to reduce and eliminate excessive CO² emissions right now. This is, in fact, one of the top 10 reasons our clients tell us they are interested in solar/renewable hydronic installations; to reduce fuel consumption and lower their carbon footprint.

Use what we have

Each of us in the plumbing, heating, cooling or construction industry may not be able to eliminate the CO² problem, but we can all contribute to the solution by using what we know already. We don't have to wait for some future technology to sustain our efforts to reduce CO² emissions today. The transition to lower carbon emissions will not happen overnight, since it took over a hundred years to become established at its current level. But the transition has begun on many fronts, and with a variety of existing and proven technologies.

For example: The renewable electric power industry is gaining ground around the world with larger and larger zero-emissions generator capacity using photovoltaic (solar electric), wind turbine farms, CSP and other technologies joining the existing hydroelectric capacity (which has always been zero emissions).

Architects and builders are showing more and more commitment to high-efficiency “green” building construction with high LEED and HERS ratings and even net-zero designs that require much less energy for heating, cooling and lighting.

Appliance manufacturers are earning high Energy Star ratings on devices that have ever-increasing energy efficiencies. This includes boilers, water heaters and circulator pumps used in our industry.

All of these examples have been gaining momentum for years, and are stronger than ever today. It is only logical that the hydronic industry do its part, and indeed it is. That is, in large part, the subject of this column every month. Here is a brief summary of some of the most successful existing technologies we have been using in recent years to achieve higher efficiency and lower carbon emissions.

Combustion efficiency

In the hydronic heating industry, we have been steadily going in the direction of higher and higher efficiency over time. Higher efficiency means lower fuel consumption, which means lower CO² emissions. Every time we specify or install a modulating/condensing hot water boiler, we can certainly feel good about the combustion efficiency of these newer burners. The new technology burns 10 to 20 percent less fuel (or better) than the old school boilers, and that is a significant improvement. This might translate to a savings of 2.5 tons of carbon per year in one of our typical residential projects. The same kind of improvements are also available in wood burners using wood gasification technology.

Electric efficiency

Heat pumps that perform with better heating efficiencies than ever before in cold climates are now available. When a new heat pump replaces conventional electric resistance heat, it can reduce electrical consumption roughly by a factor of three or better. This can translate into a much larger fuel savings at the power plant, since most combustion-fueled power plants burn roughly about four units of fuel for every unit of electricity delivered to the end user. So, when a heat pump saves three units of energy for the end user, it means 12 units of fuel were not burned at a combustion-fueled power plant.

One word of caution: when changing from combustion heat to electric heat, the former emissions at the job site may just be transferred to become new emissions at the power plant. “Elsewhere emissions” are not the same thing as zero emissions.

Waste heat recovery

Engine generators and localized power plants can burn a lot of fuel and generate a lot of waste heat. Only about 20 percent of the energy embodied in the combustion fuel from a generator is delivered as electricity, while 80 percent is typically lost as waste heat. If a simple heat exchanger is used to recover half of that heat, the fuel utilization of the equipment is improved by a factor of three and other hot-water fuel can be avoided.

Another common source of waste heat is from air conditioners in buildings. The waste heat from any refrigeration cooling system can be used to make hot water, but common practice is to waste this heat by exhausting it to the outdoors. We have found that when this waste heat is captured with heat exchangers, a large water heating potential can be tapped while conventional fuel and its CO² emissions are greatly reduced.

The solar heat bonus

Any one of the efficiency improvements mentioned above can be made even better by adding solar heat. We have found that in our solar/hydronic heating installations, the annual fuel consumption can typically be cut in half by the proper application of solar heating technology. And a carefully designed project can achieve much higher fuel savings than that.

For example, in a recent retrofit installation, a bank of solar heat collectors was added to provide about half the heat needed by the building, for a savings of 52 million Btu annually. The remainder of the heat was provided by a mod/con propane boiler. If we refer to Figure 80-2, the CO² saved by the solar collectors amounts to at least 3.5 tons per year and the savings from the new boiler can be added to that. This is a good example of an existing building where net-zero design was not possible, but cutting fuel consumption and emissions in half was not hard to accomplish.

We find these opportunities literally in every project we do, and we encourage everyone to simply do what they can with what they know to improve energy efficiency and reduce carbon emissions on every job.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion, and do not constitute any recommendation or endorsement. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a bachelor of science in mechanical engineering and he is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit for more information.

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