Radiant Design: Reviewing Renewables & Beyond
Here, the author draws on experience to review a number of heating sources and fuel sources with regard to efficiency, pollution, maintenance, and cost. Is a hybrid design selected for the application’s parameters your best path to a balance of performance and expense?
The use of radiant heating continues to be a viable system choice for commercial green buildings because it is clean, quiet, comfortable, and efficient. Another desirable aspect of radiant technology is its heating and fuel source flexibility. This article will serve as an application guide to radiant heating source selection and examine the potential benefits and drawbacks of differing heating technologies for commercial radiant systems.
The selection of a heating energy source is one of the most important decisions you will make for your radiant heating system. A properly designed radiant system should use the most efficient heat source that is practical. Some of the choices for heat source include boilers (natural gas, propane, oil, coal, biomass, electric), water heaters, heat pumps, solar thermal, and waste heat. Each has it benefits and drawbacks.
Because radiant heating operates at low water temperatures, it is ideally suited for a wide variety of heating sources. This heating source flexibility is a great asset, since one or several heating sources and/or fuel sources can be used in combination to select the most appropriate system for the project.
Boilers and Boiler Fuel ChoicesBoilers are by far the most predominant heating source for radiant systems. They are generally efficient and dependable.
Gas is readily available and is produced mostly within the United States. Its price has increased in the recent past, but continues to be less expensive than electricity. It is clean enough to use with condensing flue gas technology and will be a minor contributor to global warming as compared with oil. Condensing boilers often are a practical choice because they recover additional heat from condensing flue gas, boosting efficiency into the range of 88% to 99%, and payback periods on the incremental cost over a base-efficient 80% boiler are very short. And since return water temperature from the radiant system will always be below 130°F, the boiler will always be in condensing mode.
Oil is a polluting heat source, and its efficiencies are typically less than gas units. On the other hand, oil may be less expensive than gas in some regions and has more heat value than propane. But most of our oil is imported, and use of oil increases our dependence on foreign sources.
Waste-oil boilers also exist and are a suitable choice when sufficient waste motor oil or cooking oil exists on-site or at reclaim facilities. The waste oil can be recycled during periods of low heating demand. These systems often supplement the main heating source for the building, as waste oil demand fluctuates and the building’s heating requirement is often more than the waste oil heating source can produce.
Coal has pollution issues. The use of coal-fired heat sources in commercial buildings has practically disappeared in modern times across the U.S.
Electric resistance heating was most popular in the past when electric energy prices were low. It can be a viable option when annual electric energy loads are small, when the electric heat source is very efficient (as with a heat pump), or when the electricity is available at low cost.
Water HeatersThe use of domestic water heaters instead of expensive boilers has become a tempting solution for many engineers and installers due to its low installation cost. Radiant heat runs on warm water in the range of 70° to 125° for in-floor applications and in the range of 140° to 180° for radiant wall panels. If applying a non-condensing boiler, be certain to design the system to prevent cold shocking, when a lot of room temperature water comes back upon startup, by adding a mixing valve or other such device.
Only consider water heaters if they will be high-efficiency units with long-lasting materials such as stainless steel and if they will be powerful enough for the job. Don’t design commercial combination systems that run domestic water through the radiant system to mitigate the risk of water-borne disease. Further, tankless water heaters should not be used to power a radiant system, as they are not designed for this purpose and may not provide adequate flow rate. Although you may see a manufacturer’s diagram in the installation instruction for a radiant application, many manufacturers will also not stand behind the product in this application.
Renewable SourcesBiomass, such as wood, can be an attractive option in areas of the county with higher wood growth rates. Biomass systems work best with a heat storage system to allow prolonged burn times. Labor costs are higher for biomass systems, and pollution can be an issue; typically, some form of redundant heat source will be required, since in my experience, some of these systems are prone to maintenance issues.
Solar thermal systems are a good heat source choice for radiant and typically require a heat storage component. Solar systems will almost always require a redundant heat source for prolonged periods of cloudy days. Solar electric systems can also power electric heat sources, but current economics prove this to be a pricy option in large commercial buildings. However, using photovoltaics to provide power to electric heating has been used successfully in very small, near-energy-neutral, grid-tied commercial buildings with low heating and overall building energy loads.
Heat pump systems, especially geothermal, are among the most efficient systems in the market, with heating COP in the range of 3 to 5, roughly three to five times as efficient as the most efficient fossil fuel or electric boiler. But this efficiency comes at a higher installed cost. Geothermal systems are seeing increased use in small and large commercial buildings and for radiant heating. Remember, these systems are typically electric systems, and although very efficient, they often lose capacity at low outdoor air temperatures. In heating-dominated climates, this can lead to high demand charges. Electric supplemental heat is often included with these units, but careful design can often drastically reduce or eliminate the use of this supplemental heat.
Consider hybrid geothermal/boiler systems for better economics where the geothermal system handles +90% of the heating energy needs, and low-temperature, condensing boilers handle the remaining 10% of energy needs. Solar-assisted heat pumps also exist, where the solar thermal collectors are used to charge a tank that serves as a reservoir for the water-to-water heat pump unit. Air source heat pumps also exist and have typical COPs of 2 to 3.
Gas heat pumps work similarly to an air source heat pump, except they use gas instead of electricity to fuel operation. Instead of using compressors and ozone-depleting refrigerants like electric heat pumps, gas heat pumps have an engine operated by natural gas and utilize natural refrigerants, such as ammonia and water. This produces environmental benefits, and also gas heat pumps have less harmful emissions than gas or coal-fired boilers, and their heating COP of roughly 1.5 falls between gas boilers and air source heat pumps. Power and dependability are potential disadvantages of these systems. These systems have not yet matured in the marketplace for hydronic heating.
Good applications for heat pump systems include applications where natural gas is not available or gas price is high and electricity price low, and where heating/cooling loads can be balanced to minimize the borefield cost.
Waste HeatWaste heat opportunity also exists in some commercial applications. One application is cogeneration, such as from micro-turbines. Electricity is generated from the turbines, and waste heat from the turbines is used as a radiant heat source. These systems often have longer payback periods.
LifeCycle Cost ConsiderationsWhen selecting a heat source and radiant system, consider the lifecycle costs, which include the cost to install, operate, maintain, and dispose of the system. Also consider grant or utility incentives that may be available for specific higher efficiency technologies. Computer-based energy modeling should be performed to determine the most attractive economic investment. With any system, energy logging and monitoring are important to help identify operating efficiency degradation during the operating life of the equipment.
Maintenance ConsiderationsFor fuel-based heat sources, maintenance requirements center around the periodic cleaning of the burner as well as checking safety features including valves, low-water cut-off, and water level control.
Electric boilers require less maintenance than fuel-fired counterparts. Like all boilers they require water-quality management, and controls maintenance. However, there is no combustion chamber, no tubes to clean or replace, and no burners to clean and tune. However, electric heating elements require replacement from time to time.
Heat pump maintenance is generally minimal and centers around ensuring that air is out of the system and (if it is a closed-loop system) that the system remains pressurized and the heat exchangers are clean. Refrigerants should also be monitored.
Solar energy systems require periodic inspections and routine maintenance to keep them operating efficiently. From time to time, components may need repair or replacement and ensuring collectors remain unshaded, not cracked, and are clean will result in optimal efficiency. Inspect safeties, including pressure-relief valves. Antifreeze solution quality and quantity must be maintained, and water quality monitored.