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This article discusses design advice for small load heating systems such as used in energy efficient homes or to heat small areas in buildings. We discuss how to design, choose, buy & install heaters where the heating requirements for the building are modest.
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Readers should also see our extensive list of heating system inspection, diagnosis, maintenance and repair articles at HEATING SYSTEMS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
This article gives advice on sizing heating systems for energy efficient homes or for heating small areas. Following a discussion of how to determine the size of heating systems where traditional rules-of-thumb no longer apply, heat distribution tips offer real heating cost savings.
The article discusses small heating systems (less than 40,000 btuh) and sources for this equipment. We're not sure, however, about the recommendation for use of a high efficiency water heater for space heating in a building.
The duty cycle required of the equipment when used for building heating may result in a reduced equipment life and in some applications using a water heater for space heating can void the manufacturer's warranty. Be sure to review these questions before buying a water heater to use for house heat.
The small-load heating system article concludes with a discussion of integrated systems - an ideal solution that combines ventilation, heating, and cooling in one system, such as a heat pump water heater with exhaust-only ventilation.
Spend more on conservation, it is said, and you can spend less on a heating system. Yet frequently we year the complaint that small heating systems are hard to find. And once you find them, they may be no cheaper than a larger heating system. It's like buying a four-and-a-half-foot bathtub - you pay more money for less hardware.
Advice for Sizing Small Heating Systems
Don't rely on rules of thumb to size heating systems. They can oversize by 100 percent or more in an energy-efficient house. Learn how to do a heat-loss calculation yourself (it's not really difficult) or find someone who can. (See HEAT LOSS R U & K VALUE CALCULATION). If you do your own heat loss calculation, use a full worksheet method or computer program, not a quickie heat loss estimator.
You can get a thorough workbook, Heat Loss Calculation Guide, from the Hydronics Institute or from other heat loss calculation books we recommend other at the InspectAPedia bookstore (hosted by Amazon.com). Some of these are listed at the end of this article.
Standard practice is to size a heating system to meet the design heat load, which is the heating load the system will exceed only 2 1/2 percent of the time in the three coldest months of winter. Most heating contractors oversize heating systems by a large margin - mostly for quick recovery after nighttime thermostat setbacks, but also just to be on the safe side. Other heating contractors slightly under-size the heating system to boost heating system efficiency. ASHRAE recommends oversizing by 40 percent to make up for a 10 degree F thermostat setback.
Gross oversizing of heating systems is bad practice because it hurts heating efficiency and inappropriately increases heating costs. But given all the uncertainty in heat-loss calculations, the true delivery efficiency of the heating system, and weather, I would oversize the heating system a bit. In a high-mass house, you can overcome the thermostat setback problem by timing the setbacks to allow extra time for the building to warm up.
Watch out: because some heating contractors excessively oversized heating systems so much that the results were very inefficient heating plants and high heating bills - resulting in building codes in some areas specifically preventing over-sized heating boiler installation.
Should Your Small Load Heating System be Electric or Combustion?
One school of thought holds that the economics favors electric-resistance heating in small-heating-load houses. In my part of the country, with electricity at 10 cents per kilowatt hour (in the 1980's) and rising, a 1500 square-foot house with R-20 walls and R-40 ceilings costs about $1000 a year to heat (in 1983), according to a computer estimate. Even a super-duper-insulated house (R-40 walls, R-60 ceilings) would cost about $400. a year to year.
By comparison, a mid-efficiency (85 percent AFUE) oil-fired furnace would heat the same example houses for about $300. and $120. dollars respectively using an oil price of $1.00 per gallon. HEATING COST FUEL & BTU Cost Table contains current heating cost comparisons for different fuels. The moral is that high electric rates and electric baseboard heating don't mix.
On the other hand, paying a big premium for a super-efficient condensing furnace or boiler is not a good investment in a low-energy house. You will never recoup the extra cost above a mid-efficiency heating unit because the annual heating load is so low to start with. Good-quality, correctly-sized middle-of-the-road heating equipment is really what's in order. Look for a high AFUE rating on your heating system.
Heating Distribution in Small Load Heating buildings
Here is where you can win real savings. Tighter, better-insulated homes can use smaller and simpler heating (or cooling) distribution systems.
Massachusetts custom builder Paul Bourke told me that he saved $1500. to $2000. by downsizing the heating system on a 2000 square foot superinsulated house. Nearly all the savings came from using smaller air ducts, mostly 8-inch round. The savings paid for a $1300. air-to-air heat exchanger, which Bourke hooked in-line into the heating duct system.
You can win other savings by ignoring standard layout rules that no longer apply. For example, in a thermally sound home, heating supply registers and radiators needn't go on outside walls or below windows (standard practice in the heating industry). In general, these houses need fewer points of heat supply. Often a single pipe loop or trunk duct will do. You can simplify or eliminate heating area zoning.
Centralizing heat distribution may mean fewer heating system distribution system losses, too. In a major monitoring study, the Solar Energy Research Institute found that the actual delivery efficiency of 23 heating systems averaged 49 percent - far below their 80-percent average combustion efficiencies. Researchers attributed the poor showing in part, to heat lost from ducts where they pass through unheated spaces, slabs, and wall and floor cavities.
Small Heating Systems: Specifications, Types, Fuels, Information Sources
Heating systems with outputs of 40,000 BTUH (BTU's per hour) are less hard to come by. Most U.S.-made heating systems start at about 85,000 BTUH. To some extent, both gas and oil fired heating systems can be downsized below their rated capacity - up to a point. For example on an oil fired heating boiler, you can down-size the boiler and increase its efficiency by installing a nozzle that burns oil at a lower rate.
But as we (DJF) learned at oil burner school, if you make the size of the flame in the combustion chamber too small on an oil fired furnace or boiler, the inability of the flame to heat up the sides of the combustion chamber will cause incomplete combustion of the fuel and thus a much lower boiler efficiency - in short, if you downsize a specific heating system boiler or furnace too far it will not operate properly.
If standard heating equipment sources don't pan out, try suppliers of heating equipment designed for mobile homes, motels, and apartments. Their small heating systems often have closed-combustion and through-the-wall venting, both desirable in low-energy houses. While these are generally designed as room heaters, you can sometimes add simple heat distribution systems as well.
Another promising approach for small heating systems, developed mostly to promote gas heating in condos and apartments is to heat with a high-efficiency water heater. This can work with any heat distribution system: radiant floor, baseboard, or warm air. While tapping a heating loop off of solar or conventional hot water systems is nothing new, only recently have manufacturers sought code approvals.
A good source of information on these systems is the East Ohio Gas Co. which has put together a manual for heating with water heaters. The company's Paul Swenson maintained (in the 1980's) that these systems are safe and reliable and put no added strain on the water heater. One caution: because the heating loop circulates domestic hot water, all plumbing must be new and rated for potable water (e.g. brass or stainless steel pumps are required.)
Watch out: many if not most water heater manufacturers do not recommend using their equipment for heating a building, and many water heater manufacturers specifically exclude that use from their warranty coverage, or they reduce the promised life of the water heater if it is used for home heating -- DJF. The life expectancy of water heaters and things that affect water heater life are discussed at AGE of WATER HEATERS. Here is a sample water heater warranty.
While standard baseboard convectors can be used with a water heater, runs must be longer because the water temperatures are lower. You can save wall space by using fan/coil heating convector units (we provide fan coil heater sources below). Single-room fan coil heater units to transfer heat from the water-heated heating water to air have been used in solar heating since the early 1980's in the U.S.
For larger heating loads, a 1980's product entered the market: Hydroheat. Hydroheat is a centrally ducted water-to-air heating system that can accommodate air filters, central air conditioning, and other air heating features.
East Ohio recommends heating with water heaters for loads up to about 28,000 BTUH, while Apollo rates its largest water heater unit at 45,000 BTUH. In general, you can heat a living space with the same sized water heater you would buy for domestic hot water alone. That choice should prove economical. As heating systems get larger, the economic advantages start to disappear since large, expensive and inefficient commercial water heaters are needed for this approach.
One way to treat larger heating loads, suggested to me by a fan/coil manufacturer, is to use a gas-fired tankless water heater with an inexpensive hot water storage tank. (See Tankless Water Heaters for details about these "instantaneous" water heating systems, their capacities, and uses.) Both domestic hot water (DHW) and space heating could draw off the tank, which could supply up to 60000 BTUH to the house and have ample capacity left over for showers and appliances. Since hot water storage tanks don't have the up-the-flue losses of gas fired water heaters, this heating system approach might deliver top efficiencies at a reasonable first cost.
Warning: as we suggested above there are likely to be both life expectancy and warranty reductions when you use a water heater of this type for home heating as well. -- DJF.
Incidentally, if you install a water-heater for home heating use, the heater should be treated better than most people treat the average water heater - drain the sludge from the water tank (see DRAIN a WATER HEATER TANK) and change the water heater anodes annually to improve the life of the system. Things that affect water heater life are discussed at AGE of WATER HEATERS.
Integrated Systems for Small Heating Loads
The ideal heating system, I think, will combine ventilation, heating, and cooling in one inexpensive unit.
Heat-pump water heaters with exhaust only ventilation, such as a unit produced by DEC-International, holds promise. Once stored in the water tank, the heat could be distributed in any of the ways I've described above.
Another tack is to install a heating element in the ductwork of an air-to-air heat exchanger. With its ductwork serving double duty, a heat exchanger becomes a lot more economical. Several builders have had success heating superinsulated homes with small (4-KW) electric heaters this way. Rhode Island builder Arthur Boyce used a control strategy that allows the heater and supply fan to work alone as an electric furnace or with the air-to-air heat exchanger as a pre-heater of fresh air. Apollo and other water heater manufacturers make hydronic coils of this type.
Integrated heating systems from Scandinavia have been available since the 1980's. A system used by Cherry Building Systems AB handles air filtering, negative ion generation, and electric heating in one unit. When rising demand makes these products more available and less costly, low energy housing will better meet its promise of low first costs as well as low operating costs.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
"Heating Small Loads: solutions are in sight for this surprisingly tough task" - links to the original article in PDF form immediately below are preceded by an expanded/updated online version of this article.
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