Heating Small Loads
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Small heating system designs for energy-efficient homes or for heating small areas
How to design, select, buy, & install Heating Systems for Small Heating Loads, Super-insulated Homes, or Small Areas
Use of domestic water heaters for small home heating loads
Use of tankless water heaters for small home heating loads
Heating distribution suggestions for super-insulated buildings
Comparing first-costs versus operating costs for heating systems for superinsulated buildings
Questions & answers about the most effective & efficient way to heat small areas or very energy-efficient homes that don't need much heat
Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices

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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How to Design, Select, Buy Heating Systems for Small Heating Loads & Small Areas

Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

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.

Original article:

"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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Heating Small Loads - part 2

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Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years. ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in buildings, and other topics.
DEC International, Madison WI
East Ohio Gas Co., PO Box 5759, Cleveland OH 44101. East Ohio Gas has put together a manual for heating with water heaters.
Hydronics Institute, PO Box 218, Berkeley Heights, NJ 07922 can provide a heat loss calculation instruction book.
Handbook for calculating solar influenced building heat losses (SAND),
Heat Loss from Dwellings (Building Research Station digest No. 35).
Room-sized fan coil units:
- Enerspan, Inc., 14168 Poway Rd., Poway CA 92046 - Sunvector
- Myson, Inc., Embrey Industrial Park, PO BOx 5025, Falmouth VA 22401 - Solar Vector
- Turbonics, Inc., 11200 Madison Ave., Cleveland OH 44102 - Chill Chaser
- Apollo Industries International, Inc., PO BOx 61056, Raleigh NC 27609 - Hydro Heat
Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the InspectAPedia Bookstore
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
"Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
"Heat Exchangers for Solar Water Heating Systems", U.S. DOE describes the types of solar water heater heat exchange methods between the sun and the building's hot water supply
"Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
"Solar Water Heating System Maintenance and Repair", U.S. DOE
"Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
"Scaling and Corrosion in Solar Water Heating Systems", U.S. DOE
www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12850 is the base U.S. DOE website for these articles
"Active Solar Heating Systems", U.S. Department of Energy, including
"Radiant Heating Systems" U.S. DOE
"Absorption Heat Pumps & Coolers", U.S. DOE
"Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
"Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.

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How to Design, Select, Buy Heating Systems for Small Heating Loads & Small Areas

Advice for Sizing Small Heating Systems

Should Your Small Load Heating System be Electric or Combustion?

Heating Distribution in Small Load Heating buildings

Small Heating Systems: Specifications, Types, Fuels, Information Sources

Integrated Systems for Small Heating Loads

Original article:

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair