If you build a small, tight, well-insulated home — in other words, a green home — it won’t need much heat. Since typical residential furnaces and boilers are rated at 50,000 to 100,000 Btuh, they are seriously oversized for a superinsulated home, which may have a heating design load as low as 10,000 to 15,000 Btuh.

Builders have been struggling for decades with the question, “What’s the best way to heat a superinsulated home?” Your solution will depend in part on your answers to a couple of other questions:

Are you comfortable heating the house from a single point source? If you are, the best solution might be a wood stove, pellet stove, or a direct-vent space heater. These solutions work best in compact homes with open floor plans. Of course, the tighter the home’s envelope and the thicker the insulation, the more likely that indoor temperatures will remain fairly consistent from room to room.

Do you want an all-electric house? Green builders have diverging views on this question. Builders of net-zero-energy homes often avoid gas- and oil-fired appliances, preferring to balance energy loads with electricity produced on site by a photovoltaic (PV) array or a wind turbine.

Of course, most homes still depend on grid-powered electricity, and if your local electric utility generates power from fossil fuel, then it makes little environmental sense to heat with electricity. From a carbon-production standpoint, it’s usually better to burn fuels on site rather than in a remote power plant.

What not to install
Before moving on to right-sized solutions, it’s worth mentioning that it rarely makes sense to install radiant-floor heat in a superinsulated house.

As Alex Wilson has explained, for well-built homes, an in-floor radiant system is usually overkill. Although these expensive systems are a good way to heat a poorly insulated house, they are usually a waste of money in a tight house. Moreover, many radiant floor systems include energy-wasting circulators that operate for 24 hours a day during the heating season.

A two-stage furnace may make sense
Some builders of superinsulated homes have concluded that there’s nothing wrong with a conventional natural gas furnace. The engineers at the Building Science Corporation in Westford, Massachusetts often specify a two-stage natural gas furnace (for example, the Goodman GMH95-045). Unlike a one-stage furnace, which has only two firing modes — on or off — a two-stage furnace with an ECM blower operates efficiently even under partial load conditions.

The Goodman GMH95-045 is available on the Web for between $839 and $1,050. According to Building Science principal John Straube, production builders can install a heating system using this furnace for about $2,500.

Right-sized heating systems
What if you don’t want a gas furnace — perhaps because your house lacks a gas connection, or because the smallest available furnace is still too big for your needs? Here are three other options:

• Electric resistance baseboard heat.
• A direct-vent space heater.
• A ductless minisplit air-source heat pump.

Electric resistance baseboards
If your home has an impeccable envelope, and you’ve pared your heating load to the bare minimum, it may make sense to heat your house with electric resistance heat. That’s how Katrin Klingenberg, the founder of the Passive House Institute U.S., heats her 1,450-square-foot house in Urbana, Illinois.

It’s possible to buy a 1,500-watt (4,714 Btuh) baseboard heater on the Web for $90. If your house has a design heating load of 14,000 Btuh, you could heat it with three baseboard heaters ($270 plus installation). The main drawback to this solution is that in most areas of the country, electric resistance heat is expensive to operate.

Direct-vent space heaters
Many compact homes are easily heated by a direct-vent space heater. Installed on an exterior wall, these heaters require a wall penetration to accommodate two concentric vents. The outer pipe (the donut) brings in combustion air for the sealed-combustion burner, while the inner pipe is the exhaust flue. It’s possible to buy direct-vent heaters that burn natural gas, propane, or kerosene.

Direct-vent space heaters are affordable. A quick Web search reveals that a natural gas Empire MV145 heater (20,000 Btuh input) can be purchased for $500. Other options include the Rinnai 263 (9,100 Btuh output) for $909 and the Monitor GF1800 (16,000 Btuh output) for $1,150. If you want a heater that requires no electricity, you might choose the Robur TS2000 (7,400 Btuh input) for $509.

The first net-zero-energy house in the country — a superinsulated Habitat for Humanity house in Wheat Ridge, Colorado — is heated with a natural gas direct-vent space heater in the living room, supplemented by electric resistance heaters in each bedroom.

Ductless minisplit air-source heat pumps
If you’ve traveled to Mexico, the Caribbean, or the Mediterranean, you’ve probably seen ductless minisplit air conditioners. A ductless minisplit includes an outdoor unit (the condenser) and at least one wall-mounted indoor blower (a fan-coil unit); the two units are connected by copper tubing that circulates refrigerant.

A ductless minisplit is a type of air-source heat pump, so it can be used for heating as well as cooling. In recent years, Japanese and Korean manufacturers (including Daikin, Fujitsu, Mitsubishi, and Samsung) have significantly improved the heating efficiency of ductless minisplits. These units can now be used for heating in very cold climates — even in Vermont, where winter temperatures reach -20°F.

According to Andy Shapiro, an energy consultant from Montpelier, Vermont, Asian heat pumps are much more sophisticated than U.S.-made equivalents. “The Japanese are eating our lunch with these units,” Shapiro told me.

These ductless minisplits are so efficient and dependable that they are the heating system of choice for many low-energy buildings, including Passivhaus buildings. Shapiro used ductless minisplits for a heating system he designed for the newly completed net-zero-energy gymnasium at the Putney School in Putney, Vermont. He calculated that the capital cost for the building’s ductless minisplits and a photovoltaic (PV) array big enough to power them was cheaper than a ground-source heat pump system — even though the ductless minisplits require a bigger PV array. At the Putney School, the ductless minisplit system cost $100,000 less than a ground-source heat pump system, while requiring only $35,000 more in PV modules than would have been required to power the ground-source system.

According to Shapiro's estimate, which is based mostly on manufacturers' specifications, a ductless minisplit heating system can be expected to operate at an average coefficient of performance (COP) of 2.3 — that is, 2.3 times as efficient as an electric resistance heater. Shapiro's monitoring data reveal that the typical water-to-water ground-source heat pump system has an average COP of about 2.5. (Remember, the higher COPs claimed by ground-source heat-pump manufacturers are artificially high because they exclude pumping energy.) According to Shapiro, water-to-air ground-source heat pumps may have COPs as high as 3.5.

Specifying a ductless minisplit
The nominal heat output rating of a ductless minisplit is calculated at an outdoor temperature of 47°F. Since the unit’s heat output drops with the outdoor temperature, it’s important to check low-temperature performance before specifying a unit.

For example, the Mitsubishi Mr. Slim Hyper-Heat unit (model PUZHA36NHA) has a nominal heat output rating of 38,000 Btuh. According to the manufacturer, at an outdoor temperature of -13°F, its heat output drops only 21%, to 30,000 Btuh.

Another ductless minisplit, the Quaternity unit from Daikin (model FTXG15HVJU), has a heat output rating of 17,890 Btuh at 43°F. At -4°F, however, its heating output drops to 7,310 Btuh.

As long as the heating system designer sizes the unit so that it will meet the building’s heating load at the design temperature, there’s no reason it won’t keep a home comfortable — even when the temperature drops well below zero.

Ductless minisplits In Massachusetts
Carter Scott used a Mitsubishi Mr. Slim system with a nominal heat output rating of 28,000 Btuh to heat a net-zero-energy home he built in Townsend, Massachusetts. The 1,232-square-foot home has a design heating load of only 10,500 Btuh. The installed cost of the ductless minisplit system — including one indoor unit downstairs and another one upstairs — was $5,250, a price that includes both heating and air conditioning.

According to the home’s architect, Ben Nickerson, at an outdoor temperature of 0°F, the Mitsubishi unit still cranks out 10,000 Btuh at a COP of 1.8.

Can I really depend on a heat pump?
If you’re worried that a heat pump won’t be adequate when the thermometer bottoms out in January, remember:

• The coldest temperature of the year is reached for only a few hours a year.
• Tight, superinsulated homes lose heat very slowly, even during power outages. As long as a cold snap doesn’t last for many days, most superinsulated homes won’t lose much heat.
• In very cold temperatures, turning off the ventilation system will help a building stay warm.
• If you’re really worried about prolonged subzero cold snaps, one or two electric resistance baseboard units provide cheap insurance.

This blog was adapted into a Fine Homebuilding article, Heating Options for a Small Home.

Last week’s blog: “Deciphering the Tax Credits.”