Regardless of what kind of backup heating system you put in your passive solar home – forced-air, radiant floor, baseboard hot-water, or something else – you're going to need a source of heat. And that means a high-efficiency boiler or furnace.
Modern high-efficiency gas boilers use an induced-draft setup. This means that they have a fan to push the hot gases out of the combustion chamber – they are so efficient at sucking the heat out of the combusting gas that the flue gas simply wouldn't have enough heat to rise and escape without a kick in the pants!
High-efficiency gas boilers usually reach an efficiency of 80% to 85%. They come in a variety of sized and designs.
Most modern gas furnaces are induced-draft models with efficiency ratings around 78% to 85%. They minimize indoor air pollution. An important feature to look out for is a sealed combustion chamber, which prevents the release of dangerous gases into the home, such as carbon monoxide.
It is possible to get an efficiency score from 90% to 97% in a gas furnace, but this requires a condensing feature . This is when the heat exchanger is so efficient that the gas actually cools enough to condense, releasing a lot of extra energy in the process. The resulting condensate drains out and the escaping flue gas is so devoid of heat that it can be vented through plastic pipe.
Oil boilers and furnaces
Oil-based models squirt fuel oil into a chamber, where it mixes with air to make it more combustible. The best models create turbulence in the chamber, to enhance the process. Most models are between 78% and 82% efficient, though some condensing models are available which beat that. The problem is that condensing models produce a corrosive liquid as part of the process, which can damage the boiler or furnace. Self-destruction is a high price to pay for a little bit more efficiency.
Strangely, there are very few models of oil boiler or furnace with a sealed combustion chamber. This is doubly odd when you consider that cold external air getting into the chamber reduces efficiency and may impede start-up.
A very eco-friendly and increasingly popular choice, wood-burning furnaces can be installed in basements, storage rooms, utility rooms, garages, and even outside in their own sheds. Furnaces come in models of all sizes, for residential, commercial, and even industrial applications. They can be used in conjunction with almost any heating system.
Wood furnaces are much like wood stoves. They burn clean and are generally pretty efficient (though they range widely from basic to advanced models). They are easy to operate and use a renewable fuel source.
On the down side, they tend to be noisy because they need a fan to distribute hot air through ducting, which also uses electricity. Loading fuel and removing ash can be a hassle, as well. The fire hazard is low as long as the furnace is properly installed and maintained, but it is still an additional risk.
Many wood furnaces come