Updated: May 2014

Indoor Boilers

Modern indoor wood and pellet boilers are widespread in Europe and are slowly gaining in popularity in the US and can be a clean and efficient option. Automated pellet boilers (hot water) and furnaces (hot air) using bulk pellets offer homeowners virtually the same convenience as a fossil fuel furnace by feeding fuel automatically from a large storage area. New Hampshire, Maine, Massachusetts, New York and Vermont have offered generous rebates to install these systems in recent years to help transition the northeast away from outdoor wood boilers. The US also has a legacy of very basic unregulated indoor hot air furnaces. The new EPA regulations that came into effect in May 2017 finally closed that loophole by requiring all forced air furnaces sold in the US to emit no more than 0.93 lbs per mmBTU of heat output, regardless of whether they are wood or pellet units or large or small furnaces.

Modern indoor wood boilers can have oxygen sensors, microprocessors, and catalyst capabilities. This allows them to achieve very high efficiencies, in the mid 80s, HHV. They need to be loaded once or twice a day and hot water storage tanks ensure that they burn as cleanly and efficiently as possible. Hot water storage allows the full load of fuel to be completely burned, while extra heat is stored in the tank. This prevents the boiler from cycling on and off.
Modern pellet boilers and furnaces also have automated sensors to control combustion, along with automated fuel loading so that the operator may only need to clean out the ash tray once or twice a month. Pellet boilers and furnaces can be loaded daily with 40-pound bags of pellets, just like stoves. They may also have a bulk storage bin that holds several tons of pellets and is only refilled once or twice a winter.
Unregulated indoor hot air furnaces are still being sold and installed in the US and many are in the same price range as larger stoves, $2,000 - $5,000 range. While typically not as polluting as outdoor wood boilers, these units will vastly improve as they are regulated for emissions and have to report their efficiencies.
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