Indoor Boilers
Modern indoor wood and pellet boilers, already widespread in Europe, are slowly gaining in popularity in the US and can be a clean and efficient heating option. Automated pellet boilers (which heat water to be pumped through a home generating radiant heat) and furnaces (forced- air furnace which heats the air) using bulk pellets offer homeowners virtually the same convenience as a fossil fuel furnace by feeding pellet fuel automatically from a large storage bin. 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 unregulated, older outdoor wood boilers and from oil boilers.
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The US also has a legacy of very basic unregulated indoor hot air furnaces. However, the new EPA regulations that came into effect for smaller furnaces in May 2016 and for larger furnaces in May 2017 finally established federal requirements for this technology. All forced-air furnaces manufactured and sold in the US until May 15, 2020 can 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. After May 15, 2020 emissions limits are 0.15 pounds of PM per million Btu heat output for each individual burn rate. These emission limits are quite strict and the EPA list as of April 2020 has only 13 boilers and 2 furnaces that meet the 2020 emission standards.
Modern indoor wood boilers can have oxygen sensors, microprocessors, and catalyst capabilities. This allows them to achieve very high efficiencies, in the high 70s and low 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 but mainly they have bulk storage bin that holds several tons of pellets and is only refilled once or twice a winter. Efficiencies can be extremely high with scores between 80 and 90% HHV.
Older, unregulated indoor wood furnaces will take decades to be retired from homes across the US, but gradually, they are becoming more and more scarce. Often they are replaced with modern oil, gas or propane boilers, but as of May 2017, if they are replaced with a new wood unit, it will be vastly improved. Some states offer enhanced incentives to trade in old boilers for new ones.