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Hydronic heater basics
Hydronic heaters, or outdoor wood boilers, are typically located outside in small sheds with short smokestacks. They burn wood, which heats either water or water-antifreeze, which sends the heat and hot water through pipes into nearby buildings, such as homes, barns and greenhouses. However, hydronic heaters may be located indoors and may use other biomass for fuel, such as corn or wood pellets.
Source: Hearth, Patio and Barbecue Association (HPBA)
Although most units are designed to burn dry, seasoned wood, some people use them to burn green wood, which generates much more smoke and is less efficient. Other people burn household trash or treated wood, which not only release harmful chemicals and pollution, but is against federal law.
Unqualified or uncertified hydronic heaters can be substantially dirtier and less efficient than most other home heating technologies. With their smoldering fires and short smokestacks (usually no more than six to ten feet tall), hydronic heaters can create heavy smoke and release it close to the ground, where it can linger and expose people to nuisance conditions and health risks. EPA-certified models are the cleanest models available.
Qualified vs. certified
The EPA-qualified voluntary program for hydronic heaters ended in 2015, and most hydronic heaters that are "qualified" under EPA's Voluntary Hydronic Heater Program are not "certified" per EPA's Wood Heater New Source Performance Standard. Contact your state or local air quality agency for clarification on the type of wood-burning appliances, if any, that may legally be installed in your area. Find EPA-certified hydronic heaters.
Check your state and local ordinances
Hydronic heater emissions are a significant concern in many local areas. Numerous scientific studies report potentially serious adverse health effects from breathing smoke emitted by residential wood combustion. Residential wood smoke contains fine particles, which can affect both the lungs and the heart. In some areas, residential wood smoke can be a significant source of exposure to fine particle pollution.
Many local agencies have developed ordinances that ban or restrict the use of hydronic heaters, establish minimum distances to neighbors, and set minimum stack heights.
EPA-qualified hydronic heaters program (terminated)
The Voluntary Hydronic Heater Program ended May 15, 2015. This information is for historical purposes only.
EPA's Voluntary Hydronic Heater Partnership Program was first launched in 2007, providing a process for manufacturers to demonstrate models that were 70 percent cleaner than unqualified models. The program goal was to achieve early emission reductions, protecting public health sooner than a federal regulation. The program evolved to Phase 2, and EPA-qualified units were up to 90 percent cleaner than older, unqualified units.
Now that hydronic heaters are included in the New Source Performance Standard (NSPS) effective May 15, 2015, the Voluntary Hydronic Heater Program has sunset.
The list of certified hydronic heaters
includes hydronic heaters formerly included in the voluntary program which meet Step 1, and in some cases, Step 2 of the 2015 NSPS. Some models in the voluntary program are not certified under the NSPS because they were tested with a pervious hydronic heater test method (M28 OWHH). The 2015 NSPS did provide a sell-through period for these models, allowing manufacturers and retailers to sell them until December 31, 2015. After this date, the models listed in the Hydronic Heater Sell-Through List
can no longer be sold in the United States.