The following table lists OWBs certified by Vermont to meet the Phase II particulate emissions standard. After March 31, 2010 all OWBs sold or purchased in Vermont must be certified to meet the Phase II standard of 0.32 lb/mmBTU heat output.
Vermont Certified Outdoor Wood Boilers (pdf) - Updated October 2016
The Numbers Game and Choosing an OWB - What do the numbers mean?
8-hr heat output rating: If an OWB is loaded with wood and burned at a rate such that the whole load took 8 hours to burn, it will have produced heat at this hourly rate. Note that this rating may not be very useful and it does not reflect the maximum heat output which may be very much higher than the 8 hour average output rating. In most cases, burning continuously at the maximum rated output would burn the load of wood in less than 8 hours. Important Note: Purchasers of OWBs should always check with the manufacturers, a knowledgeable boiler dealer or a heating specialist to determine proper sizing for their heating needs.
Emission Rates: The following describe the tested particulate (i.e. smoke) emission rates for the boilers listed in the table. In all cases, the lower the number, the "cleaner" the boiler. Vermont has adopted a "Phase II" particulate emission standard for OWBs. Vermont's "Phase II" emission standard is 0.32 lb/mmBTU of heat output and is the same as the EPA standard for OWBs adopted in 2015. OWBs meeting this standard are cleaner and more efficient, requiring less wood to heat your home.
- Average Emission Rate (grams/hr): A gram is a measure of weight and this number describes the weight of solid particles (particulate matter) emitted per hour, as determined during a specific laboratory test. The greater the particulate emissions, the denser the visible smoke emitted from the boiler. For comparison, the recently adopted EPA standard for indoor woodstoves is 4.5 g/hr. Most new woodstoves on the market do much better than this with some emitting less than 1 g/hr of particulate.
- Average Emission Rate (lb/mmBTU heat output): A BTU (British Thermal Unit) is a unit of heat. Each cord of good dry hardwood fuel contains about 30 million BTUs of heat but some heat is lost to gases going out the stack. The "lb/mmBTU heat output" is a measure of the pounds of particulate emitted for each million BTU of heat output (the heat that can be used to heat your home and water). The cleaner the OWB, the less lbs of particulate (smoke) is emitted per million BTUs (mmBTU) of usable heat produced. This is the critical number to compare to Vermont's Phase II standard of 0.32 lb/mmBTU of heat output (see column 7 in the table). All certified OWBs must emit less than this standard during laboratory testing. OWBs with the lowest number in this column not only have to be clean burning, but are also good at transferring the heat into the water that gets pumped into your house (i.e. they are most efficient).
Choosing an OWB
Lifestyle: Choosing a wood heating source is an important decision. Consider having an energy audit and increasing the efficiency of your home no matter what device you choose. Burning wood is labor intensive and you want the most benefit from your efforts. There are many wood heat options including indoor woodstoves, indoor wood boilers, masonry heaters, pellet stoves or boilers and outdoor wood and pellet boilers. Burning regular firewood is not only labor intensive but requires a great deal of good storage space to dry the wood properly. Wood that is not dried properly will not give the maximum heat and will produce more particulate matter. Pellets can be purchased in smaller or larger quantities and may be easier to handle, though supplies may be uncertain. Woodstoves can heat without power but pellet stoves and outdoor units require electric power to operate. Also consider that some energy is lost in the underground piping for outdoor installations and some energy is required to run the pumps and other electrical equipment on OWBs.
Sizing a wood burning device: In general, for any wood burning device the smaller the better. Oversized units tend to burn at lower temperatures (less efficiently) much of the time. One exception is the use of very large water tanks to store the energy from your boiler. In this case, the heating device can burn at a maximum rate (usually the most efficient rate) and the excess energy is stored in the water tank for use over time. Your heating requirements are unique to your house and your family, especially if you're heating domestic hot water. It is best to discuss your heating needs with a professional. If you have been burning oil or gas, you can calculate the total yearly energy use from those fuels as an estimate of your heating demand. It is important to include the overall efficiency of the heating devices when performing these calculations.
Location: Exhaust from any combustion device should be considered a potential problem. Locate your OWB so there is little chance that the exhaust will impact on your house, your neighbors' homes or where children are likely to play. Even the cleanest wood burning device will emit toxins, including potentially high levels of carbon monoxide. Locate the OWB reasonably close to your house to minimize the loss of heat from the underground piping. The location should also be readily accessible even when the snow gets deep. Good covered wood storage near the boiler is also essential. By Vermont regulation, any uncertified or Phase I OWB installed after October 1, 1997 must be at least 200 from any neighbor's residence. It may not be legal to install an OWB in a tight residential neighborhood. The setback requirement for installation of Phase II OWBs is 100 feet from another person's residence, school or healthcare facility.
Questions to ask your dealer:
Is the OWB model certified by the State of Vermont and the U.S. EPA?
How long is the warranty and what is covered?
How long have the units been on the market?
What is the thermal efficiency of the unit?
What kind of maintenance is required?
What are the installation requirements?
How large a space will the unit heat?