Special Considerations for Outdoor Hydronic Heaters

Special Requirements for Outdoor Hydronic Heaters

What are Outdoor Hydronic Heaters?

Outdoor hydronic heaters (formerly referred to as outdoor wood boilers or OWBs) are residential or small commercial wood-fired water heaters that are located outdoors or are separated from the space being heated. The wood burned in the large fire boxes heats water that is circulated into the home through underground pipes. The energy may be used to heat houses, shops, domestic hot water, greenhouses, swimming pools and spas.

Older, uncertified outdoor hydronic heaters can cause dense smoke, as many are equipped with limited exhaust systems that do not disperse smoke efficiently. The smoke emitted can negatively affect the health of the homeowner, nearby neighbors, and the environment.

How are Outdoor Hydronic Heaters Regulated?

In response to complaints about outdoor hydronic heaters in the early 1990s, the Vermont Air Quality & Climate Division adopted regulations (Section 5-204 of the Air Pollution Control Regulations) that apply to outdoor hydronic heaters installed after October 1, 1997. This regulation established set-back and stack height requirements for outdoor hydronic heater installations but did not set an emission standard that would reduce the air pollution emissions from these units. Subsequent amendments to the regulation in 2007 and 2009 established a certification program for outdoor hydronic heaters that required them to comply with first a "Phase I" and then a "Phase II" particulate emissions standard to be sold in Vermont. With the adoption of  a new federal Environmental Protection Agency (EPA) regulation, effective May 15, 2015, outdoor hydronic heaters were regulated nationwide, as well as in Vermont, and may also be regulated  by your town. The newer outdoor hydronic heaters, certified previously as "Phase II" outdoor hydronic heaters under Vermont’s regulation and now under the federal regulation, can be operated much more cleanly and with greater efficiency than uncertified outdoor hydronic heaters.

With the latest revision to Section 5-204, effective December 15, 2016, any new outdoor hydronic heater sold in Vermont must be certified by the EPA but does not need a specific Vermont certification. The set-back and stack height restrictions continue to apply to uncertified outdoor hydronic heaters and new outdoor hydronic heaters are still subject to a 100-foot setback requirement from the nearest residence, school or healthcare facility not served by the outdoor hydronic heater.

The Numbers Game and Choosing an Outdoor Hydronic Heater - What do the numbers mean?

8-hr heat output rating: If an outdoor hydronic heater 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 outdoor hydronic heaters 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 typical tested particulate (i.e. smoke) emission rates for hydronic heaters. In all cases, the lower the number, the "cleaner" the boiler. Vermont has adopted a "Phase II" particulate emission standard for outdoor hydronic heaters. Vermont's "Phase II" emission standard is 0.32 lb/mmBTU of heat output and is the same as the EPA standard for outdoor hydronic heaters adopted in 2015. Outdoor hydronic heaters 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 outdoor hydronic heater, the less lbs of particulate (smoke) is emitted per million BTUs (mmBTU) of usable heat produced. All certified outdoor hydronic heaters must emit less the current EPA standard of 0.32 lb/mmBTU during laboratory testing. Outdoor hydronic heaters 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 Outdoor Hydronic Heater

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 cordwood and pellet woodstoves, indoor cordwood and pellet hydronic heaters, masonry heaters, and outdoor wood and pellet hydronic heaters. 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 outdoor hydronic heaters.

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 hydronic heater. 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 outdoor hydronic heater 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 toxics, including potentially high levels of carbon monoxide. Locate the outdoor hydronic heater 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 outdoor hydronic heater installed after October 1, 1997 must be at least 200 from any neighbor's residence. It may not be legal to install an outdoor hydronic heater in a tight residential neighborhood. The setback requirement for installation of Phase II outdoor hydronic heaters is 100 feet from another person's residence, school or healthcare facility.

Questions to ask your dealer:

Is the outdoor hydronic heater 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?