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Outdoor Wood – Fired Hydronic Heaters
Outdoor wood-fired hydronic heaters (OWHHs) are fee standing meal fireboxes with stacks that are commonly separated from the space being heated. They are known by various names, including outdoor wood boilers, outdoor wood heaters, water stoves, and outdoor wood furnaces.
Outdoor hydronic heaters are used to provide heating and/or hot water to homes and outer buildings as well as to heat agricultural operations such as green houses and dairy farms. Outdoor hydronic heaters may be the right heating solution where there are multiple buildings to heat and there are no neighbors living nearby.
However, these heating units can poise a serious problem in populated areas because their technology does not allow for efficient combustion on a consistent basis. Inefficient combustion occurs when there is not enough oxygen present in a heating system, producing a greater volume of by-products, some of which are harmful to the humans and the environment alike. Unlike wood stoves and pellet stoves, OWHHs are currently not regulated in British Columbia.
How They Work
A large firebox heats water contained in a jacket surrounding the firebox. Heated water is then passed through a system of underground pipes to supply heating and/or hot water to homes and buildings. When the water circulating in the heater reaches an upper set point, air supply to the fire is cut off. When the water temperature drops to a lower set point, air supply is reintroduced to the system, thus starting the cycle all over again.
Reasons For Continued Use
Despite the health problems associated with outdoor hydronic heaters, they remain in use because gas and electrical heating costs continue to rise and many people have easy access to free or cheap wood. Outdoor hydronic heaters are also easy to control with an internal thermostat, and they may in fact save a homeowner insurance costs by being located outside the house and presenting a low risk for fire. Finally, outdoor wood fired heaters have an advantage over indoor installations because they leave no mess from ash and soot within the home.
Reasons For Concern
When the combustion temperatures inside the heater is kept relatively cool by the water jacket surrounding the firebox, a process known as cool combustion take place, producing smoldering, smoky fires.
OWHHs have dampers that operate Cyclically, opening or closing depending on demand. When the damper is closed, fire smolders, causing very poor combustion and heavy, foul-smelling smoke. Smoldering fire also causes creosote to form on the cool internal surfaces that ignite at the start of an “up” cycle; creosote results in additional smoke emissions. OWHHs emit hundreds to thousands times more particulates air pollution then oil and gas appliances and at least twenty times more emissions than Environmental Protection Agency (EPA) emissions – certified wood stoves. Fine particulates can be carried deep into the lungs and contribute to such human health problems as cardiovascular disease, chronic lung conditions, and premature death. Much of the smoke emitted by outdoor hydronic heaters is fine, condensed, organic material that does not burn under cool, oxygen – deprived conditions. Toxic pollutant such as benzene, formaldehyde, dioxins, and polycyclic aromatic hydrocarbons are produced, all of which may cause cancer.
Unlike most home heating devices, OWHHs are designed to provide heat and hot water all-year round, so that their effects are continuous. Fuel for outdoor hydronic heaters is also easily obtainable as even unseasoned, wet wood can be used.
Stacks on OWHHs are usually less then 3.5 meters from the ground, resulting in poor smoke dispersion and causing extremely smoky conditions in surrounding areas. Low – level heavy smoke often lingers and seeps through nearby ventilation systems, cracks in windows, doors, etc, exposing people to various nuisance conditions and health risks.
Large fireboxes allow people to burn a variety of toxic materials, including garbage, tires, plastics, and treated wood. In additions, their size allows fireboxes to accommodate wood of longer lengths that are difficult to season properly, remove excess moisture from, and burn efficiently. Fireboxes also encourage the burning of back yard waste like leaves that could otherwise be turned into compost, which further contributes to smoke emissions.
The EPA Voluntary Program
In the spring of 2007, the US Environmental Protection Agency (EPA) started a voluntary program that encourages OWHH manufactures to make cleaner models of outdoor hydronic heaters available in both the US and Canada.
Fourteen manufactures have signed and agreement requiring then to make at least one model that has a particulate matter (PM2.5) emission rate of no more than 0.258 grams per mega joule (g/M) of heat input. Heat input is directly related to the amount of fuel burned.
This new emission rate standard is expected to reduce emissions by 70 percent. Outdoor hydronic heaters that meet the new rate standard will be tagged as cleaner units at the point of sale.
What The Local Government Can Do
Stay passive and do nothing. OWHHs have an estimated lifespan of 20 years. Uncontrolled units installed today will contribute fine particulates and other air toxins to the air shed for a long time.
Develop public education programs. Public education programs should educate citizens about outdoor hydronic heaters and their associated health risks, help the public make informed choices and raise awareness about the importance of cleaner burning/heating methods.
Establish property line setbacks and stack height requirements. To protect neighboring properties from nuisance smoke and odors, set backs should be at least 150 meters away from the building and the stack height should be 1.5 meters from the roofline of nearby buildings. Incorporating these setback and stack height requirements into the building cold would help ensure compliance with them.
Set emission standards. An OWHH emission limit that is even greater than the EPA’s voluntary standard should be set. The Northeast States for Coordinated Air Use Management (NESCAUM) recommends that a Phase I emission limit be set at 0.189 g/MJ of heat input effective March 31, 2008, and another for Phase II, be set at 0.137 g/MJ of heat input effective of March 31, 2010. Note that the Phase I emission limit is still four to ten times dirtier then the EPA woodstove emission standard. NESCAUM has developed a model regulation available at www.nescaum.org/topics/outdoor-heaters. Vermont was the first state to adopt these emission standards.
Enforce bans and prohibitions. The sale or installation of OWHHs that do not meet the emission standards of CAN/CSA-B-415.1 (equal to EPA’s emission standard) should be prohibited. As no OWHHs currently meet these standards, this strategy would effectively ban all outdoor hydronic heaters. Banning OWHHs would also help reduce the burning of prohibited material. The Regional District of Central Okanogan has a by-law requiring all indoor and outdoor solid fuel-burning appliances, furnaces, or boilers to meet CSA emission standards. Environment Canada’s Model Municipal By-law for Regulating Wood-Burning Appliances outlines how municipalities can apply this strategy. The information is available at www.ee.gc.ca/cleanair-airpur/.
How To Deal With Existing OWHHs
Develop public education programs. These programs aimed at owners of OWHHs should educate them about alternative heating methods that are safe for both humans and the environment.
Set limits on use. Restrict or set the limit the use of existing OWHHs during the non-heating season, and/or when the air quality conditions are poor. This strategy would address high emissions events but not day-to-day pollution.
Establish setback and stack height requirements. Require the existing OWHHs meet the same setback and stack height requirements as new OWHHs within a set time frame. The local government should follow up and cite OWHH owners for non-compliance.
Enforce bans. A ban should be imposed on all OWHHs that do not meet the emission standard by a certain date.
Require removal of OWHHS upon transfer of property. When a wood-burning appliance that does not meet applicable emission standards stands in a property that is being sold, the heater should be ordered removed before the sale is completed or the property transferred. The District of Houston and the Town of Smithers have set requirements for wood heating appliances, including OWHHs.
Information Provided By: British Columbia Ministry of Environment The Lung Association of British Columbia
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