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Small Wood Boilers


Changes to Maryland's Regulations

Maryland's rule that regulates small wood boilers (COMAR has been repealed effective February 12, 2018. Since 2015, new residential hydronic heaters sold in the U.S. have been regulated by the U.S. EPA under 40 CFR Part 60, Subpart QQQQ.

The repeal of COMAR removes any redundancy or potential conflict with the federal regulation - this includes the certification requirement for manufacturers who wish to make their units available for sale and use in the State. New residential hydronic heaters made available for sale and use in Maryland are subject to all requirements and standards under 40 CFR Part 60, Subpart QQQQ. For more information, please refer to the following:

As a reminder, state, local and county governments have the authority to enforce applicable provisions under 40 CFR Part 60, Subpart QQQQ, as well as any other applicable state, local and county regulations and ordinances - including nuisance complaints that may arise from the operation of residential hydronic heaters.

Frequently Asked Questions

What are residential hydronic heaters?


Hydronic heaters (also called outdoor wood heaters or outdoor wood boilers) are typically located outside​ the buildings they heat in small sheds with short smokestacks. Typically, they burn wood to heat liquid (water or water-antifreeze) that is piped to provide heat and hot water to occupied buildings such as homes, barns and greenhouses. Hydronic heaters, however, may also be located indoors and they may use other biomass as fuel (such as corn or wood pellets).

Why are residential hydronic heaters regulated?


​Smoke from redidential wood heaters contains fine particle pollution, also known as fine particulate matter or PM2.5, along with other pollutants including carbon monoxide, volatile organic compounds (VOCs), black carbon, and air toxics, such as benzene. Residential wood smoke can increase particle pollution to levels that pose serious health concerns, and in some areas constitutes a significant portion ofthe fine particle pollution problem.

The fine particles in smoke can get deep into the lungs, harming the lungs, blood vessels and heart. People with heart, vascular or lung disease, older adults and children are the most at risk.

What considerations should I make when choosing a residential hydronic heater?


Consider lifestyle, size of the unit and location. Also consider local ordinances and regulations or permitting requirements that may apply. Some communities restrict the use of residential hydronic heaters, while some cities, towns or municipalities may require permits for operation and/or installation.

Questions to ask your retailer:

  • ​Is the outdoor hydronic heater model certified​ by 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?

What if I own or purchased a boiler prior to the new regulations?


​Owners of pre-existing units will be allowed to continue to keep their units provided the units are operated in​ accordance with state and federal rules​ and do not pose a public nuisance.

What should I do if my neighbor's hydronic heater creates excessive smoke and/or odors?


  • ​​Seek medical care if you are experiencing symptoms associated with smoke exposure. Symptoms may include coughing, shortness of breath irritated eyes, etc.
  • Contact your local municipal or county government. There may be local ordinances, enforcement codes and regulations pertaining to residential hydronic heaters
  • Contact your local health department to inquire if the situation constitutes a public health nuisance
  • Contact the Maryland Department of the Environment (MDE). MDE is the lead State agency with regards to air quality and responds to complaints, provides compliance assistance and pursues enforcement actions when necessary. You may call (410) 537-3215 to report nuisance complaints


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