Wednesday, April 9, 2014

EPA Lists Efficiencies of Qualified Outdoor Boilers from 39% to 78%

Updated: April, 2015

In April 2014the EPA posted efficiencies for outdoor wood and pellet boilers, also known as hydronic heaters.  Efficiencies range from 39% to 82% as measured using higher heating value.

The average qualified outdoor wood boiler is about 63% efficient and the average pellet boilers is 74%.  However, of the 40 units qualified under the EPA’s voluntary program, efficiencies were only listed for 21 units.  Nineteen of the units do not have efficiency numbers available because they were tested using EPA’s previous Method, which resulted in excessively high efficiency numbers.

The highest efficiency boiler in the EPA’s qualified program is a Central Boiler pellet unit at 82% HHV.  The lowest is Marway Welding’s Phase 2 – 200 at 39%.  Another boiler has 47% efficiency, which shows how boilers can qualify for the EPA’s voluntary program emission standards without having high thermal efficiency.  Thermal efficiency measures heat transfer from the combustion chamber to the water that heats the home.

The posting of these efficiency numbers was welcomed by the Alliance for Green Heat who has long advocated for consumers having access to reliable efficiency data.  The listing of reliable efficiencies makes hydronic heaters the first class of wood heating appliances to provide efficiencies to the general public.  It may take years for the public to get reliable efficiencies on most wood and pellet stoves, as most manufacturers have been unwilling to share that information with the public until it is required by law to do so.  To date, only 10 stove manufacturers have provided third party efficiency numbers for 27 stoves for the EPA to publicly post.  

These boiler efficiencies show that most boilers are not higher in efficiency than top performing wood stoves.  They indicate that there is even a wider range in boiler efficiency than wood stoves, including several that are very low efficiency units. 

The saga to provide consumers with such data has taken many twists and turns.  In 2011, the EPA removed efficiency numbers that were in the 90% range after state regulators questioned their accuracy.  Scott Nichols, who sells European indoor boilers that are not part of the EPA voluntary program, is one of few to write about these issues that have faced EPA and the boiler industry. 

The EPA requested outdoor boiler manufacturers to stop using the discredited high efficiency numbers in their advertising, but few stopped using them, leading to a letter from the EPA in 2013, which strongly urged manufacturers to stop using the efficiency numbers.  The Alliance for Green Heat reported on that development and documented numerous cases where outdoor boiler manufacturers were engaging in other misleading advertising practices

The EPA again sent a letter to qualified boiler manufacturers on January 31, 2014 because “misleading information has appeared on several partner manufactures and retailer websites.”  This information included language such as EPA “certified” or “approved” boilers and claims that their boilers were up to 90% efficient.


  1. Note that you have to burn twice as much wood in the 39% boiler for the same heat output as the 78% boiler. If both boilers had the same particulate (PM) emissions factor in terms of grams of particulate per kilogram of fuel (g/kg), the inefficient boiler would have twice the emissions measured in g/h at the same heat output, or in lb/MMBtu at any heat output.

  2. Nice post on outdoor boilers. I would like to know about the proper maintenance of boilers and their scheme. I came to know about UK boiler scheme from
    Azko Energy . It was helpful..