Bulletin #7230, Maine Home Energy: Sizing Your Biomass Boiler to Fit Your Needs

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Developed by Andrew Plant, Extension Assistant Professor, University of Maine Cooperative Extension. Reviewed by John A. Belding, P.E., Director, Advanced Manufacturing Center, University of Maine; Kathryn Hopkins, Extension Professor, University of Maine Cooperative Extension; and Donna Coffin, Extension Professor, University of Maine Cooperative Extension.

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If you are considering purchasing a new pellet or biomass boiler, there are several considerations you will want to take into account prior to your purchase.

For many of us who are interested in the economical and environmental benefits of using biomass-based heating sources for our homes, often times we consider the downside risk of these systems in relation to convenience. So, for most consumers, we seek the comfort of using our existing heating systems as a backup plan for when we want to take that week (or two) vacation away from the cold winters that we experience in Maine.

Too often, when we are in the decision-making process of making the purchase, we consider, and many times are recommended, biomass boilers that are over-sized, especially if they will be operating in conjunction with your current boiler or space-heating systems. To maximize your fuel-use efficiency, and in effect your energy dollar efficiency, we need to scrutinize the numbers.

Sizing Your Boiler

Currently, a typical recommendation is to size a boiler to your peak-load needs; in other words, the rating of the BTU/hour output of the appliance needs to be sufficient to meet the BTU/hour needs of your residence during the coldest part of the year. This recommendation is sufficient if one is considering a total replacement of their current centralized heating system (heating needs will rely solely upon the output of the biomass boiler), but this tremendously exaggerates your needs when the system is coupled to current heating systems or back-up systems. The result is inefficient use of fuel, the dollars spent on the fuel, and the system itself. A good comparison of this situation would be the purchase of a Formula 1 racecar just to run your normal, everyday errands.

How do we determine what our heating needs are? The best way is to have a certified professional do a heat-loss audit on your residence. They should take into consideration the following1:

  • The local climate
  • Size, shape, and orientation of the house
  • Insulation levels
  • Window area, location, and type
  • Air infiltration rates
  • The number and ages of occupants
  • Occupant comfort preferences
  • The types and efficiencies of lights and major home appliances (which give off heat).

We can also get a rough estimate of our needs on our own based upon our current usage of fuel and some local climate calculations of Heating Degree Days (HDD). Average yearly (1971-2001) HDD accumulations for some selected cities in Maine are:

Yearly HDD
Fort Kent10,07960

You can use the numbers listed above, or if you want specific data for a location and a particular year, visit Weather Underground’s Weather History or NOAA Online Weather Data, where you can do a personalized search of weather data at your closest weather data station2. If you choose to find specific numbers for your area over a given year, it’s suggested that you do so over a three or four year period to determine an average.

Here are some calculations you can do (based upon heating oil usage).

Boiler Sizing Worksheet (Excel)

What was your annual fuel consumption? ___________ gallons

What was the Heating Degree Day accumulation for that year? ________ HDD (or use data from above)

Divide the number of gallons used in that year by the number of accumulated HDD for the same year. This will provide a rough estimate of the number of Gallons/HDD that was used.

________ gallons / _______ HDD = ________ gal / HDD

What was the MAXIMUM  HDD for any one day? _______ HDD (you can use data from above)

Multiply the ________ gal / HDD by the _______ Max HDD = ________ gallons at Max HDD.

Divide ________ gal @Max HDD by 24 (hours per day) = __________ gallons/hr @ Max HDD

Multiply _________ gallons/hr @ Max HDD by the BTU value of the fuel (138,000BTU/gal heating oil) = __________ BTU / hr at Peak Load.

As an example:

If you used 1000 gallons of heating oil per year on average living in Houlton, ME:

You would take 1000gallons / 9,261 HDD = 0.11gallons / HDD

Your average Max HDD is listed as 54 HDD, therefore we would use 54 HDD x 0.11 gal / HDD = 5.94 gallons of heating oil per day on the coldest average day (Peak Load Day).  To get to your BTU/hr at Peak Load we divide 5.94 gallons by 24 hours and then multiply by 138,000 BTUs/gal of heating oil.  5.94 gal / 24 hrs = 0.25 gal / hr, 0.25gal / hr x 138,000 BTU / gal = 34,500 BTU / hr. Your peak load BTU / hr output is 34,500.

To proceed from here, you will have to make the decision as to whether your new system will act alone or will be coupled with an existing system. If it will be a standalone, you will want to size it to meet the peak loads that you have calculated and have a professional confirm your numbers.

If you want the system to be used in tandem with other heating appliances, you need to calculate the sizing of the boiler in accordance with these other systems. Ask a professional to help.  In no way should the new system (given its expense) be regarded as the back-up system. You want to utilize it as much as possible throughout the year in order to maximize the efficiency of your fuel and dollar investment. Early adopters are finding that the recommendations for sizing and use strategy are to size to peak load, and shut the system off to run on the back-up system during “shoulder” periods or warmer months of the year.

In typical situations such as this, where boilers will be coupled to other heating sources, boiler sizing should follow the “50/90” rule3.  The rule is a general guiding principle based upon peak versus annual heating loads. Data has shown that sizing boilers to 50 percent of the peak heating load needs results in meeting 90 percent of the annual heating needs. So logic follows that sizing to 100 percent of peak (in a tandem situation) is a largely inefficient investment, in which payback scenarios of your new boiler purchase can jump from 6-7 years payback to 20+ years payback. Instead of shutting your new investment off, keep it running.  Let the back-up system help it attain peak needs.


1 United States Department of Energy. 2009. Sizing Heating and Cooling Systems. www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12340

2 Historical Weather Data. 2010. www.wunderground.com/history/ or www.weather.gov/climate/xmacis.php?wfo=car

3 Kohler, A. 2010. Boiler Sizing — Partial Bin Analysis. www.kohlerandlewis.com/WebPage/Technotesboilerloads.html

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

© 2011, 2012

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