Harry “Dutch” Dresser, Ed.D.
Maine Energy Systems
Bethel, Maine

Since Maine Energy Systems began working to make automatic wood pellet boilers a heating alternative in the US in 2008, much has changed for the end-user.  For most of the last 6 years, MESys has been largely alone in the marketplace.  A few, older bag-fed hobbyist boilers were in the market, but there were few, if any, difficult choices to be made by those who wanted to heat their homes with central wood pellet boilers.

In the last two years, or so, the northeastern US has experienced an influx of pellet heating appliances that is completely consistent with patterns of growth in European countries as market leaders opened new marketplaces.  There are now a fair number of pellet boilers available in the northern marketplace, some of them of European design and/or construction and some homegrown.

This is good for a healthy marketplace even though it places a burden on potential buyers to clarify their needs and expectations and budgets so they can match them against available products.

As I’ve watched this market develop behind us, it has become clear to me that there is a metric that cuts through marketing hype and does a fine job of providing categories of boilers from which potential customers can choose.  That metric is the length of time the owner can expect a boiler to reliably operate without any human intervention.  For fun, I call it UROP (Unattended Reliable Operating Period).

UROP (Unattended Reliable Operating Period) Limitations

There are three factors that can limit UROP (the length of time the boiler operates without human intervention): burner malfunction, need for ash removal and/or burner cleaning, and fuel supply limitations.  Let’s explore each briefly.

            Burner malfunction

Well-designed and engineered burners are quite tolerant of the natural variability in pellets, so burner malfunctions needn’t be a limiting factor in the length of time your boiler runs without interruption.  However, as with all products, there are lower quality, lower priced pellet boiler systems whose burners are less tolerant of differences in pellet quality, and these burners can fail unpredictably for a number of reasons.

            Ash removal and/or burner cleaning

Probably the biggest differentiator among pellet boiler systems is the manner in which ash is handled and heat exchanger and burner surfaces are kept clean.  The most automatic systems clean their burner surfaces nearly continually without interrupting the heating process, clean their heat exchanger surfaces daily to maintain efficiency, and remove the ash from their own fireboxes continually.

Less automatic systems, often also lower in quality and price, require the homeowner to occasionally clean the burner surface, regularly clean the heat exchanger surfaces, and routinely remove the ash from the base of the firebox after shutting down and cooling the system.  Various combinations of what you have to clean and how often are found in systems currently in the market in the Northeast.

The shortest required cleaning cycle can define the UROP for any given boiler system.  If you are required to empty the ash every two weeks during heating season, and you have plenty of fuel for two weeks of operation and the burner is tolerant of varying pellet quality then it is reasonable to consider your UROP two weeks.

If, however, you are confronted with manual recommendations to clean this or that “as required” or “if necessary,” or something similar, you’ll have to gain experience with the system to know what that really means.  It might mean this burner is finicky, so you will need to clean it manually whenever the boiler shuts down, or it might mean, this burner rarely causes a fault but it might occasionally.  The best way to learn about a boiler’s habits before you buy it is to talk with a few present owners.

            Fuel supply limitations

Boilers that are self-feeding from storage units holding several tons of pellets, or more, generally do not have their UROPs limited by fuel supply limitations.  Boiler systems that are bag filled or are attached to small storage units may have their UROPs limited by fuel quantity available.

Pellet boiler categories

            Base of the Convenience Pyramid, very limited UROP

Using UROP (Unattended Reliable Operating Period) to categorize the equipment currently available in the northeastern US, I consider the equipment easily divisible into three broad categories.


The category with the clearest limitation on UROP is comprised of boilers that are hand-filled, either in a hopper on the boiler or in a very small external unit, or boilers whose manuals report that their burners may require attention at irregular intervals.  Regardless of the other properties of the boiler system, these systems require the homeowner to be present to fuel the boiler at short, regular intervals. If you don’t plan to leave your house unattended, or you have a back-up heating system that is reliable for a longer period, these devices can work for you.  They should be priced very favorably given their short UROPs and their low convenience.  Because of their short UROPs, these boilers are not recognized as primary heat sources by insurance companies.

            Middle of the Convenience Pyramid, limited UROP

The actual limit to UROP among pellet boilers in the middle of the Convenience Pyramid is a bit more difficult to ascertain.


Boilers in this category can either have small storage units and/or regular cleaning regimens that limit their UROPs.  Often boilers in this category require manual cleaning of the firebox or emptying of an ash container within the firebox at intervals determined by pellet consumption.  UROP then is determined by variable heat demand; it could be measured in days or weeks for the same boiler under different weather conditions.

If the boiler requires burner cleaning, and the interval isn’t specified, UROP can be constrained by that factor.

If the burner is reliable, and the ash removal cycle is greater than the amount of ash produced by the quantity of stored pellets, then the storage unit’s capacity becomes the limiting factor on the UROP.

If you are willing to clean your boiler’s firebox, heat exchanger, and/or burner, these boilers can suit those willing to work on their boilers if their price reflects their moderate inconvenience.

            Top of the Convenience Pyramid, lengthy UROP

Boilers that sit toward the top of the Convenience Pyramid are the most automatic of the pellet boilers.  They clean their own burners regularly; they clean their own heat exchangers regularly; and, those at the very top of the Convenience Pyramid de-ash their fireboxes into external containers that can be emptied without shutting down the boiler.


Generally these boilers are connected to storage units of three tons, or greater, capacity, so fuel supply isn’t usually the limiting factor on UROP.  Ash removal is the limiting factor on length of operation without intervention for these boilers.  However, for those that remove ash to external storage units, such as the MESys/ÖkoFEN boilers, more than two tons of pellets can be burned before the external ash storage box requires emptying.  Two tons of pellets has the energy equivalence of 240 gallons of oil, so a house left unattended on maintenance level thermostat settings can be left for signifant periods, often measured in months, without fear of exceeding the boiler’s UROP.

Get what you pay for

Each of these boiler categories contains examples of boilers that could meet the expectations of different kinds of end-users.  The challenge for new consumers of pellet boilers is to understand what level of convenience they expect from their pellet-fired central heating systems and learn before buying what level of convenience boilers you consider can actually deliver.


Once you’ve decided, get quotes that outline exactly what you’ll be buying and at what cost.  Many of the middle tier boiler systems are sold “a la carte,” with a very simple basic boiler quoted initially to get your attention and the pieces to make it operate as you expect available at additional cost.  Take care to pay prices that reflect the level of convenience and reliability you have chosen.  Don’t pay top-level prices for bottom- and middle-level products.

Happy heating, Dutch Dresser

Dutch Dresser is the Managing Director of Maine Energy Systems, manufacturers and distributors of MESys/OkoFEN boilers.