We have been asked a number of times recently to explain why we don’t require a large accumulator tank in heating systems served by our ÖkoFEN boilers. It’s a great question that generally comes from people who have taken the time to do some learning and some comparing.
Accumulators are quite common in cordwood fired solid fuel systems. In those systems, the purpose of the large, highly insulated water tank is thermal storage. The boiler is fired, produces lots of heat, which heats lots of water, and that hot water is stored for distribution as the heat demands of the house dictate over time. This system allows for batch firing of cordwood boilers so that the homeowner doesn’t have to try to continually match boiler output to heat demands.
Some pellet boilers use accumulators in a similar fashion. The boiler system responds to demand by coming to full fire to replenish the stored heat in the accumulator when the water in the tank reaches a preset low temperature. The boiler ceases firing when the temperature in the accumulator tank reaches the preset maximum temperature. As the heat needs of the house change, the heated water from the accumulator is circulated through the house to meet that demand. The somewhat cooled water returns to the accumulator after circulation gradually cooling the water in the accumulator tank leading to the next cycle.
This sort of accumulator use reduces the number of start/stop cycles the boiler would have to go through to meet the changing demands of the house if there were no thermal buffer in the system. By providing a large mass of water to be raised in temperature a significant amount, the boiler can work at full power for a reasonably long time before shutting down to await the next replenishment cycle.
Other pellet boilers achieve the same start/stop cycle reduction without a massive thermal storage system. These boilers modulate their heat production to achieve long run times while meeting the changing heat demands of the house as they arise. When demand, determined by comparing boiler water temperatures and outdoor temperatures, calls for maximum heat, the burner runs at peak output. As the demand diminishes, fewer pellets and less air are fed to the burner reducing heat output to match actual demand. The end result is the same: lengthened run times and reduced numbers of stop/start cycles.
This reduction is desirable because boilers are most efficient when they are run as nearly continually as possible. To think about that consider the difference in fuel efficiency between a car as it travels in stop and go traffic in the city and the same car operating at a more or less continuous speed on the highway. We all understand that the car driving highway miles does much better on fuel mileage; the same is true for boilers.
The reason for the difference in operational practice among pellet boilers has to do with the manner in which the manufacturer has chosen to minimize emissions levels for their boilers. Some boiler designs operate cleanest at full fire, so these boilers prefer the accumulator tank system configuration. Other boiler designs can operate cleanly through a significant range of output levels, so they generally modulate during operation and are used without accumulator tanks.
ÖkoFEN boilers have a certified ability to operate very cleanly through output levels from 100% down to 30%, so they are configured to modulate throughout that range during normal operation and are commonly used without accumulator tanks.