Wood Pellet Heating: Installing thermal storage with your wood pellet heating system

What is thermal storage?

In the winter, your heating system is typically not running at full capacity all the time. For most homes, the heating is turned on high in the morning when your family wakes up and then in the evening when you are back from work or school till you prepare to go to bed. Even from the start of winter to its end, the heating load varies, with lower heating needs in early December compared to late January.

Thermal storage is another term for storing the heat generated by your system for later use. The most common thermal storage system is a simple insulated water tank that is installed in the basement along with the wood pellet heating system.

As discussed in the comparison of different kinds of biomass fuels, the most efficient way to run any heating system is to maximize the ‘steady state’ of the system. However, running at steady state means can pose some problems: if the heat needs of your home change, it's hard to stay at steady state while meeting your home comfort needs. With thermal storage, this situation is avoided: a hot water tank ‘stores’ the excess energy and when the heat load changes, the hot water is pumped at different rates to meet the needs and comfort of those occupying the space.

In some cases, the heating device will have modulate (reduce output from high to low) or have to switch off entirely. In the process of the heating system modulating (turning up and down in terms of heat output), some of the heat will end up being lost because the wood that is burning cannot immediately be turned off like you would just switch off the gas on your stove. This energy too can be saved with the help of thermal storage.

Thermal storage currently can only be integrated with hydronic (hot water) devices. The way it works is that when your heating system is turned down, the excess heat is used to heat the water, which is stored in the tank. When you need to heat your living or work space, the hot water first comes from the thermal storage tank while the heating device is cranking up the fire to heat more water.

This is a very simplified explanation, and every home’s heating system and needs will be slightly different. Talking to a professional is highly recommended and others here in the WePOWR communities may have answers to your questions.

Do I need to install storage with my wood pellet system?

Whether or not you will need to install a thermal storage will be based on a variety of factors. These include the design of the current distribution system, the space available around the heating device, the expected heating load profile of the home or office, the modulation capability of the heating device, and even the state and federal rules for devices or eligibility to the rebate programs.

Generally speaking, it's hard to say outright whether you should install storage or not, as each home's heating needs and system will be different. Professionals who install wood pellet heating systems will attempt to make the entire system (the device and distribution systems as a whole) work as efficiently as possible. Thermal storage may or may not be needed for this: some wood pellet heating systems have a high range of modulation, able to turn down to as low as 1/10th of maximum heating capacity.

Furthermore, your state government might have some requirements placed on each device for efficiency, emissions, as well as certifications. For example, in order to be eligible for a wood pellet boiler rebate through the state of New York, you must install storage along with your system. For rebate eligibility in Massachusetts, thermal storage is required unless the wood pellet heating device meets more stringent criteria (and an additional rebate is available for the storage system).