John Seigenthaler: Renewable Heating Design

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What characteristic do solar thermal collectors, hydronic heat pumps and thermal storage tanks supplied by biomass boilers have in common?

One of the best things about hydronic heating systems is that it’s easy to integrate some method of domestic water heating. This combination has been used for decades in systems where a boiler was the sole heat source. It’s also possible when a heat pump serves as the heat source.

I’ve covered pellet-fueled boiler applications in several past issues of PME. All of them have involved hydronic distribution systems. While such applications are certainly the prevailing way pellet boilers are used, they are not the only option. It’s possible to couple a pellet boiler to a forced air distribution system.

All hydronic systems designed around renewable energy heat sources — as well as those designed around conventional boilers — have at least one controller that measures and responds to temperature. Common examples are temperature setpoint controllers, outdoor reset controllers, mixing controllers and differential temperature controllers. Complex systems that operate in multiple modes, or use multiple heat sources, often have several temperature-based controllers.

As global energy planning moves away from fossil fuels and toward electricity, an increasing number of hydronic heating systems are being supplied by heat pumps. Some use water-to-water heat pumps supplied by geothermal earth loops. Others use air-to-water heat pumps.

Heated floor slabs, without floor coverings, have one of the lowest supply water temperature requirements of any radiant panel system. This makes them well-suited for use with renewable energy heat sources such as air-to-water heat pumps, water-to-water geothermal heat pumps, biomass boilers and solar thermal collectors.

This month I want to deviate a bit from a purely renewable energy topic to one that’s important across the entire spectrum of hydronics technology. It’s a topic that likely gets exercised on a daily basis in any engineering office where water-based HVAC systems are conceived.

Over the last several years, I’ve been reviewing submittals for proposed heating systems using pellet-fired boilers. The submittals come from heating professionals ranging from contractors to professional engineers. They’re required for a state incentive program that offers significant rebates to encourage growth of the biomass heating market.

Nearly all systems using cordwood gasification or pellet-fueled boilers perform best when the system includes a thermal storage tank.