John Seigenthaler: Renewable Heating Design

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When designing hydronic circuits, most engineers focus on what’s necessary for that circuit to absorb thermal energy at a heat source, carry it along like a conveyor belt and drop it off at one or more heat emitters.

In 2012 the price of #2 fuel oil in upstate New York was approaching a previously unheard of $4 per gallon. This spurred many pending heating system projects to consider the use of cordwood or wood pellets.

As with most things hydronic, there are multiple approaches, and the “best” approach for each installation has to consider cost, aesthetics, access to the existing piping, available wall space and the goal of how the overall system will operate based on existing or newly created zones.

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.