John Siegenthaler: Hydronics Workshop

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Many air-to-water heat pumps are combined with boilers. In retrofit projects, it’s likely an existing boiler; in new projects, it’s likely a mod/con boiler. The boiler can provide supplemental heat during very cold weather. It can also serve as a backup heat source should the heat pump become inoperable.

Upcoming DOE regulations and rising energy costs are creating new opportunities to improve hydronic distribution efficiency through smarter circulator strategies.

Air-to-water heat pumps are different from boilers. As the outdoor air temperature decreases, their heating output decreases. That’s a characteristic of all air-source heat pumps, and it runs opposite to how heating load changes with decreasing outdoor temperature. 

In hydronic systems, the circuit usually passes through the boiler, which adds heat. When using a block heater and the boiler is off, it dissipates heat from the circuit. This heat loss must be considered when measuring the circuit's true "net" heating output.

Measuring the flow rate through a circuit has always been more of a challenge compared to measuring temperatures. Very few circuits are equipped with permanently installed flow meters.

Last month, we reviewed the basics injection mixing. It’s a method for controlling water temperatures by regulating the flow of heated water “injected” into a circulating distribution system, while simultaneously removing an equal flow of cooler water from that distribution system. Any method that controls the rate of injection flow also controls the rate of heat transfer into the distribution system.

For boilers operating on natural gas or propane, flue gas condensation begins at inlet water temperatures below about 130 ºF. Boilers operating on low sulfur #2 fuel oil have lower dewpoint temperatures in the range of 110 ºF.