John Siegenthaler: Hydronics Workshop

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A building with slab-on-grade heating from a cast iron boiler plans to expand. A monobloc air-to-water heat pump will provide heating and cooling for the new space via a fan-coil unit, with the existing boiler serving as backup.

Air-to-water heat pumps are gaining popularity in North American hydronic systems, but many practitioners are discovering that they operate differently from boilers.

As electrification reshapes the HVAC market, hydronic-based heating and cooling systems stand ready to offer many design and performance benefits. One of them is the potential for thermal storage. After all, water is one of the best materials on earth for storing sensible heat.

The availability of heat pumps allows designers and installers of hydronic heating to include cooling. A simple method involves using a single air handler with a chilled water coil for a ducted distribution system, providing single zone cooling that can complement multi-zone hydronic heating in average houses.

Most heat pumps in North America extract heat from outside air to warm interiors, which is why we call them "air-to-air" heat pumps. However, this term is incomplete, as heat pumps can also provide comfort without relying on forced air delivery.

Last month, we started with an air-to-water heat pump for space heating and added an auxiliary heat source and chilled water cooling. This month, we’re adding domestic water heating, similar to last month’s setup.

Standardized piping layouts for air-to-water heat pumps can improve acceptance by facilitating a basic heating system that uses a monobloc heat pump with a 30% propylene glycol solution for freeze protection.

Hydronically-heated concrete floor slabs offer comfort but can be slow to warm up. A proposed solution uses a heat pump and variable-speed circulators to provide rapid heating through air handlers, ensuring quick warmth during cool days while maintaining energy efficiency.

In 1986, I installed a 1.5-ton geothermal heat pump in my home using two 500-foot horizontal HDPE earth loops. After digging a trench, I connected the loops to the pump and opted for a 2 HP swimming pool pump to fill and flush the system, as a garden hose couldn't provide the needed flow rate.

In 2024, the North American air-to-water heat pump market saw significant expansion, with nine new offerings introduced at the January AHR show by companies new to this category. By year's end, at least 18 companies either have or plan to offer these pumps in the U.S. Read the list of these companies as of mid-2024.