ASK THE EXPERTS: Hydronic Heating

Solar hot water system.

I’m trying to design a long-term cost-effective heating method for my 32-by-48-foot uninsulated shop. I’m considering a stand-alone solar water heating (SWH) system that uses aluminum wall plates and tubing or radiators and fans.

Spray foam should take care of insulation and make the building more rigid, but I’m wondering if an SWH system, along with a 1,000-gallon storage tank, would make the effort worthwhile. I could go natural gas heat since the house is connected to a gas line, but trenching to the line would be needed.

Wayne Gale • via

When considering a solar heating system in a cold climate, two major factors contribute to its effectiveness—a high-performing building envelope and a heating distribution system that matches up well with solar. Radiant distribution is the best match for a SWH system since it is typically designed to maintain a building’s comfort with much lower temperatures (90°F to 100°F). Other types of hydronic distribution, such as baseboard elements, need 180°F to 190°F water.

Radiant distribution is typically embedded tubing in a concrete slab, but there are other options. John Siegenthaler provided a comprehensive view of many of these options in “Renewable Hydronic Heating” (HP152), including a thorough explanation of the details that make radiant systems an excellent match for solar.

Trying to retrofit insulation under a slab or embed tubing in an existing floor is a lot like trying to put toothpaste back in the tube. You’ll need to do the best you can to maximize the floor’s efficiency and find another way to distribute heat than a radiant slab. Since a majority of a slab’s heat loss occurs along its perimeter where it is most exposed to the coldest air temperatures, you should make sure that the edge of the slab (or the perimeter frost wall) is insulated. For heating distribution, you could consider a radiant wall or a radiant ceiling using PEX tubing and the aluminum distribution plates you mention.

The system depends upon what is driving your decision-making process. If it is solely cost and your conventional alternative is natural gas, it will be difficult to make the numbers pencil out for a traditional solar system. The combined cost of the solar equipment and the retrofit of the heating distribution system will be substantial when compared to natural gas.

There may be some solar solutions that could work for your application. If you use the space occasionally, a solar air heating system could be an excellent solution. Gary Reysa’s article, “Low Thermal Mass Sunspaces” (HP158), and his website,, are excellent resources. These systems are simple, very DIY-friendly, and can be installed on a shoestring budget. There are also off-the-shelf solar air-heating systems available.

Another solution is to use solar-electric arrays combined with minisplit heat pumps. By accruing net-metering credits with your utility in the summer and using electricity to power an air-source heat pump in the winter, you can offset much of the heating demands in the space with solar energy. Since the performance of heat pumps diminishes as outdoor temperatures drop, you might need some other type of heat to maintain comfort.

Vaughan Woodruff • Insource Renewables, Pittsfield, Maine

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