Solar and Hydronic Heating Work Well Together

Controls authority Carol Fey on using solar heating together with a conventional hydronic system

The prospect of tapping solar energy is exciting — whether to save money or the environment. But there are doubts. Can solar do the job? What if there's not enough heat? What's the payback on the investment?

An all-or-nothing approach rarely leads to the best answer. Rather than asking if solar can do the whole job, let's ask what part of the heating job it can do. Looked at as a supplement to conventional heating rather than as a replacement, solar is promising.

With the requirements for lower water temperatures for high efficiency condensing boilers and for radiant heat — both in-floor and under-slab snow melt — the lower Btuh output of solar can be a plus.

Solar and hydronics

The primary difference between a solar heating system and a conventional hydronic system is the source of the heat. In a conventional system, the boiler fuel is the energy source, and the heat exchanger is a boiler. In a solar system, the energy source is the sun. The heat exchanger is the solar panel. Both are heating water.

A perfect configuration is for solar to be the primary heat source. If more heat is needed, the conventional heat source is there to supplement.

A logical place to start applying solar is domestic hot water. The hot water provided from the solar panels can be run through a coil in the top half of the water heater. It can satisfy low demand situations by not having to heat the whole tank. If more is needed, the secondary heat source can be fired.

Two things make DHW a good application. First, it is relatively inexpensive, requiring just a couple collectors. Second, it is a year-round application. The capacity of a system is limited by the ability to dissipate its Btu's in August as well as in January.

The controls and accessories of solar and hydronic systems are more alike than different: a controller, sensors, expansion tank, air eliminator, system fill valve, and storage tank. As with a boiler, shut off the air eliminator and fill valve after start-up. Solar need not be more complex.

Even though the output of a solar system will likely be fewer Btu's than from a boiler, the parts need to withstand higher temperatures and pressures. That's because of the potential for higher temperatures from the collectors. On a sunny day the water produced by a vacuum tube collector can exceed 300°F. Steam is common when the vacuum is released. For the same reason, piping needs to be copper, not PEX. System fluid also must tolerate extreme conditions. The storage tank needs to be bigger, because of the adage “make hay while the sun shines,” and the fact that the sun doesn't shine all of the time.

Types of collectors

There are three types of solar collectors.

The largest seller is rolls of unglazed plastic, which are used for pools in climates such as Florida.

Flat plate collectors have the advantage that they can be installed flat, so installation angle is not an issue. A disadvantage is that they weigh around a hundred pounds, so they can be a challenge to install on a roof.

Evacuated tube collectors are best for winter high-load applications. Even though they cost more, when labor is factored in they may cost the least for roof installations. The collector is assembled of individual 1½ pound tubes and a 30-pound manifold.

Will there be enough energy? The adequacy of solar energy depends upon several factors:

  • The amount of energy needed. For example, for DHW, what is the household's peak hot water usage?

  • The weather. The sun shines different amounts in different climates. Colorado annually has 300 sunny days, with perhaps 1,800-2,000 Btu's per day. But much of the rest of the country has to expect much less.

  • The percentage of total energy expected from solar. 100% is not feasible; 10% is.

Customers have differing expectations from solar. In one household, one person might want to save money, while another wants to save the environment. Solar can contribute to both.

It's important for a contractor to determine how big and fast customer expectations are. For a family of four, there might be a 10-year payback on an initial cost of $15,000 for solar DHW. At the same time, the system could be expected to save 100 million Btu, 8 tons of carbon dioxide (CO2), 1,500 pounds of sulfur dioxide (NOX) and 100 pounds of carbon monoxide (CO).

Solar energy can be a perfect partner for a hydronic heating system. The key to success is to be realistic about what the customer expects, what the system can deliver, and the time frame when the results will happen. Solar cannot deliver 100% of the heating demands of an ordinary household. But it can supplement, saving both money and the environment.

This fall and winter technical trainer Carol Fey will be working in the boiler rooms of Antarctica. You can check out her blog at She also has a new e-book entitled, “Carol Fey on Electricity, Controls, and Troubleshooting,” a collection of 58 articles, available at

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