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Improper & Proper Placement of Cold-Air Return in a Solar Heating System
Improper & Proper Placement of Cold-Air Return in a Solar Heating System

I’m researching solar space heating options for my home. Solar air heating seems simple enough, but when I search the Solar Rating & Certification Corp. (SRCC) database, only three collectors make an appreciable (though not stunning) number of Btu per day.

However, those three companies have pretty sketchy websites (like circa-1990s designs), with no real distribution network. I’m skeptical.

Why does it seem that solar water heating systems are used more prevalently for space heating?

Mike Taylor • via Facebook

There are at least two reasons why solar air collectors aren’t as popular as liquid collectors and solar water heating systems. One is technical; the other economic.

First, air collectors are not as efficient as liquid collectors. Heat transfer is an important facet of collector efficiency. Air has lower density and lower specific heat, so it is not as good a heat-transfer fluid as water or an antifreeze solution. Air has a measure of heat content called specific heat (SpHt) of 0.24, while water has an SpHt of 1.0. Both characteristics are important for good heat transfer.

Many liquid collectors have efficiencies of about 75% (or slightly more), while air collectors don’t exceed 65%. However, in air systems, the collector’s efficiency is offset by the overall system efficiency, which is higher, since these systems operate at a lower temperature (~70°F) compared to a domestic hot water system (120°F to 140°F). See ”Getting Into Hot Water” in HP123 for additional explanation of interpreting collector performance.

Air collectors are less expensive to manufacture than liquid collectors—they do not need expensive copper piping to contain the heat-transfer fluid. But their cost advantage and higher system efficiency isn’t enough to offset the economic disadvantage of solar space heating, since it usually is required for only about half the year. Consequently, the system isn’t generating a return on investment (ROI) in warmer weather. Also, the winter solar resource is much lower when compared to the yearly average resource, so space-heating systems have an even greater economic disadvantage compared to solar water heating systems. The bottom line: Since water-heating systems are working all year long, they have a better average solar resource that increases the ROI.

Lastly, many air collectors are incorrectly installed, with the cold-air return located in the ceiling. Air collectors have two duct openings—one for the supply air and one for the return air. The hot outlet air is usually installed in the home’s ceiling, making ductwork short and allowing strong circulation with a small, energy-efficient blower. But installing the cold-air inlet also at the ceiling results in the heated air circulating only at the ceiling—not at the bottom half of the room where it is needed. Proper placement of the cold-air return to the collector—at floor level—is the key to a correctly installed system.

Air collectors can also be installed vertically on a south-facing wall. In this configuration, it is difficult to install them incorrectly, as they have openings on the bottom and top of the collectors, which thermosyphon the air. The cold air enters at the bottom (floor level) and, as it rises through the collector, is warmed and exits at the top. Many installations incorporate a small fan or blower to increase the circulation.

There are thousands of air collector systems in the western United States, both wall-mounted and correctly installed rooftop systems. The high desert and mountain locations in these states have four-season climates with relatively good winter solar resources. But even when well-designed air collectors are installed correctly, the systems still face the economics of approximately six months of use with varying degrees of diminished solar resources.

Anyone evaluating the benefits of solar space heating should consider passive solar design—with new construction, this is normally the most cost-effective solar heating solution. It can also be an economical retrofit. Incorporating a structure like an attached sunroom, Trombe wall, or other passive design feature is always possible with new homes, and usually feasible with many existing buildings. The economical passive do-it-yourself solar projects described in detail by a frequent Home Power author Gary Reysa on his website ( and Home Power articles are good places to start (see HP98, 99, 109, 116, 117, 153, & 158).

Chuck MarkenHome Power thermal editor

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