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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

Comments (3)

Fred Golden's picture

I was reading a book called Passive Solar House: The Complete Guide to Heating and Cooling Your Home by James Kachadorian. In this book, he described how to store a huge amount of heat in the foundation of his home in Maine. He even had a patent for the flooring system that expired many years ago.

With the bricks laid on their sides, north to south, and air entrances near the south and north walls, air passively enters these long channels, and flow slowly through the concrete blocks covered with 4" think concrete floor. This cement weight is around 1/2 cubic foot per square foot of floor area, and can hold 30 Btu's per cubic foot of concrete. If one modified his plan slightly, and installed a air heat collector on the south wall, below the foundation (assuming that the land drops off to the south by around 4' below the finished floor grade) then it will heat the home wonderfully, even with minimal glass surface area. Electric automatic insulated air dampers can shut off heat flow when the collector is cooler than the floor air or when overheating is going to happen.

So yes, Air solar collectors can heat air and warm a room.

I know someone with such a system in northern Illinois. Their collectors are only around 20 square feet each, there are two of them. A small fan comes on when it is cool in the home, and the air box is warm. Usually they set back their electric furnace to only 55F in the winter when they are at work, so there are many times the fans will warm the home in the winter. While not ideal for a retirement home, where the owners are home more days than away, it works great for them!

I do agree, that active solar hot water collectors are much better at storing massive amounts of energy in a small area. They are also much easier to 'document' the energy stored, or created by sun power. So as far as rebates are conserned, they are easy to document what a 'evacuated solar panel' will collect in a given area of the United States in 365 days. But a south wall of a barn, covered with 100 square feet of 1/8" thick plastic and the collector is spray painted black under that, with electric dampers high on one side and low on the other, that will be several hundred Btu's of heat per day, but not really anything you can 'document' for a rebate.

While really warm air is 'great' to warm a room that is 10 - 20F below the desired room temperature, it is not an 'exact' temperature like a hot water heating system with a 200 gallon storage tank, and tubing embedded in the concrete floors. And as discussed, it is easy for a air heating system to overheat the room before sunset.

Back in the 70's there where homes built with hot rock heat storage. There is a 'reason' that is not discussed much anymore. They are not efficient, not low cost to install, and offer a certain 'smell' if mold ever started growing there.

Captron's picture

I think a couple of points raised need further consideration.

The density of the two mediums is paramount, however the real challenge is to store the energy.

As a rule, in an energy efficient environment with optimised passive solar, there is no cheap and easy way to store or concentrate heated air. The only place this can happen is to use the house as your storage device. There is little carry over for dark rimes unless some sort of thermal mass is also incorporated. Basically heating air is a poor heating medium. After all when the sun is out, you least require ancillary heating. Even forced air furnaces have low efficiencies even if the flame efficiencies are optimised. A flame thousands of degrees is stepped down to room temp, not efficient at all. And, don't get me started on GHG emissions.

Fluid however is energy dense, and can easily be stored in short term or long term storage, moved around, and recovered. It has be benefit of being much more efficiently collected, and hydronic heating does have many advantages such as being quiet, dust free etc.

Long term storage is the biggest advantage and it can take advantage of low cost materials as well as integrating PCM or phase change materials. A quick Google of "solar thermal heat core storage" or "solar heating with long term storage" or even "Seasonal thermal energy storage" will give you some background on the subject as well as links to relevant articles from HomePower

Fred Golden's picture

I took a solar class in High School back in 1983. All the students made a simple hot air collector out of 1X6 pine sidewalls, with 1/2" plywood back and 2" of fiberglass insulation inside. I used some galvanized metal over the fiberglass, and had a sheet of plexiglass to cover it all. In the middle, the instructor suggested some scrap steel (he was also the welding instructor and had lots of scraps of steel sitting around) to hold some heat. All of it was spray painted flat black, then the cover installed.

We took these up to the roof (yes they had a staircase up to the roof) and mounted them, installed a digital temperature probe, and got extra points for collecting the warmest air.

It is not that difficult to build a solar wall to the south wall of your home. By using a small doggy door for the inlet at the bottom, and equal size opening at the top about 8' away, this can be opened to allow room air into the collector, and get it back into the house once warm. A fan assists airflow. 2X2's can direct airflow to the left and right as it goes up the wall, to get full coverage side to side. Then paint it all black, and cover with sheets of PVC to keep in the heat that you have captured.

Sure glass would be a better heat transfer material, however it will also give up some heat on a cold and windy day, so it pretty much works out similar both ways. And you can afford a 64 square foot plexiglass cover, while a glass window 8' square is kinda expensive!

Good luck on your project!

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