Solar Hot Air Collectors

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Solar Hot Air Collectors

Does it make sense to install one of these $1,600 gadgets on a south-facing wall?

Posted on Mar 20 2015 by Martin Holladay

A solar hot air collector is basically a black box with glass on one side. Instead of heating fluid that circulates through tubing, a solar hot air collector is like a parked car. When the sun shines on the collector, the air inside gets hot. A solar hot air collector usually includes a hot air duct connection at the top and a return-air duct connection at the bottom. To improve efficiency, most solar hot air collectors have a black metal baffle or screen behind the glass that allows air flow on both sides.

Several manufacturers sell solar hot air collectors, including Your Solar Home of Aurora, Ontario (manufacturer of the SolarSheat collector) and Environmental Solar Systems of Methuen, Mass. (manufacturer of the SunMate collector). The SolarSheat 1500GS measures 43" x 87" (a little smaller than a sheet of plywood); it sells for $1,150 (without a fan) or $1,650 (with a PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.-powered fan). The SunMate is smaller; it measures 34" x 77". Equipped with a thermostatically controlled 100-cfm fan (“turns on at 110°F and off at 90°F”), the SunMate sells for about $1,590.

These days, this type of solar collector is usually installed vertically on a south-facing wall. This makes sense, since vertical installation takes advantage of low sun angles during the winter. Manufacturers usually advise installers to drill a couple of 5-inch diameter (or larger) holes in the wall, penetrating the siding, sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , insulation, and drywall.

[Image credit: International Energy Agency - SHC]

One of these holes is used for the hot air duct, and the other for the return air duct. These ducts terminate at indoor grilles installed on the drywall. In most cases, the hot air duct includes an inline DC fan that is powered by a wall cube transformer or a small PV module. The fan is usually controlled by a thermostat.

During the summer, when heat is unwanted, the fan is turned off.

Roof mounting is also possible. Because of the need for bulky ductwork, roof-mounted solar hot air collectors (even those installed by factory-trained contractors) usually have an unattractive jury-rigged appearance.

Do they work?

When sun shines on a vertical solar hot air collector during the winter, the air behind the glass heats up. Once the collector is warm enough, it delivers up to 100 cfm of 100°F degree air to the room on the other side of the wall. In this sense, the collectors work.

Bernard M. Waxman is a resident of St. Louis, Missouri, who wrote a blog describing his installation of two vertical solar hot air collectors on a south-facing wall. According to Waxman, “the panels will provide heat from about 9:20 a.m. until after 2:00 p.m.” (In other words, under good conditions the panels can provide heat for 5 hours out of 24.)

On another web page discussing solar hot air collectors, Waxman reported, “As long as your expectations are not too high, these units perform well.”

Needless to say, you need to pay attention to winter sun angles before installing this type of collector. A homeowner named Rosemary Kelly posted another comment on the page mentioned above: “The southernmost corner of my home stays completely sunny in the summer, so this is where I had my solar panel mounted. I found out the hard way that the southernmost corner of my house does not stay sunny in the winter months when the sun is lower in the sky, so my solar panel has not worked at all during the month of December.”

An abbreviated history of solar air collectors

Inventers have been experimenting with solar air collectors for at least 150 years. After the first energy crisis in the early 1970s, alternative energy enthusiasts launched a solar energy boom. Most solar hot air enthusiasts designed systems that stored heat in giant rock bins installed in their basements. Meanwhile, solar hot water enthusiasts stored hot water in large insulated tanks or mini-swimming pools in their basements.

Sitting on the sidelines, the photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. enthusiasts didn’t do much until about 1979, when a few off-grid pioneers began investing in the first PV modules that broke the $10-per-watt barrier.

[Image credit: Mother Earth News]

Do-it-yourself hobbyists naturally gravitated to solar hot air collectors, since these inefficient devices are so easy to build. Mother Earth News, the 1970s magazine known for optimistic articles with exaggerated claims, published a classic 1977 article hyping solar-air collectors. The article, “DIY Solar Heating with the Heat Grabber,” advised readers to assemble a box made out of Thermax, stick on a piece of glass, and insert the angled device in a half-open double-hung window. (Believe it or not, back in the 1970s, many people thought that this would be a good way to save energy.)

After a few years of tinkering, the solar pioneers discovered a few facts. First, they discovered that solar hot air systems didn’t work as well as solar hot water systems. (Among the reasons: per unit of volume, the heat capacity of water is much higher than the heat capacity of air; and ducts carrying air are usually leakier than pipes carrying water.) A little later, the pioneers realized that investments in envelope airtightness and extra insulation made better economic sense than investments in active solar equipment. And later still, solar enthusiasts realized the investments in PV equipment make better economic sense than investments in solar hot water equipment.

I’m grateful for the solar pioneers. Without their efforts, we wouldn’t necessarily know all these facts.

[Image credit: The Complete Handbook of Solar Air Heating Systems by Steve Kornher and Andy Zaugg]

An anecdote from Joe

I recently had a conversation about solar hot air collectors with building scientist Joseph Lstiburek.

“A long time ago, we had to fix an elementary school that was all messed up in Kearsarge, New Hampshire,” Lstiburek told me. “It had vertical solar collectors on the south wall of the school — I think they were site-built. The school was pre-conditioning the ventilation air by pulling it through these collectors. The collectors were painted black, and the air in the collectors got very hot. These solar collectors were giant VOCVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production. emitters. I remember the comments from the school superintendent — he said, ‘Every time we run it, the kids get sick. What should we do?’ I said, ‘Don’t run it.’”

Should I buy one?

There are several downsides to installing vertical solar air collectors on a south-facing wall:

  • Drilling two 5-inch-diameter holes in your air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. and insulation layer creates a thermal bridge.
  • In most cases, these holes represent air leakage paths.
  • In some cases, these poorly flashed holes provide a path for rain to enter the wall assembly.
  • These heating devices provide no benefit in the summer, and unless equipped with very tight dampers, may increase air conditioning bills.
  • This type of solar space heating system includes no storage, so once the sun moves away from the collector, heat output drops to zero.
  • During the hours when these collectors provide heat — between the hours of 9:30 a.m. and 2:30 p.m. on sunny days — most well-designed homes don’t require any supplementary heat, because they have enough solar gain through south-facing windows to keep the home comfortable. Moreover, during the work week, many homes are unoccupied during these hours.
  • One of these collectors costs more than a window. Interestingly enough, a south-facing window acts a lot like a solar hot air collector when the sun is shining. So why not just install a window, since a window serves more functions and provides a view?
  • A great many energy reduction measures will provide a better return on one’s investment than one of these devices.

When the topic of solar hot air collectors was discussed on the GBA forum, solar energy veteran Kevin Dickson noted, “(1) Extremely few rock bins have been built since the 1980s; (2) Solaron, the biggest air-based solar company in the ’70s, was installing 98% water systems by 1983 (I worked there); (3) Why would you spend $2,000 on a SolarSheat that sits idle all summer, when you could apply that money to a solar domestic hot water system that runs all year? (4) Operational results from air-based whole-house solar systems were so bad that most of the systems were torn out.”

Who’s buying these things?

So why are equipment manufacturers still trying to sell solar hot air collectors at $1,600 a pop?

Solar air collectors are basically expensive toys. They’re kind of fun, because on a sunny day you can feel (barely) warm air enter your house through a vent. Sold as is, without any instructions mentioning the usefulness of a rock-filled storage bin, these collectors are a bit like the human coccyx — a vestigial tail. It isn't functional, but it reminds us of something obsolete from the distant past.

These collectors provide a warm fuzzy feeling, reminding purchasers of more elaborate solar-air systems devised in the 1970s, without requiring homeowners to invest in the complicated storage devices that were included in the most effective systems of that era.

Invest in your thermal envelope, and then invest in PV

If you want to buy some solar equipment, your best investment will usually be a PV system. If you can afford to spend at least $6,000 or $8,000, you can probably get a PV system installed for $4 per watt. You can buy 400 more watts of PV for the same price as one solar hot air collector. (If you are installing the PV system yourself, you can buy PV modules rated at 1,600 watts for the same price as one solar hot air collector.)

Manufacturers of solar hot air collectors seem to live in a time warp, as if the 1970s never went away. If any evidence of this claim is needed, consider the photos proudly posted online by the manufacturer of the SolarSheat collector.

The photos show a rooftop installation. It might be possible to comment on the aesthetics of the installation — but I won’t.

Instead, I’ll comment on the photos of the attic ductwork that the company installed. What do you think? Did the homeowners who invited these duct installers into their attic make the right choice? Or is it conceivable that owners would have been better off investing $2,000 in attic cellulose rather than $2,000 in a roof-mounted box that blows a little 100°F air into the living room for a few hours on sunny winter days?

Martin Holladay’s previous blog: “A Balanced Ventilation System With a Built-In Heat Pump.”

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Image Credits:

  1. Image #1: Environmental Solar Systems

Mar 20, 2015 8:30 AM ET

I Have Only One Comment
by Bruce Palmer

. . .about the last photo. Yikes.

Mar 20, 2015 11:23 AM ET

by Malcolm Taylor

Several things brought a smile to my face. The description of Mother Earth news as "the 1970s magazine known for optimistic articles with exaggerated claims". What Martin - do you really not believe you can bake bread for a family of eight in five minutes a day? And of course that roof mounted collector. What a fantastic find!

Mar 20, 2015 12:09 PM ET

Edited Mar 20, 2015 12:18 PM ET.

Martin, good article. Your
by Eric Habegger

Martin, good article. Your right, it is very fun to experiment with this type of heating even if it isn't practical. Right now during my overhaul of my home I'm doing my own experiment. I've painted the entire bottom surface of the roof sheathing flat black. The idea is to get a low resistance path for the roof to reradiate heat downward. I've then covered the rafters in a radiation barrier to contain that radiation. It seems to work really well since even with the flat black paint my house was much cooler this past summer than previous summers.

I've connected that radiation barrier to two points at the very peak of the roof. The first is to a motorized damper vent that can open or close to release or contain the heated air to the atmosphere. The second is to a large (12" diameter) insulated duct that routes to the living space with another motorized damper in line with it. This is an open loop system, unlike the ones in your article, and is a high volume, low temperature system that works in connection with incoming air from the soffit vents. With temperature sensors installed the previous winter it worked on days when it was clear and the high of the day was only 46F it reached a temperature of 80 degrees F.

This is a long term experiment and won't be finalized and determined if it can be effective for many years. It is a system that only has the "potential" to work if used in a temperate climate like California or other southern tier states. The advantage is that it doesn't effect the look of the house in any way and can be decommissioned if necessary simply by opening one damper and closing the other. And it is also using the radiant barrier for cooling as well as heating. I'll see if it has any promise in the years to come.

Mar 22, 2015 10:45 PM ET

when done right, it works right
by Paul Kuenn

Fun story Martin.

However, our Solar Sheet is airtight (proven during blower door test) with an electric damper that was improved upon by me and run by our solar PV system. Faces directly south and looks very nice between the door and the only two windows. At -15F it blows a consistent 85-90F air from 9am to 3pm in winter (NE Wisconsin). A good 3 pane tilt and turn window cost about the same. Our Solar Sheet does not loose heat during the cold evenings but keeps the house at 65F all day long without any other heat. Old house built North to South so not much choice on solar gain using windows.

There's always more to a story when it's done in perspective to the homeowner. I do agree that most advertising is a bit far fetched. You don't want to invest in these if your home isn't air tight and super insulated.

Mar 23, 2015 5:56 AM ET

Edited Mar 23, 2015 5:58 AM ET.

Response to Paul Kuenn
by Martin Holladay

I'm glad that you are happy with the performance of your solar hot air collector, and that your were able to modify the damper to make it more airtight.

I'm not convinced that it was a good investment, but your attention to airtightness is admirable -- and that approach should certainly be emulated by anyone contemplating the installation of one of these devices.

Mar 23, 2015 10:10 PM ET

Forgot to mention
by Paul Kuenn

I did enclose the Solar Sheet within the last layer of 3" insulation so the sides are not exposed to weather and I'm sure that is why the performance is so good. While at it, I re-wire the 24V PV panel at it's top to do some extraneous work in the off season.

A trellis with pole beans placed in front of it love the extra warmth of it in summer. The soffit shades almost half of it during the summer months. Little extras always go a long way.

Mar 23, 2015 10:58 PM ET

South Wall Air Heaters
by Jim Baerg

Enjoyed your article Martin. It brought back some painful memories from the 70's when, sometimes, technical naivete and sales enthusiasm prevailed over efficiency or payback. I think that's where my long term insistence on collecting data after the project is completed comes from.

I do think that there is a place for retrofitting larger (cheaper?) South wall air collectors on metal shop buildings, warehouses and other commercial buildings. Building use and available hours of sun coincide quite nicely. Again, some monitoring is in order to improve designs and to assure cost effectiveness.

Apr 2, 2015 10:17 PM ET

Commercial Solar Wall Installations
by Kohta Ueno

Great column Martin--no disagreement here. But as a point of discussion--solar walls do make some sense in commercial applications for preheating ventilation air when you have really high ventilation rates. I remember that an aircraft hangar was a good application of one commercial product. Also, there was a 2011 ASHRAE article talking about using it to preheat ventilation air in a school, where it goes through an ERV.

Designers assessed an interesting way of taking advantage of the available passive solar energy using a solar wall system. The installation of a 316 ft2 (29 m2) metal-clad solar radiation collector on the southbound wall of the school provides 14.5 MWh of solar energy annually, close to 10% of the energy needed to heat outdoor air. In addition to providing renewable energy, heat loss from the wall covered by the solar panel is recovered and used to preheat the air, doubling the R-value of the wall to about R-50. Also, the solar wall diminishes the particulate levels in
the air drawn through it, which reduces filter maintenance.

Of course, these commercial applications have maintenance folks working on them; also, they are not trying to really heat the building, as much as temper ventilation air (i.e., only "low quality" heat needed).

2015-04-02 ASHRAE Excerpt.jpg

Apr 3, 2015 4:36 AM ET

Response to Kohta Ueno
by Martin Holladay

Thanks for your comments. My article focuses on residential applications; you're right, of course, that solar hot air collectors sometimes make sense for conditioning ventilation air for large commercial buildings.

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