Does anyone has and quantifiable data for a solar heat air system?  It seems the state of Minnesota has been investing into this technology but I have not yet found any good data I could use regarding possibly BTU's gain that could be expected...   Talk about passive energy!





Tags: Passive, heat, heating, solar

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Luis - have you tried nonprofit RREAL (Rural Renewable Energy Alliance) at ?

L: DOE: Solar Heat


in either case, there is virtually no quantifiable or verifiable data on solar, just some people who say they can make it work without backup heating!

There is a great deal of data, engineering, white papers, and published work on solar air heating.  It can work, but you need a structure designed to use it.  That means a structure that is air tight and well insulated so that the losses will not exceed the heat gains.  Just adding solar air heat panels alone is generally not sufficient to meet your heating needs during the winter time.

You also need good sun exposure... trees that shade the collectors during the winter time will make any collector in-effective.

For raw data about solar insolation:

See the various NOAA and weather station summaries.  The major MN universities are also likely to have data for solar heating potential.

For history,  ideas of what works, what doesn't, possible future... John Perlin's book "Let It Shine, The 6,000-Year Story of Solar Energy" is a very good starting point.  Copyrighted 2013, published by New World Library.  ISBN number is 978-1-60868-132-7

If you want to understand solar radiation and what the measurements mean, then the book:

"Solar and Infrared, Radiation Measurments", By Frank Vignola, Joseph Michalsky, Tohmas Stroffel published by CRC Press, Copyright 2012, ISBN 978-1-4398-5189-0

For understanding solar thermal systems, two technical textbooks that are gems are:

"Solar Thermal Systems,  Successful Planning and Construction", by Dr Felix A. Peuser, Karl-Heinz Remmers, and Martin Schnauss. The publishers are James & James   /  SolarPraxis  Copyright 2002, the English versions ISBN number is 1-902916-39-5,

"Solar Engineering of Thermal Processes", by John A. Duffie and William A Beckman.  Published by John Wiley and Sons,  third edition copyrighted 2006.  The ISBN is 978-0-471-69867-8.

There is also an IEA  collection of papers on solar thermal systems.  I believe task 24 or 26.

The books are good if you are trying to engineer/design solar thermal collectors.  The starting point if you are thinking of adding solar thermal heating systems... make sure you've had an energy audit and fix the building envelope!  Solar thermal systems work... but only if the building envelop efficiency is high.  Passive Houses are good examples - tight efficient building envelopes, low heating requirments - and in fact they can obtain much of their needed heat gain through the windows (and of course summer time those windows need shading or you will overheat).


My company specializes in large scale solar air heating systems, and solar air heating in general. We don't do much with small modular glazed collectors (although we've installed about 40 of them), but we did just install a demonstration project in late winter and began some basic metering at a university. I don't have data back on this yet, but will happily share it if you'd like me to (when available.)

We have metered around 6 commercial installations, and have found that what manufacturers and energy models tell you for btu gains/energy savings can vary quite a bit. Most of our commercial installs use preheating of large ventilation loads using either glazed or metal collectors with a cavity behind them. We've tried both preheating and space heating. Preheating is much more effective since you are constantly using the sun to preheat outdoor air loads (vs waiting for the collector to get to say 72F before turning on the fan).

You could try using RETSCreen energy/feasibility software as a guide for what you might expect to see. It's sometimes a bit aggressive in it's proposed savings, but it's a good ballpark.

For low air flow, high temp gain systems used for space heating - we're seeing between 70,000btu's to 100,000btu's per sf of wall installed here in Maine/New Hampshire. This is between 2cfm to 5cfm per sf. (For higher air flows, the btu gains will increase a bit more.) - Keep in mind, these are larger installs (2,000sf to 3,000sf) like Gymnasiums etc

If we were doing residential collectors primarily, I think I'd focus more on passive house designs where we could connect to and preheat the ventilation air to the ERV and just about do away with the heat load of the house (since this is the main heat load).

I'm happy to share as much information as I can about this subject, and you're welcome to contact me directly at or

Best of luck!


Consider using sections of roof for the main thermal collection where it's not practical to add a greenhouse or other collection system, use a data logger in the attic close to the peak between rafters to know daily temperature curves to know what to expect in gain in heat or cold, this can be used to cool in summer storing 'cold' at night.

The main construction is creating the air flow for thermal volume as a ductwork next to the roof sheathing with a 3/4" gap using 1x1's tacked in next to the rafters and 1" thick insulated board with caulking to seal it, about 13"x3/4" the air gap between rafters. I don't have heat-transfer figures for all this but am working on equations to use to fill in real values later, but the main deal is capturing the air in a narrow gap to grab maximized temperatures with a lot of roof area using what's there so expecting a lot of roofing nails sticking through and some of that may need snipping when putting in the 1x1's and sealing it all so it acts like a collector.

There are plenty of ways to collect heat or cold but to store it consider using water in pipes hung below the floor between joists for the thermal-mass, using ducting to gather from the attic and disperse below the floor [the air circulation pattern fitting into existing ducting].

There is a lot of added deadweight doing this so the joists may need added girders, insulation board used below the joists to seal them for airflow (or added between joists for saving headroom in a basement). A blower is used to move the air when conditions are favored to store enough therms of heat or cold.

Summary: Use a confined thermal space right below roof sheathing between rafters with inlet air choice of outside or recirculated then blown to below the floors where water in pipes is used for the thermal-mass to store enough therms for the daily cycle, that air exhausts outdoors or recirculates; ductwork design is very important to costs & efficiency, caulked insulation board is used to create "ductwork" between rafters and joists as part of the duct system design.

There are a several of issues with using the roof as solar collector.  If carefully engineered it could work,  done wrong and your house and pocket book will forever suffer.

First,  is heat, roofs and attics do indeed get very hot.  I've measured summer time temps of 140F+,  winter time temps 80+   Trapping heat for use -- means you've done just that, you've trapped heat and slowed down the desired air movement from under the roof sheathing and out the top vents.  That means the roof materials (asphalt based?) are likely to exceed the recommended temperature.  Ice dam membranes can be damaged and the shingles or roofing material MAY have a shorter life.

Second, if you use the wrong sheathing material (foam), you've just increased --- substantially -- the risk of fire and the possible risk to your home.  Fire codes in nearly every jurisdiction require that foam that can be exposed to flame (and that includes attics) either include some kind of fire retardant - or they should be covered by some other material - like sheetrock.  Ignore this and if you have a house fire (even a kitchen fire that makes into the attic) and your insurance may refuse to cover the loss.  You could choose to use a mineral fiber material similar to a Roxul ComfortBoard product, or use plywood.  But both add weight to the roof, and you really should check to verify that you would not be exceeding the limits for the structure.  The cardboard and thin foam duct boards that are used to hold back insulation when adding more into the roof, might work... they are both light weight AND they are supposed to be treated or made with fire retardant materials... but I dislike both and doubt that they would hold up overtime.

Third,  the air in the attic  is generally not the cleanest and free of dust, molds, rat, poop, or other nasty stuff.  Even if you create a duct that you pull the air through,  you would need to filter the air through some kind of MERV filter and pull as much crud out of the air before it is mixed in and re-used anywhere.  One of the common problems in the hot air rock storage systems in the 70's and 80's was the "clean & washed" rock eventually collected dust, other stuff, and moisture... and the air that moved through the rocks would sometimes then smell like air that had been filtered through a fresh load of gravel.   Making stuff work right is  always more difficult than the initial concept.

Fourth, bringing warm humid air through the roof rafters seems great in the daytime... but as night falls  and if the conditions are right, you've just set yourself up for some attic rain as the humid air condenses against a roof deck (or the nails poking through) that would result during the night... If you use the attic as heat collector, you need to flush out that warm air and return the rafters to a ventilation system at night and not hold in the moisture.. you'd need to work through the design and be careful...

The solar air systems that worked and remain working today were very carefully designed systems.  The designers spent a lot of time working through the details.   There are probably as many solar air systems that used the rafters that have since been torn out and replaced by the owners (or new owners) because some of the important details were missed.

I bring up the points because anyone that doesn't have a lot of design experience  and wants to try doing this should first consider at least reading John Perlin's book for some of the history of successful projects and the ones that didn't quite work out.   You want to learn from the successes and learn from the mistakes...

Jed mentioned RReal, they make a  commercial solar air collector, they've done the engineering, and their unit avoids the nasty rafter problems - by being a  self contained air collector.  You'd still need to look at structure requirements if you place  it on a roof top, but the other issues about fire, moisture, etc are handled by the manufacturer.   A factory engineered solution also means nearly zero of the insurance  hazards... and you'd have far fewer issues with any code inspections or future house inspections (when house is sold).

I was being specific, using the roof is for where nothing else is allowed for collectors, HOA's or whatever.

The key issue is thermal-mass, without that you have nothing but an energy sink.

There is no fire hazard, for this use one would specify a fire-rated insulation board so it'll take longer to burn through to roof sheathing than the rest of the roof.

The dust, dirt and odors shouldn't be a problem as you can clean & seal the area before putting up the board & duct, so the air will be separate from the rest of the attic.

The objections stated don't seem to produce a coherent energy system at low cost to the owner and using as little energy to maintain comfort zone as possible.

The suggestions for this system are based on common building methods and materials used for decades and include thermal-mass, the air is applied to that, exhausting it into the rooms is an option, you want to be able to recirculate the air on some days as well.

The key is the attention to the details,  you really have to seal any "roof" ducting.  It can't be leaky where it would allow air elsewhere from the attic or under the eaves to be pulled in -- unless it is filtered before use. The design also implies that the roof has plywood over the rafters... not all roofs do that.  It depends on the design of the roof and the materials.  Title roofs may not use plywood.  Some cedar roofs use a light weight lathe structure for the roofing.

Insulated roof sheathing may use materials that out-gas when heated.  Since the manufacturers do not expect the air to be re-used within a house, (material rated for external use), you might find that you are bringing in VOC's and aldehydes that you had not expected.  A design of an undermounted roof air heat system really does require a lot of effort.  It is often just easier to make sure you are at or exceed the prescriptive insulation levels,  keep the building air tight and fix the obvious problems.  And honestly if you want an solar air system to work - those items are critical first steps anyways.

As for common building materials used for decades,  remember that for decades some of those building materials used glues that were high in formaldehyde.  Nearly all the flexible self adhering membrane materials outgas VOC's.

I've looked at and seen a number of solar air heat houses in the past, and I've tried some of what you suggested.  It takes a lot of work to do it right.  Generally not something that an average owner can do without making a mess of the house. 

Under the average floor for a home with a crawl space is air you don't want inside the home as well as most attic air.

This method puts in pipes for thermal-mass and provides good aeration in the joist space then seals it from the crawl space with insulation board to have a closed air duct. It does the same thing to part of the roof and the air being moved isn't in contact with the attic air, they don't mix, you're not drawing attic air, you're drawing heated air in the duct system.

Leakage happens with the best ducting so having some leakage with this isn't very critical to the thermal gain, nearly all of the collector air ends up below the floor and alters the temperature of the thermal mass. Heating-cooling the thermal-mass is the job, not circulating air to a room, the room comfort is controlled by the thermal mass in the floor.

The attic air is used below the floor and I'll repeat >>>>>> the exhaust from the joist area can be vented to outdoors and never enter the living space.

It happens that as these materials and their ducts get ventilation and age the air gets much better so after a while it'll be fine to use in the rooms & in winter this may be desired to recycle the warmer air, there is a dust filter on the blower intake.

And keep in mind, this is a technique for where a homeowner can't put anything on the roof due to restrictions of some kind, so while it's not perfect it's certainly a practical way to gain thermal energy and store it to use to maintain comfort zone instead of energy inputs of some other kind converted to heat or cold.

Another aspect is that the crawl space heats up all winter and "cooled" in summer at about the same temperature when storing night coolness. What this does over a few years is reduce the heat loss to the crawl space because that dirt heats up to the average, it heats the foundation as well, you're changing the thermal relationships to the dirt from the floor.

Overall this is a practical, inexpensive way to integrate thermal gain & mass to an existing home, if you can use separate collectors it doesn't alter the need for thermal-mass when using heated air for storage.


   I agree with Jed.  Get a hold of RREAL in Minnesota.  We installed a solar furnace/collector on a client's home in Illinois with their help.  They were able to give us a BTU rating for the size collector that was installed and even a good estimate of the payback for installing such a system.  As others have mentioned, it will also depend on the amount of available sunlight, the construction/R-value of the collector itself, the insulation value of the home's shell, etc., but at least they may be able to give you a good estimate and idea of the BTU potential.  


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