# Opinion about Air Solar Heater



## Hammerjoe (Oct 25, 2014)

Today on Ask This Old House show in their segment "What is this?" they showed this device that collects the sun to heat a room.
I went to their website and found out the company and their website:

http://www.solarinfrasystems.com/

This thing seems to sell for about $250.00
I am curious if this is a just a gimmick or is it something that works?

What are the opinions of the experts here?


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## bmblank (Oct 25, 2014)

It's a gimmick. The sun that that thing catches and converts to heat would otherwise end up in your house and eventually get converted to heat. Unless of course your house is full of mirrors and all that light just gets reflected back outside.


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## sesmith (Oct 25, 2014)

Solar air heaters definitely work.  Check out all the solar air heater ideas on the Build it Solar web site.

That said, putting a solar air heater in an existing window nets you nothing.  The sun coming through the window is already warming the room without the solar heater.  A solar heater only makes sense if you can use it to add glazing area by having the solar heater separate from your existing windows.


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## BrotherBart (Oct 25, 2014)

Infrequent poster precaud installed solar heating areas in his house and benefits from them.

Don't remember the details.


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## Circus (Oct 26, 2014)

Hammerjoe said:


> solarinfrasystems,  I am curious if this is a just a gimmick or is it something that works?


 
Gimmick.  
It's schemes like this that killed solar in the 70s. The only way to make an existing window more solar efficient is to lower losses when the sun doesn't shine.
Real solar air panels, that don't sit in windows, work fine as long as they seal when inactive.

Costs about the same but takes much more effort.


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## peakbagger (Oct 26, 2014)

Many folks are not aware that This Old House is a profit making enterprise that get paid a healthy fee for every product that is shown on air.

A wall mounted solar air heater will cost a lot less for the same amount of heat.


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## Hammerjoe (Oct 26, 2014)

I know it is using the inside air that has been heated by the sun, but what I am failing to understand is that if the air in the room is say 75F and the devices heats it up to 80F isnt that a net gain?
Or does it mean that the device does not produce a higher temperature than whats in the room?


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## woodgeek (Oct 26, 2014)

Should be illegal gimmick.  Heat would enter window anyway.

Real commercial solar air heaters are expensive.  They outputs are usually not stated or overstated, but their 'simple payback' time on reduced energy bills is usually 30-100 years!

DIY versions can be made....try build it solar.  Heat output of a larger unit could heat a good sized room if/when the sun is shining.  

Generally speaking in the US east of the Mississippi river, however, there is little usable sun in the winter....solar heating is tough.  West of the Mississippi is a different story.  Don't know about the Maritime provinces...you should look up solar resource versus month for your area.


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## Grisu (Oct 26, 2014)

Hammerjoe said:


> I know it is using the inside air that has been heated by the sun, but what I am failing to understand is that if the air in the room is say 75F and the devices heats it up to 80F isnt that a net gain?
> Or does it mean that the device does not produce a higher temperature than whats in the room?



Check the temperature of the wall part that would be in the shadow of the unit. It is likely a bit less than what the surrounding wall shows which still gets sun. Net gain will be zero. Practically, the only net heat generated will probably be the electricity powering the fan. Add one or two incandescent lightbulbs and you have the same heat gain plus light for about $2 in cost.


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## Circus (Oct 27, 2014)

woodgeek said:


> east of the Mississippi river, however, there is little usable sun in the winter.



Not true. When it's 30F, it's cloudy but when it's -10F, it's sunny. You can't get rid of your furnace but you can save a lot of the fuel.

PS Hammerjoe, save your money.


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## woodgeek (Oct 27, 2014)

Show me your average hours of sun in Dec, Jan and Feb.  Preferably numbers for thermal solar, rather than PV.  

Then compare your numbers to Boulder, Colorado.

Air heater eff plummets at very low outdoor temps, as most are designed to not fail when 'stagnant' in June, i.e. single glazing.  In comparison PV will make >50% of rated energy on cold days with 50% cloud cover.  Air heaters will not, as their eff will fall close to zero....they might even be unable to heat to above room temp.

Taken together, in the 'East' you might get some decent BTUs in the shoulder seasons, but if you sized to get lots of winter heat, you will overheat in the shoulders.


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## woodgeek (Oct 27, 2014)

Ok.  I looked up the specs on a real, commercial solar air heater, the 'Solar Sheet'.  This product retails for $2000 form Northern Tool.

Discussion here: http://forum.solar-electric.com/showthread.php?10213-quot-SolarSheat-Solar-Air-Heaters-quot-vs-DIY

It has a 2 m^2 collector area, and is intended to be mounted vertically south facing for winter space heating.

The SRCC rating page for it is at: http://www.altestore.com/mmsolar/Others/SRCC_1500G.pdf

It says that when the outdoor temp is 36°F colder than indoors, it will generate 10,400 BTU per sunny day.  If we assume a sunny winter day has 4 hours of full output power, this suggests it delivers ~2600 BTU/h, about 760W.

Looking at PVWatts for a vertical southfacing collector, the PV hours of resource are about 100 hours per month during winter months. If all those hours were full clear sun, then the 'solar sheet' would deliver 250 kBTUs/month in the winter time.  In a 6 mo heating season, that is 1.5 million BTUs, worth $30-50 per year, depending on your fuel. Simple payback on the $2k product is 40-70 years.

It looks like the stagnation temp for direct sun is ~100°F.  That is, the efficiency of collection drops linearly to zero as inddor-outdoor delta T approaches 100°F. So at -10°F in WI, the output would be less than 25% of the numbers above, or 2500 BTUs per very cold sunny day.  The same as burning 150 grams of wood.  Or about $0.50 of conventional heat at $20/MBTU for each of those days.  You would only need 4000 such days to pay off the unit.

My previous estimates showed that the best, single-glazed DIY panels had similar BTU output per unit area to the above product.  While you can make a 2 m^2 panel for less than $2000, it will still only provide ~1.5 MBTU/yr from places with a 100 hr/mo winter resource.  To get a more reasonable fraction of your heat in a superinsulated house, say, 15 MBTUs, now you are looking at 20 m^2 of collectors, a non-trivial amount of DIY work.

Or you can get PV grid-tie installed, and use mini-splits, much more cost effectively.


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## Circus (Oct 27, 2014)

woodgeek said:


> Show me your average hours of sun in Dec, Jan and Feb. Preferably numbers for thermal solar, rather than PV



It's a mistake to use PV calculations for solar air. Think of solar air as a really large thermal pane window without the heat loss. It just sits there quietly heating.
Your statement "Air heaters eff will fall close to zero at very low outdoor temps" is simply untrue. Here cloudiness is inverse to the temperature. True, the east isn't leeward of a mountain range, but a 50% fuel savings is realistic.
Commercial collectors are over priced. Often the same supplies if labeled solar are expensive and if labeled greenhouse are cheap.
Solar air does have some quirks. Temperature swings is the biggest. I've addressed this quirk using a thermal mass and running around in a T-shirt and shorts when it -20F outside.


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## Circus (Oct 27, 2014)

woodgeek said:


> While you can make a 2 m^2 panel for less than $2000, it will still only provide ~1.5 MBTU/yr


 
I have 100 sq ft (>9 sq m.) of collector on my house. The materials for the collectors and ducts were $300. Building the collectors were fast and easy. Mounting the collectors and fabricating-installing the ducts was the difficult part.


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## woodgeek (Oct 27, 2014)

Using PV numbers provides an *over-estimate* of solar thermal resource.  Solar air heaters don't lose heat from your interior when they are off, true, but they do lose most of the solar heat they collect to the outside before it even gets inside your house.

Even in completely clear conditions, single-glazed solar air heater (commercial or DIY) will lose >70% of the potential solar heat it collects in single digit or colder outdoor temps.  Any reduction of input from thin or broken clouds, and the unit will never heat enough to even turn on....this is why PV provides an overestimate of sun hours/month....it will still provide usable power in thin or broken clouds.

After the solar heating debacle of the 70s, the SRCC was formed to rate equipment like this to avoid scammers.  The PDF I gave you tabulates output and says a single-glazed, well-engineered 2 m^2 collector would yield a measly ~10,400 BTUs per completely clear day when it was heating by +36°F, corresponding to a modest outdoor temp around freezing.   Under 'mildly cloudy' the output drops to 5900 BTU/day, about the same space heating as provided by the body-heat of a 6 year old child.

The same SRCC table rates the 2 m^2 panel as providing a pitiful 1600 BTU/day under clear skies with a lift temp of +90°F, roughly what you could expect at your -10°F outside example.


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## woodgeek (Oct 27, 2014)

Circus said:


> I have 100 sq ft of collector on my house. The materials for the collectors and ducts were $300. Building the collecters were fast and easy. Mounting the collectors and fabricating-installing the ducts was the difficult part.



That's cool.  My estimate is that they are providing you <7.5 million BTUs per year (10 m^2) if single-glazed.  Do you disagree?  Did you double-glaze?


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## Circus (Oct 27, 2014)

woodgeek said:


> Did you double-glaze?


 
Twin walled 8mm polycabonate greenhouse glazing.
I installed an electric clock to the furnace blower. Logged the run time prior and after solar. Now heating cost more in November than February. Weird.

Update: 
The clock data didn't pan out to be as accurate as I would have liked. Propane suppliers like to gouge their customers (the reason I built the collectors).  In reaction, I switched modes of heating not recorded by the clock.


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## woodgeek (Oct 27, 2014)

Twin-wall PC is the way to go:  R-2 versus R-1, high temp compatible, good transparency, unbreakable, cheap.

With data you can estimate BTUs/yr....10 MBTU, 15 MBTU??


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## sesmith (Oct 27, 2014)

I can.  A few years ago I experimented with a solar air heater to supplement our heat.  Scott Davis put up some pictures and data I sent him on his web site here:

http://www.n3fjp.com/solar/solarhotair.htm

Mine is the larger  downspout collector (labelled Scott S. from NY).  It was approx 100 Sq feet of glazed area.  Worked great even in our less than stellar area for solar (near Ithaca, NY).  It produced lots of heat even with outside temps. close to zero.  Collector efficiency was in the low 40% area.  A good air collector will be around 50% efficient.

According to some calcs. I made, in the 2009-2010 heating season, my collector produced approx,
14632800 Btu. of heat (about 20% of our heat that year).  It cost me approx. $10.80 to run the fans on the collector to produce that heat.

The good, was that the thing really worked.  It was an excellent supplement to the wood heat we were mainly using to heat the house, at the time.  The bad, was that it was ungainly to put up every year, and after a back injury, I had, it was retired to storage.  It was also a little ugly, if I say so myself.  I now heat the house with a GSHP, and it really isn't worth the trouble to set up the collector.  You really need a lot of glazing area to make a significant amount of real heat.  Also, there are cheaper ways to make an air heater than the downspout idea I tried.  I also had some issues with ducting that was undersized.  This affected efficiency some and caused some issues with the fans.

The bottom line was that it was a lot of fun to do, and well worth looking into for supplemental heat as a DIY project.  You just have to be realistic about how much heat you expect to get and how much glazing area it really takes to get it.  The commercial collector shown in the beginning of this thread is a joke.


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## woodgeek (Oct 28, 2014)

Nice.  Looks like you are single-glazed, but the airflow is all ducted within the panel, so no flow against the glazing.

Is there a hat around here somewhere I can eat?


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## Circus (Oct 28, 2014)

woodgeek said:


> the airflow is all ducted within the panel, so no flow against the glazing.


 

It's better having the absorber cooled only from the backside is a myth. A cooler collector is more efficient and having air circulating on all sides of the metal doubles the heat transfer to the air. The glazing is warm in either case. If any heat is being transfered it's probably from the glazing to the air being heated.


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## woodgeek (Oct 28, 2014)

A stagnant air film is about R-0.7 or so.  With forced flow (or wind) that is lost.  With single glazing there are big eff differences for different air heater designs, depending on the flow and temp of the air on the inside of the glazing.  The nice thing about double glazing, is it simplifies the internal flow design issues.


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## Circus (Oct 28, 2014)

woodgeek said:


> A stagnant air film is about R-0.7 or so.


 

but the radiance from 140F sheetmetal is far higher than 90F sheetmetal. Collectors designed correctly need no fans.


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