In Part 20, we addressed (hopefully satisfactorily) a question from a visitor to DailyHomeRenoTips.com who wanted to know about the temperature of the air intake component when the solar air collector was not on.
Today, we address what to me is the most significant question about the performance of any solar air heating unit in our quest for energy conservation within our home by way of this type of supplemental home heating device; namely, what is the temperature rise?
First, what is temperature rise?
As I have come to learn, the term temperature rise means the increase in temperature of the air. In the case of a solar air heater it is meant to be the difference between the temperature of the air when it enters the unit and the temperature of the air when it leaves the unit re-entering the home / building.
Now, please remember I am not a scientist, nor an engineer; I’m just a simple home owner. So I use the tools that I have (unless someone wants to pay for any more accurate measuring devices and send them my way 
).
OK, here we go.
In Part 20 we saw that the temperature of the blower assembly inside the house early in the morning was around 50 degrees F. This is likely too low as the starting point.
Here is a picture of me taking the temperature reading of the ceiling vent at the same time on the same day (around 7 am). The vent is only connected to the flexible duct line coming from the Solar Max 240. If you don’t believe me, take a look at the pictures of prior articles in this series.
The reading in the above picture is 77 degrees F. This seams to high to me to use in the temperature rise calculation. Why? Well, this is one of the two ceiling vents. The component of the solar air heater which takes in air is just above the bottom of the floor. Warm air rises. So, basic science tells us that the air will be warmer towards the ceiling of any basement than it will at the floor level.
The above is another reading a minute or two later, slightly less. Here’s a reading from the wall around the in-take component a minute or two later.
Tell you what; I’ll use a range as the base, say between 65 (~ 1/2 way between the temperature readings of the ceiling vent and the area by the air in-take component) and 70 degrees F.
Next, here is a series of pictures I took of the same ceiling heat vent cover during the middle of the day when the sun has been shining and the Solar Max 240 had been running for a while.
What do you think? You can see temperature readings of 135, 136, 140 and 136 degrees Fahrenheit. I don’t know about your, but I am quite frankly very pleasantly surprised with such high readings.
Additionally, remember that this is the ceiling vent which is 24 feet away from the solar air heating unit; i.e. it is the ceiling vent that is in the basement recreation room, not the vent that is a mere 12 feet or so away in the basement bedroom. Even with using an insulated flexible duct line, I have to believe that at least a couple of degrees of temperature is being lost as the air travels that 24 feet to the ceiling vent.
Even though we did achieve a maximum reading of 140 degrees F., for purposes of calculating the temperature rise, we’ll again use a range of temperatures, this time between 130 and 135 degrees F.
Well, where does that leave us? If we use simple math and calculate the difference between the upper and lower temperature ranges, we have a temperature rise achieved by the Solar Max 240 of between 60 degrees F. (130 - 70 ) on the low end and 70 degrees F. (135 - 65 ) at the high end.
Wow. Now, we would be remiss if we didn’t remind you that a truly scientific analysis would include a study of the temperature rise each hour while the unit is in use. Solar air collectors do not all of a sudden start to pump out 140 degree F. temperature the minute they start nor the last minute they work. If I get time to do such a study I will.
As well, one needs to measure how much air is actually being ‘pumped out’ of the unit. This is a different type of measurement and quite frankly I have no device to undertake such measure. I’ll have to figure out how to do this in a way that makes sense to me. And, once we have this measure we can then, I think, calculate BTU’s which those in the know look to for a measure of the benefit from solar air collectors. At this point it will likely not be until some time over the holidays when I will have a a ittle time for this and will write about it some time in Janaury, when we our focus will be on energy conservation in the home.
Next time, however, I want to talk about an issue that I did not pay sufficient attention to during our installation process so you can be sure to select the most beneficial location for your solar air collector device and not repeat our mistake.
To continue to the next article in this series, simply select this link to Part 22.
13 responses so far ↓
1 A // Dec 23, 2008 at 1:42 pm
Hmmm… interesting.
Couple things.
First, for air flow use the garbage method. Basically, stick garbage bag over the vent and time how long it takes to till (or empty). I know Jon Eakes (http://joneakes.com/) has it in his database and I believe CMHC also has numbers.
Second, so how does the Solar Max compare to the furnance? …say a reading of the cold air return and a reading of a vent or two after the furnace has been running awhile.
Not that this is in any way a proper scientific comparison but I know you’re just looking for ideas to play with your new toy.
Cheerio!
A
2 Dan // Dec 24, 2008 at 12:41 pm
Hi A,
Yep, someone else (from New England) mentioned the garbage bag method as well.
I’m in the middle of writing a series on home energy conservation tips reviewing the 240+ that are on our ENERGY CONSERVATION page for January; however, once I finish that series I will return to writing about our solar air collector from a few different perspectives, what we didn’t expect in choosing the installation location that we did, etc. and will include an article on airflow.
As far as ‘compare to the furnace’ I’m not sure what information it would provide to take a reading of a cold air return and one of a vent connected to the furnace but sure, for you who have been a long time
visitor and commentor to our site I’ll do that.
Dan
3 mark // Feb 1, 2009 at 4:09 pm
Now you have had it for a while how is it working, I know you said earlier that you may need to change unit location
4 Dan // Feb 1, 2009 at 4:24 pm
Hi Mark,
I don’t think I ever said that I ‘may need to change unit location’.
However, to your question, it is working fine.
Once our energy conservation tips series of articles is completed (late February), I wll write some more articles about our experiences with the unit, including shade considerations, etc.
Dan
5 Bill B // Apr 19, 2009 at 5:08 pm
Very nice report! My question is how much solar energy goes right thru a window, versus the collector. I would think nearly as much energy would go thru a window ( if not low - e coated) as would a solar collector. Thus on my house I have low E windows everywhere but the south facing windows. The trees shade the windows in our hot summer, and provide alot of solar heat gain in the winter.
6 Dan // Apr 19, 2009 at 5:26 pm
Hi Bill,
I’m no scientist, so I have no idea.
However,solar heat collectors like this one contain cylindrical interior columns making the surfact exposed to the sum more than a flat surface, allowing a longer period of time for the interior components to receive direct UV rays which causes them to become / maintain heat.
As well, the interior components have a dark paint applied to attract more heat.
You might want to contact the manufacturer directly with your question. You can access their site by clicking on the picture of theunit installed on our south wall over in the right side of our site.
Dan
7 A. J. // Sep 21, 2009 at 11:51 am
As a solar energy professional for over 20 years, I just want to comment that you did a nice job installing the solar collector.
I would also like to point out that the product you installed will deliver as much heat to your house (not directly to the basement) as a window of the same size. This amount of heat would be worth about $25 a year if supplied by a natural gas furnace - somewhat more if supplied by an oil or electric furnace.
Unfortunately the manufacturers of these types of solar collectors grossly overstate the value of the heat they can physically deliver..
I prefer having a window myself:)
8 Dan // Sep 21, 2009 at 12:08 pm
Hi AJ,
Thanks for your comment.
While I do not disagree that some manufacturers of solar air collectort may overstate their value, I would have to disagree on your comment producing the same amount of heat as a window of the same size.
First, if that were the case a window does not come with a fan to blow the air heated by the sun throughout the house as do solar air collectors (at least the ones with which I am familiar).
Second, different models / manufacturers product different amounts of heat, so it’s not fair to lump all of them in the same category.
Earlier in this series I provided a link to a Natural Resources Canada page indicating efficiency ratings on different mondels of solar air collectors, including this one which we purchased and installed.
Third, back to the fan aspect, a window will only cause the sun to heat the air at and around the window, whereas a solar air collector will cause the sun to heat cooled air by way of the same fan pulling into the unit cooled air from within the house.
Fourth, as far as the generalized $25, it will depend on a whole lot of factors. I am not ‘in the business’ as you indicate you are, however, this number appears way, way low.
Cheers,
Dan
9 Vasile // Sep 23, 2009 at 4:15 pm
I agree the energy captured by the panel is the same as a equally large window. In the end, it’s the same incoming solar radiation.
But with a window, the heat is not as concentrated and suitable to be transferred via ducts to another part of the house, as you did. Instead, the flooring and maybe furniture will heat and later transfer that heat to the room.
In my opinion, the great advantage is that you can put the panel on the south side and use it in the winter, while disabling it and avoiding the overheating in the summer. You may get the same result by covering the window with a reflective screen in the summer, but it doesn’t look anywhere close as nice.
As for the fuel economy you can get, my *extremely* optimistic scenario (7000 BTUh for 6 hours/day and 180 days/year) gives around $200, so I presume real-life to be around $100. And if your furnace is highly efficient, this figure goes down to probably $60 (I considered a 60% efficiency in my calculations).
I agree there is more to this than pure money. It’s a cool thing which heats
and I really like it.
But here’s the deal breaker: our lifestyle means that we’ll get the heat during the daytime, when none of us is at home (and when the thermostat is already low). So maybe the end result is even lower, more like $40/year. A 50+ year break-even point it’s not something I would like to put my money into
I can imagine though that it may work better if you have small children or older people staying at home all day.
Regards,
Vasile
10 Dan // Sep 24, 2009 at 10:28 am
I agree, Vasile, that lifestyle does impact financial payback. The financial payback would be greater for those families with ’stay at home moms’, or with ‘retired’ (if one every truly is retired these days
), family members who are home most of the time, vs ‘commuter’ families where everyone goes to school & work during the work day.
With the price of Natural Gas being so low right now, yes the financial payback is long. However, Many areas of North America, e.g. New England, New York State, Pennsylvania, etc. see the majority of homes use home heating oil or electricity to heat their homes. In these cases, the financial payback is much shorter.
Every situation is just that little bit different.
And yes, there is no argument around the positive environmental impact of solar air heaters.
Dan
11 mark // Oct 4, 2009 at 3:06 pm
I have installed one for my in laws and their furnace has gone from cutting in on a sunny Jan. Feb. day from 4 times an hour to twice, now I am know heating expert but if that does not save $$$ then I do not know what will.
12 peteathome // Oct 23, 2009 at 7:17 pm
As a physicist and an engineer, I have to agree with the others who say it collects only as much energy as a window of similar size.
I also agree, though, that this energy is not as concentrated and so would be harder to vent to where you want it. But a small fan near the areas being warmed by the window would spread it around.
Any light coming in the window will get turned to heat even if there are no black surfaces. It just bounces around until it gets absorbed.
BUT - there is a major advantage of this approach over to a simple window, in addition to the concentration effect:
At night, a window is a liability, leaking a lot of heat unless really insulated ( high-e curtain and window low-e gases and coating). That’s one reason we don’t simple cover walls with windows for heating. That and the summer overheating that this approach also avoids.
13 Dan // Oct 24, 2009 at 11:17 am
Hi Pete,
Thanks very much for visiting our site and for your comments.
You provide a valuable perspective.
Dan
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