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# Does anyone know

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Member Posts: 189
they list the btu output in btu's/sq ft/day

If I want to convert to btu/hr, how many hours do they figure are in a day? 24? 4? 8?

Thanks, Mike Dunn

• Member Posts: 1,935
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sun up to sun down

would be my guess if you are tracking maximum solar gain/day
• Member Posts: 53
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Solar window is approximately six hours.
ie 9 am to 3 pm with the collector oriented true south.
There is some minimal solar gain outside those hours but it is because of the angle of the sun. The air temperature also affects output of flat plate collectors.

Joe

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• Member Posts: 3,403
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I'm sure HR knows, but...

... I think it is simply the total daily BTUs captured. So, BTUs will change hour to hour. If you could adjust your math to need only daily total, it could be good.

Yours, Larry
• Member Posts: 22,541
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Here is the SRCC sheet

from a sample collector. It does show some snap shots under various "sun" conditions and various operating temperatures. The right side is in BTU.

The best way is to plot the Y axis and slope, located on the bottom of the SRCC sheet, on a graph like this one Siggy generated. Then plug in the various operating conditions. Then you can apply the panel to your exact conditions and see how performance changes with return temperatures to the panel and ambient air temperature changes.

Pic 9 & 15 show the panel operating at a high 170F and a lower 95F return. This shows how important the operating temperatures are in selecting a collector.

"Where it's at, and what you are asking it to do" are the two key questions.

hr
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 189
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Thans

HR for the response.

Another question. On the SRCC ratings they mention the test fluid is water. How do I compensate the btu outputs for a solar fluid of 50% glycol?

Thanks,

Mike Dunn
• Member Posts: 65
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You would deduct

Mike,
You would deduct 5-10%. I have seen higher numbers so you would probably be best to plan on at least 10% when compared to water.

Mister T
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