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Degree Days, Btu's and effiency stuff HELP

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Posted by rob on December 18, 2004, 10:30 pm
Hi from 45N lat

Today the outside temp was -9C and temp from air heater was
30C....475 cfm....and it ran for 5 hrs....how many Kwhs or btus did I

If the heater was 100 % efficient how many sq ft of heater do I have?

I have from 2000 to now electric consumption and daily degree days. I
adjusted degree days monthly totals to coincide with electric meter
reading dates and also subtracted a "fixed" kwh(lights , tv,
refridgeration) so I have an guestamate of 'heating only kwh' per
degree day. can anyone tell me the "normal" relationship...i know it
is not linear... im trying to fix bumps in elec consumption from

Should we store heat in massive unused brick chimney or in a 6x7x8
water room or both as we are now doing?


Posted by Gary on December 19, 2004, 5:24 am
rob wrote:

Heater Power Output =

Where (TemperatureIncrease) is the increase in air temperature from
the heater inlet to the heater outlet.   IF the heater takes in
outside air at -9C, and it exits the heater at 30C, then the power
output would be approximately:

=(475ft^3/min)(0.075 lb/ft^3)(30 - (-9))C*1.8 F/C)(0.24 BTU/lb-F)
= 600 BTU/min or 36,000 BTU/hr

If you had this this level of output for 5 hours, then:

Total Heat Output for 5hr = (33,000 BTU/hr)*(5hr) = 180K BTU for day

If the heater takes in air from the heated space instead of outside,
then you need to redo the calculation above with the correct value for
TemperatureIncrease = (HeaterExitTemp - HeaterIntakeTemp)

Since the solar input at noon is about 260 BTU/sqft, and at 9am and
3pm the solar input is down to about 160 BTU/sqft, your panel output
was probably not constant over the 5 hours centered around solar noon.

Full sun on Dec 18 at 45 lat on a collector tilted at 45 degrees is

1400 BTU/day per sqft of panel
Peak around solar noon is about 256 BTU/hr per sqft of panel

For a vertical collector, the numbers are almost the same at this time
of year.

You can use these numbers and your measured numbers to estimate


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