# Degree Days, Btu's and effiency stuff HELP

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 Please Register and login to reply and use other advanced options Posted by rob on December 18, 2004, 10:30 pm  Hi from 45°N lat Today the outside temp was -9°C and temp from air heater was 30°C....475 cfm....and it ran for 5 hrs....how many Kwhs or btus did I produce?...40K? 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 construction... Should we store heat in massive unused brick chimney or in a 6x7x8 water room or both as we are now doing? Thanks Rob Posted by Gary on December 19, 2004, 5:24 am  rob wrote: Heater Power Output = (VolumeFlowRate)(AirDensity)(TemperatureIncrease)(SpecificHeat) 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 about: 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 efficiency. Gary This Thread Bookmark this thread:               Subject Author Date   Degree Days, Btu's and effiency stuff HELP rob 12-18-2004 please rate this thread Please Register and login to reply and use other advanced options Latest Posts