Settle an argument


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Old 09-09-10, 01:35 PM
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Settle an argument

How would the return air temp and humidity affect the temp rise on a new furnace install?? If your return air temperature was higher than normal and the humidity was high(because new furnace installed and tested in summer) would you get a higher or lower temperature rise? Or would your temp rise be a constant?
 
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Old 09-09-10, 06:44 PM
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Temperature Rise

You should get the same amount of rise. The total amount of moisture in the air remains the same. Only the RELATIVE humidity changes. The discharge air temperature will be higher but after deducting the return temperature to determine the temperature rise, the delta t (temperature rise) should remain constant once the furnace reaches "steady state".
 
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Old 09-10-10, 03:48 PM
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This is an interesting question from a theoretical standpoint. We know that conductance of heat is quicker in humid conditions as more or denser molecules can transfer heat faster. (Think Finnish sauna, throwing water on the hot rocks. Been there, done that - temp rises instantly). Also you have another factor where heat transfer is greater between disimilar temperatures. The closer to equilibrium the lesser and slower the heat transfer.

Then -if say in theory that 140 degree air entered the cold air return, (furnace has a 200 to 240 degree high temp limit) and say furnace has a normal rise of say 50 degrees - I'm not so sure it go to 190. But if it did, it might take an abnormal time to reach that? I'm not 100% sure on this, but I'd hate to take the bet without first consulting some heating engineer or physics prof first.
 
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Old 09-10-10, 04:35 PM
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Overall the temperature of the air would be the same, whether it took in cold air or hot air.
If you took in 78 degree air and heated it to 82 that's a smaller rise than taking in 65 degree air and heating it to the same thermostat setting.

Or am I misreading your intended meaning of "rise?"

If it heats hot air it'll heat cold air.
 
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Old 09-10-10, 04:41 PM
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The "rise" Broods is refering to is the amount of temperature increase between the incoming & outgoing air. Not the room temperature. All furnaces have an allowable range of temperature rise & a maximum outlet temperature.
 
 

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