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Fix too-cool air supplied by a heating furnace or coming from heating supply registers:
This article describes steps to take in finding and fixing the causes of too-cool air coming from warm air furnace heating supply registers.
Warming up the air supply coming from your heating system may be as simple as re-connecting a loose or disconnected return air duct that is drawing too much cool air from the wrong place in your building.
How to Improve the Warm Air Supply from a Heating Furnace
Reader Question: Heating Furnace Runs but Delivers Air that is Cool or Not Warm Enough
8 Feb 2015 brianbe said:
I have a Rheem/Rudd model UGDG05EAUER up flow LP gas fired warm air furnace rated at 50,000 BTU's and 80% efficient. I am not an HVAC guy, but I have always felt that the output temperature of the furnace was too low. I started pouring over the manual and did some research on the internet.
[Click to enlarge any image]
Sketch at left illustrating where to measure temperature drop across a cooling or heating coil was provided courtesy of Carson Dunlop Associates.
I started with checking the heat rise. The furnace is rated for a 20 - 55 degree F rise. I initially measured a supply temperature of ~99 degrees F and a return temperature of ~57 degrees F.
I used a CFM calculation I found on line. I used 1.08 x the 41 degree F rise measured and divided the output BTU's of 42,000 by the previous result of 41 arriving at a CFM rating of ~948.
According to the data charts for this furnace, a speed setting of medium high would give me a CFM of ~900.
I changed the taps and checked the rise again. There was no appreciable difference. I then dropped the heating speed to low and measured again.
Now, the supply temp was ~110 degrees F and the return remained at 57. The furnace is rated for a rise of 25 - 55 degrees F. With the heat rise now at 52 I was still in the rated heat rise for this furnace. I have read that the typical supply temperature of this type of furnace is `110 - 135 degrees F.
The maximum supply temperature rating for this furnace is 155 degrees F. How can I increase the supply temperature of the furnace while staying within the rated temperature rise of 25 - 55 degrees F?
I was going to check the gas supply pressure before and after the furnace regulator next. The furnace is in an enclosed crawl space that is at ~45 degrees F. The ducts are not insulated. Your help would greatly be appreciated.
I am measuring temperatures in the ducts as the air leaves (supply) and as the air returns (return) to the furnace. I know that I need to insulate the ducts.
Reply: things to check when air temperatures are too low at or leaving the warm air furnace supply plenum
Regarding "How can I increase the supply temperature of the furnace while staying within the rated temperature rise of 25 - 55 degrees F?"
I am guessing you mean the supply air at the registers in the occupied space is too cool. It would make sense to insulate your heating ducts.
Thanks for the clarification.
If air leaving the supply plenum is too cold then I would look for:
A dirty and partially blocked heat exchanger in the furnace - air is not being adequately warmed as it passes through the heat exchanger.
Wrong fan speed (too high): air flow rate exceeds the CFM design rate for the furnace in heating mode.
Improper burner adjustment or flame too small (rare but possible; before measuring or messing with pressure - which is dangerous - just look at the flame to see if it looks normal or weak).
Obviously if there were *no* flame at the gas burner then we'd look for an out-of-fuel problem, power off, thermostat not calling for heat, a bad thermocouple, or a bad flame igniter (on some models this is referred to as a "hot surface igniter).
See NO HEAT - FURNACE
Excessive introduction of outdoor or make-up air
Improper return air ducting, not recirculating building air but rather always taking "new" supply air from a space that is not receiving heated air (illustrated by my ugly old furnace photo at above left and by the open un insulated crawlspace ducts in my photo at left).
Return duct leak openings where return ducts run from the occupied space through unoccupied space such as a cold attic or crawl space can give the same effect.
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