One of the final tests technicians are trained to perform for either a service call or new startup is to check the temperature rise. These days, it seems like newer technicians are no longer learning this crucial test.
The test is simple. First, fire up the furnace or rooftop unit, let it warm up and wait for the fan to come on. Place an accurate thermometer in the return air plenum or duct, close to the heating equipment but not through the doors. Now place a second thermometer in the supply plenum or drop close to the unit, but again not through the doors.
If the return temperature is 70 degrees, and my supply temperature is 110 degrees, then 110-70 = 40 degree rise. Is this good? If you look on the ID tag of the equipment it will show you a temperature rise range, for example 30 to 50 degrees. Forty degrees falls in between the range of 30 and 50, so this temperature rise would be good. If the temperature rise in our example was close to or over 50 it would be too high, and if the temperature rise was close to or below 30 it would be too low, and neither would be unacceptable. Each piece of gas fired heating equipment will have the temperature rise listed on its ID tag.
High temperature rise is almost always an indication of poor airflow, or possibly too high of return air temperature. It could also be the burner is overfiring, so the gas pressure should be checked. If not corrected limit lockouts or failures will occur. Modern gas fired equipment will not cycle regularly on its limit like older units would. This explain why with some replacements with poor ductwork the old unit works but the new unit does not.
Low temperature rise may be an indication of too much air flow, or possibly too low of a return air temperature. Low gas pressure or a poorly operating burner could also be the cause. This must be corrected or such things as burner sooting, condensation or poor performance will result.
Temperature rise is a good simple test that will give us an idea of how the equipment is operating. A temperature outside of the acceptable range, either too high or too low, is an indication of a problem that must be addressed or other problems may arise.
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