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During the summer, one of the typical checks a technician makes is the indoor
temperature drop across the system.
Usually it is taken at the supply and return grilles subtracting the Supply temperature
from the Return temperature.
Ex:
Return temp 78
Supply temp 56
22 degree temperature drop.
Getting the split is the easy part, knowing what it is telling you, is more important.
The Relative humidity of the return air is needed to help with the evaluation. A
Psychrometer is as important a tool in the AC markets as a voltmeter.
Moisture is a form of heat (Latent heat) and the system has to remove it to make our
customers comfortable.
If the humidity is high, like here in south Louisiana, the unit has to work hard to remove
that moisture and cannot lower the temperature as well.
Ex: 90% RH = 8-12 degree drop typical
But in a much drier climate, like Arizona, the unit works less on removing humidity and
can therefore remove more “Sensible heat” or measurable heat.
Ex: 20% RH = 21-25 degree drop typical
If the system is charged properly and running in high stage the readings below should
provide good target numbers to work from.
If the system has a 22 degree temp drop and the Return air RH is 80% the temperature
drop is higher than the typical readings, therefore you can also look for:
- Low indoor airflow
- Dirty blower wheel or coil
If the system has a 14 degree temp drop and the Return air RH is 40% the temperature
drop is lower than the typical readings, therefore you can also look for:
- High indoor airflow
- Heating element staying energized
- Leaking ductwork
- Poorly insulated ductwork
Click here to download the .pdf version of this blog post.