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Ideal Delta Temperature

Ideal Delta Temperature

#1
06-30-08, 09:56 PM
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Join Date: Aug 2006
Location: USA
Posts: 82
Ideal Delta Temperature

I'm trying to determine if my 12 year old split level unit is working properly (it seems to take too long to cool down our house). I measured our return temperature right at the intake filter in our hallway and it reads 78 with the thermostat set at 78. The supply temperature read at a register close by is 63 and at one further away it reads 68. Is this the right way to measure delta and is a 12 degree (average) delta healthy? I'm trying to determine if I just need to recharge my system.

#2
07-01-08, 05:25 AM
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Join Date: Oct 2005
Location: USA
Posts: 1,017
Delta T is not a constant. It varies with load conditions and other factors. 78 degrees at low humidity will give you a higher delta T as opposed to the same temperature on a humid day.

Ideally it should be measured at the coil. This represents the true value. Measuring temperatures at a distant supply vent will give you incorrect values, especially if the ducts are located in a hot attic.

While some people may think a high delta T is good. It could be a sign of trouble, such as: a dirty coil, blocked filter, undersized ducts, etc.

Delta T is just one piece of the puzzle. And, in itself, is not a substitute for hooking up gauges, taking superheat/subcooling readings, and checking other conditions.

#3
07-01-08, 04:51 PM
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Join Date: Sep 2001
Location: Rochester, Minnesota
Posts: 16,984

Reading on mine just above the coil, I get a 20˚ drop across the coil. It may be 15˚ on a higher load.

#4
07-04-08, 07:52 AM
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Join Date: May 2008
Location: Wisconsin USA
Posts: 664
Originally Posted by cultcab
I'm trying to determine if my 12 year old split level unit is working properly (it seems to take too long to cool down our house). I measured our return temperature right at the intake filter in our hallway and it reads 78 with the thermostat set at 78. The supply temperature read at a register close by is 63 and at one further away it reads 68. Is this the right way to measure delta and is a 12 degree (average) delta healthy? I'm trying to determine if I just need to recharge my system.
To ball-park a temp-split we need the Return Air dry bulb plus the Return Air Wet bulb &/or Relative humidity.

At 400-cfm per/ton, & the RA @ 78-F with a 63-F wet bulb, or 43% RH, the target split would be 20-F.

At 78 & 72-wet bulb 75% RH, the target split would be 12.3-F. Humidity levels are a huge factor in the degree of split!

- HVAC RETIRED