Ideal Delta Temperature


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Old 06-30-08, 09:56 PM
C
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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.
 
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Old 07-01-08, 05:25 AM
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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.
 
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Old 07-01-08, 04:51 PM
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jim made a good point..

Reading on mine just above the coil, I get a 20˚ drop across the coil. It may be 15˚ on a higher load.
 
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Old 07-04-08, 07:52 AM
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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
 
 

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