Low Delta T --> Old system or something wrong?

Old 07-08-20, 06:21 PM
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Low Delta T --> Old system or something wrong?

I have an upstairs ac/hydroair unit that covers all of the upstairs and most of the downstairs. Both blower and condenser are pretty old Rheem units that probably don't have too much life left in them. Let me know if you need specs and I can get them tomorrow. The house can reach target temps but its unclear if the timeline to do so is okay. I have recorded a delta T between supply and return of 10 - 13 degrees depending on the registers tested.

Here are some details and things I have checked:

1) The suction line runs inside a cedar conduit up the outside of the house. Insulation on the exposed 3 feet before the conduit was rotted away, so I replaced it today. I assume the insulation inside the conduit is in bad shape as well, but wont have time to check it until the weekend.

2) The unit originally supplied air for two bedrooms upstairs and three rooms downstairs. After several additions/conversions, it now feeds 5 rooms downstairs and 3 bedrooms upstairs. I assume it was after these additions that a whole house return was added in the upstairs hallway with a filter inside the grate (16x25x1). Weirdly, whoever did this work left the two return ducts active in the two original bedrooms. Each are 6x6" and tap into the return lines POST filter. Air temps at these two ducts is 3-4 degrees cooler than the whole-house return duct.

1) Is this low delta T a concern or expected with very old equipment? Our weather has been upper 80 outside, but the house has maintained 72-74 during the day and 69 at night without problem.
2) could bad insulation in the conduit on the suction line be the cause?
3) could these two 6x6" returns be the cause since they are drawing in cooler (near target temp) air without a filter?
4) If I take the 6x6" return ducts offline, is the 16x25x1 single return in the hallway enough?
5) is there a specific temperature the suction line should be just before it enters the blower unit?

Sorry for the long post. thanks for the help.

Old 07-08-20, 10:11 PM
Join Date: Aug 2008
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Old system does not cause Low Delta-T, while 10-13 is not good, but is not that bad either. that is why you can still cool the rooms down to the target temperature. Normal delta-t is around 16-19. may be just a little short on Freon, The large suction line needs to be insulated all the way. the 6x6 returns should not affect the delta-T result. You will need an AC pressure gauge to figure out what the problem is.
Old 07-09-20, 12:39 PM
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so I just completed a few tasks:
  1. cleaned out the condenser coil. it wasn't in great shape but it wasn't awful either.
  2. Inspected ductwork in the attic for any leaks -- none found
  3. inspected the evaporator coil. seemed okay. It isn't accessible for cleaning unless I completely pull it out.
  4. inspected the blower fins. some dust but seemed okay.
  5. shored up some insulation on a few spots along the suction line
  6. adjusted the attic exhaust fan stat from 120F to 105F. It wasn't coming on prior to the adjustment even though attic is in the sun all day and its been about 90F during the day.
Some observations:
  1. No sign of freezing at the coil
  2. Parts of the blower (located in the insulated but unconditioned attic) had condensation on the outside. I am not surprised but is this normal? It is pretty humid today.
  3. should I take the two 6"x6" returns offline since there is already a 16x25x1 return in the hall way <15ft from each of those returns? Obviously an unfiltered reutnr is bad. But I don't want to disrupt comfort or limit air intake too much by removing them. I can always try to place a filter in line.

I imagine that if these don't resolve the mediocre deltaT then its probably that I am slightly low on freon, as clocert suggested.


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