AC won't keep up, lots of temps provided for diagnosis


Old 06-11-15, 03:00 PM
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Red face AC won't keep up, lots of temps provided for diagnosis

Hello all,

I've got a 13 SEER 2.5 ton matched split system that appears to be fairly new at the house I just moved into. I'm finding that the AC just isn't keeping up on these 90+F days we're having lately. It's like from the point it hits 90F outside, my air will never shut off if I have the thermostat set to something lower than 74-75F. Coming from an apartment that let me efficiently keep my place cooled to 65 with normal cycling times, this new house is disappointing, and I'm really afraid that once it hits 100F, I'll be stuck with a $400 electric bill and still be suffering 80F+ temps inside.

So here are my numbers, feel free to ask if you need any more info. I do have some refrigerant gages that I haven't hooked up yet, no refrigerant though even if it was low. But my thinking right now is to clean the condensor coils first and see if that makes a difference.

One thing that's bugging me is that in the evaporator closet inside, there's no drywall or insulation above the unit at all, you can see into the attic and the underside of the roof. You can also see some air leakage blowing through some cobwebs attached to the plenum and ducting above the plenum.

So look at my numbers and tell me what you think is going on. Thanks!

Also, the airflow is rather good inside the house. The filter I pulled out before replacing with a high quality filter wasn't very dirty, maybe a month or two's worth of regular dust.

1) Helpful: condenser 'Tonnage' & SEER Rating of Unit &/or model number: 13 SEER 2.5 Ton Matched Split System

FIRST; We need to know the 'Indoor' % of Relative Humidity 47-50 %

2) TXV or, orifice metering device? _______. Only if U know; not critical…

3) Outdoor condenser’s discharge-air-temperature 106.7F

Subtract Outdoor air temperature: 94.0F

Outdoor Condenser Air-Temp-Split 12.7F

4) Need the ‘Indoor’ percent of relative humidity - in the middle of the rooms or, at Return-Air inlet grilles 47-50% This was already answered above, the OUTSIDE humdity is at 60%

5) Indoor Return-Air Temperature 75.3F

Return Air Temp 75.3F Subtract Indoor Supply-Air Temperature 61.3F
Indoor temperature-split = 14.0F

Last edited by sv650; 06-11-15 at 03:21 PM.
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Old 06-11-15, 03:20 PM
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A delta T of 14* is a little low. Is the duct work in the attic?
Old 06-11-15, 03:23 PM
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Yes. Adding some characters to hit the 25 character minimum.
Old 06-11-15, 03:23 PM
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What type of house (two-story, ranch, etc.) and what is the approximate square footage? It's possible that the A/C is undersized for the size of the house.
Old 06-11-15, 03:27 PM
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One story brick Ranch, approximately 1300SF, which should be a good fit for a 2.5T system.
Old 06-11-15, 03:28 PM
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Is the duct work metal or flex? If it's metal make sure it's insulated.
Old 06-11-15, 03:33 PM
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Ductwork appears rigid, but is wrapped well in insulation from what I can see from the evap closet with no ceiling.
Old 06-11-15, 05:51 PM
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Ok, so I went and washed out the Condensor coils from the inside out with a water hose. My ΔT inside went from 14 to 15 just 10 minutes after firing the system back up. I'll let it run for a day and measure it again tomorrow, and if it's still not much better, I'll hook my gages up to it and see what pressures I'm getting.

How big of a difference, if any, would it make if I sealed off the evaporator closet ceiling? FWIW, the return air vent is beneath the closet and not exposed to attic air as far as i can tell. I know a lot of newer houses have the evap unit installed in the attic, so it doesn't seem likely that having the same situation, but with the unit lowered down below the ceiling into a closet with a door on it would make much difference if it's exposed to attic air.

A couple other observations...

I installed a different thermostat when I moved in. It's a Honeywell 5/2 digital that worked fine at my last residence. The wiring was a little different because this place has a gas fired furnace, but everything works fine as far as I can tell. The only quirks it has are that the AC, when coming on after resting for a while, will blow a bit of warm/hot air. It also is a bit weird in that the fan continues blowing after the thermostat has clicked off and presumably the condensor has also stopped. I'm thinking behavior like that is normal for heat operations, but I've never seen cooling cycles where the fan and condensor did not start and stop together. Is that normal, or should I revisit the wiring? And is the brief hot air at the start of a cool cycle a sign that attic air is entering the ducts, or is that normal for ducts in a hot attic to just soak up some heat, perhaps from being insulated metal wall ducting?
Old 06-11-15, 06:16 PM
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it's been about 30 minutes since my last readings, and my ΔT is now at 17.0 and the register is blowing 57.5, so I'll report back tomorrow and hopefully it will still be doing well in the heat of the day. I'd love to hear some more input though about the things I've posted, to see if I can get this working even better.
Old 06-11-15, 07:07 PM
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On many systems the compressor is purposely delayed in coming by a delay timer to protect the compressor from short cycling. Your blower may also have a delayed/timed shutoff.

It is very hard to completely insulate the ductwork in an attic to keep the heat from infiltrating into it.
Also .... the hot air in the room rises into the ductwork and gets blown back out when the blower starts.
Old 06-13-15, 08:12 AM
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OK, so it's much cooler today at 69F, but I put the gages on my condensor and found some more info to share. My low side was high, at 82psig, and my high side was low, at 140psig.

As best as I can recall, low side should be between 60-75, with 68-72 being sort of an optimal level on most 85-95 typical summer days here. And the high side should correspond to around 30F over the ambient temperature. The low side on a mild 69F day like this should be close to 60 I believe, and my high side, which equals like 82F on the R22 ring, should be like 99F on the R22 ring, or around 190psig.

I used to have an HVAC bible but I think I've lost it over the years. Someone please remind me what high side low, low side high means. Googling it has proven difficult.
Old 06-13-15, 08:22 AM
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Found a couple sites that point to a bad compressor and another forum both say that my compressor is probably bad, which is funny, because all the manufacturer and installer "official" sites say that the 94F-61F=33 degree difference between outside ambient and register air temps is "Excellent".

Can anyone verify that my data points to a bad compressor? If so, I'm going to have a fun time trying to explaining to my landlord that I need a new compressor when my AC isn't completely shot and leaving me with a house that's 90F inside.
Old 06-18-15, 06:45 PM
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At 50% indoor humidity the indoor temp-split should be about 18F; it is only 14F
The outdoor split is 12.7F it would usually be 15 to 16F, therefore it is 'probably not' drawing heat from an outside source into the Return-Air.

Could be a problem within the refrigerant system.
System may not be charged right; superheat & subcooling methods need to be used to check the system & to properly charge it if not a mild restriction in the system.

Is the indoor coil clean?

I didn't use your latest numbers after cleaning the condenser because the condenser's temp wasn't taken.

Everyone should be using my chart because most A/C systems are NOT performing to near their Rated nominal performance; this way U can find that problems exist & a contractor then needs to be contacted.
Old 06-19-15, 08:54 AM
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65F is totally unreasonable, wasteful, and unhealthy and the ac in your previous place was probably grossly oversized.

Could be a bad or incorrect metering device or a compressor with bad valves.

Look into getting this system pumped down - if it doesn't pump down you have a failing compressor. If it does pump down maybe the orifice is the wrong size.

Check compressor amp draw. Liquid and suction line temps should be checked.

Indoor split alone means nothing. High airflow or high humidity can cause a low split even if there's nothing wrong with the system.

To determine if the outdoor split is reasonable you need to look up the cfm rating of the condenser unit and measure compressor amp draw.

BTU heat removed from house = ((1.08 x split/delta-t)x condenser CFM) - (compressor amps x volts x 3.412)

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