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Super recently replaced my wall PTAC after my complaints, but the new one is still unable to cool the room to specified temperature. I've set it to 60 F but it stays at 70 or so indefinitely. Outside weather here in NYC has been 80s-90s.

I think I've ruled out internal issues with blower wheel, filter, compressor, refrigerant, etc. by measuring the CFM and wind temperature using the steps below, but let me know if my reasoning is incorrect:

  • Measured air discharge rectangular grille area: 28in * 5in = 140, ~1ft^2
  • Measured wind speed from cheap Amazon anemometer: approx averages 400 ft/min
  • Estimated high cool CFM: 1ft^2 * 400 ft/min = 400 ft^3/min, which is about manufacturer specs (375 CFM)
  • Anaeometer also measures wind temperature which matches the setting (set AC to min 60 degrees F, measures 61-62 F)

Based this, I can rule out any any internal issues (refrigerant, compressor, fan, filter, etc.) right? Also ruling out thermostat issues as room temperature is a 1-2 degrees close to separately measured.

Some specs:

  • Super replaced old unit with a rebuilt one,
  • Room: Studio Apartment, L-shaped living area, ~300sqft. PTAC and window located bottom of L near corner.
  • Old unit: Islandaire EZNY12A2GAS44AA, filter was never cleaned, drain pan filled with muddy water. 1200 BTU, might have been too big for studio.
  • New unit: Islandaire EZNY07_29AS44AA, rebuilt, serviced recently for "overload rpl (rp/?)". 7000 BTU, seems more appropriate for room. Receptable number: nema 6-20r socket (208/230 line voltage)
  • Specs above found in Islandaire's EZ series engineering manual pdf I found online

Based on what I've found above, can I safely assume that the AC is working as normal, but the reason why my room is still never cooling succesfully to the set temperature likely due to air leakage? Otherwise, are there any factors I still haven't ruled out?

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  • You have said nothing about the location of the apartment in relation to the outside, how well insulated it is, or other potential heat loads that your unit is competing against. It's possible that so much heat is coming through from the outside or other sources of heat that your AC just can't keep up with that continual heat load coming in.
    – Milwrdfan
    Commented Aug 2 at 4:13
  • Apartment is 9th floor facing south to a courtyard on a mid rise building. Right, as mentioned above I haven't ruled out other heat sources. For this question I wanted to rule out any issue with the AC unit itself before digging around for air leaks which seems much more difficult.
    – bo1000
    Commented Aug 2 at 5:01
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    AC is a close loop system. Measure th air temp at the return and compare to the exit. It should be 20 F
    – DIY75
    Commented Aug 2 at 5:41
  • @Traveler has it. If it is cooling the air then it just isn't a large enough unit for the heat load. Frankly 70 degrees is just fine.
    – Tiger Guy
    Commented Aug 3 at 3:08

1 Answer 1

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Important related issue is outside temperature. Typically you can expect an air conditioner to cool well up to ~ 20 degrees below the outside temperature. Meaning if you have 95 degrees outside then it may well struggle to get the room down to 70 degrees.

I've set it to 60 F but it stays at 70 or so indefinitely. Outside weather here in NYC has been 80s-90s.

So actually, you are doing OK. Likely you'll find some parts of the room hotter than others, but basically you can't expect to get much below 70 when it is hitting the 90s outside, at least not with a typical reasonably sized unit. Obviously in a hotter place the unit will be sized larger to compensate, but getting to 70 indoors on most days is generally considered good enough - normal comfort level.

Also keep in mind that if the output is 60 then it really can't cool the room to 60. It has to be a lower temperature in order to compensate for air leakage, heat coming in through walls and especially through windows, all the 98.6s living in the room, etc. But output at 60 when the outside air is 90 is not that bad.

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  • Would I have wanted a higher BTU rating to cool faster?
    – bo1000
    Commented Aug 2 at 3:38
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    Maybe. More BTUs = more/faster cooling. However, you also want a good duty cycle. Certain newer types of heat pumps will run at variable speeds, and then it is a non-issue. But with typical PTACs and most ordinary air conditioners, they are either ON or OFF at any point in time. If they are too large (too many BTUs) then they might cool so fast that they don't have to run much even on really hot and humid days, and then they don't dehumidify enough - as they say, its not the heat its the humidity. In theory you want something that on the hottest day will run nearly all the time and keep
    – manassehkatz-Moving 2 Codidact
    Commented Aug 2 at 3:47
  • the place cool and dry, and on not-quite-as-hot days (e.g., 80s rather than 90s) run 1/4 or 1/3 or 1/2 the time as needed.
    – manassehkatz-Moving 2 Codidact
    Commented Aug 2 at 3:47
  • Appreciate the detailed responses so far. Interesting point about duty cycle, not sure how to measure that (and it's not in the manual). As for too many BTUs issue causing too much humidity, what if I just got a separate dehumidifier? Would that make the room cool fast and solve the issue? And BTW it's 80 degrees outside, and AC still can't go below 70. IIRC even in the 70s AC couldn't ever go below 68.
    – bo1000
    Commented Aug 2 at 4:58
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    This is wrong, as anyone who lives in the deep south understands. People in Phoenix do not have homes that at 90 degrees when it i shot out. The spec is 20 degrees colder than the return air.
    – Tiger Guy
    Commented Aug 3 at 3:07

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