Central AC won't come on but "nothing is wrong"
July 17, 2021 2:27 PM   Subscribe

Our problem- central air is turning on very intermittently and getting nowhere near our temp target. Landlord had someone come and look at it yesterday and they said nothing was wrong with it, but it's goddamned hot in here.

-AC unit was replaced last summer so it is brand new. The compressor is on the roof so we cannot look at it ourselves.
-the maintenance guy checked it out and confirmed there were no coolant leaks, no debris blocking air, etc. The maintenance man was almost pissed with the landlord because everything was totally fine that he could see.
-The thermostat was just replaced - it did not fix the issue.
-We can hear the compressor turning on on the roof, but no air is coming out of the vents.
-When it DOES come on, it stays on for about 2 hours blowing cool air, then stops. The thermostat is set to cool, 75 degrees, but it never reaches 75 degrees.

I've tried to Google and Reddit to no avail, nothing I can read online describes our problem. Nothing is actually "broken" for it to be fixed. It incredibly frustrating. Any ideas?
posted by marvelousmellitus to Home & Garden (14 answers total)
Check the breaker box.
posted by rakaidan at 2:35 PM on July 17, 2021 [2 favorites]

Is the thermostat located in a spot where it is getting colder for some reason, maybe where the cold air from the AC is blowing on it?

You can also try keeping the thermostat warmer and see if that will help, try putting a heat pack near it and covering it with something to keep the heat in, like you would use on a person, nothing that will get hot enough to damage it.

By thermostat I mean the actual thermostat sensor, which might not be in the same place as the control panel.
posted by yohko at 2:58 PM on July 17, 2021 [1 favorite]

Of course something is wrong, you turn it on and it doesn't come on. You need to get someone who knows what they are doing.

There are some wires behind the thermostat. When the right pair is connected together, they tell the A/C to run.

If it does and cold air comes out, then you know the compressor and such is fine if it continues to run for longer than your 2 hour period. Problem is thermostat or wires, can be fixed by anybody inside the house.

If the air blows but isn't very cold, the compressor isn't working right. Need HVAC tech on the roof.

If the air doesn't blow, it's not getting the signal, the control board has detected a fault, or other similar things. Again, HVAC tech on the roof.

All this being said, probably a capacitor needs to be replaced.
posted by flimflam at 3:08 PM on July 17, 2021 [9 favorites]

It could be so hot on the roof that the compressor shuts itself off to keep from being damaged, or because running would actually blow hotter air into your house.

Roofs can be pretty hot places when the sun is shining.
posted by jamjam at 4:21 PM on July 17, 2021

Replace the filter? I had a similar issue years ago because the filter was so clogged with dust that air couldn’t flow.
posted by sleeping bear at 4:44 PM on July 17, 2021 [4 favorites]

Just because the maintenance guy found nothing wrong with it doesn't mean that there's nothing wrong with it- there are plenty of components that can be intermittent. Conducting tests and checking things while the system is running OK isn't going to tell you a whole lot diagnostically. Someone needs to look at it while the thermostat is calling for cooling and there's no air coming out.

When the fan isn't coming on, have you tried switching the fan setting on the thermostat to "on" rather than "auto'? If it still doesn't come on, the issue is likely the low voltage wiring from the thermostat to the air handler/furnace, the control board in the furnace, the contactor for the fan/ blower inside the furnace or the blower motor itself. I am assuming that when you say "no air is coming out of the vents" you don't hear the blower motor running?
posted by Larry David Syndrome at 4:56 PM on July 17, 2021 [3 favorites]

Sounds like the problem is in the air distribution., i.e., fan, ducts, vents... A fan problem could be with a motor or its controls, or a belt. A duct problem could be a blockage or a gap where ducts pulled apart. Vents can be closed off for winter and not be opened for the cooling season.

I'm not quite sure what triggers it, but there can be a problem with ice buildup. If so, the system would start working properly again after a wait for the ice to melt.
posted by SemiSalt at 5:30 PM on July 17, 2021

The vent covers -- where the cool air comes through, in your rooms -- come off, usually with just a couple of screws. First, check to see that the slats inside aren't levered shut. If they are open, then you can shine a light up there to look for a blocking panel. (We use foam, in the winter, to keep warm air from going up there). Also check the return vents, probably in a hallway: if they are closed up for winter, then the thing will turn off, too.

But honestly, when I hear that the air blows but never gets cold, I suspect that they hooked up the Fan wire but not the Cool wire -- or that the programming isn't activating that properly. Go online and find the thermostat manual. Read it twice and be CERTAIN that you understand the different between Cool/Heat/Auto, and the fan settings.
posted by wenestvedt at 6:10 PM on July 17, 2021 [1 favorite]

If it blows sometimes but the compressor always comes on when there is a call for cooling, I'd look at the blower first. A failing capacitor can do that. Eventually it will get bad enough that the blower won't run at all unless you open it up and give it a push.

You could try switching the fan to on instead of auto sometime when it is actually blowing and see if that makes it better. The air will only be cold when the compressor runs, of course, but at least it will work.

If the blower is running, it could be that the coil is freezing sometimes, which will block the air flow. Cleaning/replacing the filter is your first step in that case.
posted by wierdo at 7:00 PM on July 17, 2021 [1 favorite]

flimflam There are some wires behind the thermostat. When the right pair is connected together, they tell the A/C to run

Adding to this - carefully pop the front off the thermostat and take a picture of how the wires are before you disconnect anything. Figure out which wires to twist together to 'hotwire' the AC. Here is an explanation of the conventional wire color meanings in the US, but of course there's no guarantee that yours is set up conventionally. IIRC there are separate wires for AC and for the fan, so you can test each of those separately. All of this will let you either rule out or zero in on whether the fault is with the thermostat or something else.
posted by qxntpqbbbqxl at 10:03 PM on July 17, 2021 [1 favorite]

When the tech come out to check the AC, did they check both the compressor on the roof and the airhandler, which is, I assume, in your apartment? It's unclear by your description, and it kind of sounds like they may have only checked the roof unit.

Anyway...The problem seem to be not that the AC isn't cooling, it's that the airhandler isn't moving air properly. Those are two different failure modes, and if the AC guy was told "it ain't cooling" they might just check the compressor to make sure it had correct pressures, etc. It sounds, though, like they didn't actually test the temp of the cold air at the registers inside your apartment. Did they do this?

There are so many things that could be causing this. For instance, I once had an intermittent problem with the outside compressor coming on, but not the inside airhandler. Turns out, there was a loose wiring splice between the moisture sensor in the drip pan beneath the airhandler and the circuit board of the airhandler. The system thought thought the sensor had tripped and the airhandler was shut down.

You could also have a severely blocked air filter. There might be obstructions, somehow, inside the ductwork. Or, as others have suggested, there might be an issue on the circuit board of the airhandler itself.
posted by Thorzdad at 7:21 AM on July 18, 2021 [2 favorites]

My FIL is an hvac mechanic, but I am not. My house has an issue with static pressure--basically, if the air can't get pushed through the system easily enough then it will just stop running--and we have to be very diligent to replace our filter frequently, not use too "good" of a filter (we tried a hepa filter to help with our allergies and the system stopped working), and to not close too many vents at a time.

Go around your home and fully open every single vent. Turn the fan on manually then hold a piece of paper in front of each vent to see if the moving air moves the paper (air is flowing out), and in front of each return to see if it can hold the paper in place (air is being sucked in). If any vent is not working, something is blocking the airflow and your landlord should send someone out again to clean the ducts and look for other blockages.

If that all works, then turn the ac on and set the temperature as low as it can go. Go to each vent again and feel if the air is cold. Get a room thermometer and put it next to your thermostat to compare to see if your thermostat is miscalibrated, then leave it in rooms you commonly use (on your nightstand, desk, kitchen table). Log the temperature on the thermostat vs the thermometer in each room to see if your thermostat is just in a bad location and if so see if your landlord will move it or get you a remote sensor that works with your thermostat.
posted by jessica fletcher did it at 7:50 AM on July 19, 2021

Based on my own experience, a rule of thumb temperature for air exiting the vents is 60F. If its warmer than that, it's not going to do the job.
posted by SemiSalt at 12:06 PM on July 19, 2021

This happened to me recently (2 days after we had replaced the outside compressor unit) and it turned out that the fan motor in the furnace that blows the cold air from the compressor into the house had died.
posted by jenjenc at 3:36 PM on July 19, 2021

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