Central A/C: intermittent compressor engagement

Factual Questions
1

I have a problem with my Rheem central A/C where sometimes the compressor doesn’t engage. I’m hoping that there’s someone well-versed with A/C’s who I’ve helped with their computer problems once upon a time. :wink:

Here’s what happens:

When the air turns on, either automatically by the thermostat, or when I get home and do it manually, the fan always turns on, but the compressor only turns on sometimes. I don’t even have to wait to feel if there’s cold air, because my A/C has this neat “feature” where I can easily hear the difference between a working or non-working compressor, since there has always been a faint “refrigerant gurgling” sound that has always happened when the compressor first kicks in, even when the whole system was installed new, approximately five trouble-free years ago.

However the A/C turns on, if the compressor engages, everything is fine and it will always stay on until the thermostat reaches the desired temp and turns off. But, if either I, or the thermostat, turn on the A/C, and the compressor decides for whatever reason not to engage, it will always remain off indefinitely. That means, under normal operation, when the thermostat is driving everything, it will eventually attempt to turn on the A/C, but when the compressor fails to engage, the fan will turn on and never turn itself off, since there is no cooling taking place.

At this point, I have to intervene, otherwise the fan will just stay on forever blowing room-temp air. All I have to do is turn it off, wait literally a few seconds, then turn it back on again. The fan always turns on, but whether or not the compressor engages at this point seems to be about 50/50 - sometimes it won’t turn on, and I have to just switch off/on again until my 50% luck kicks in and it works. When it does engage, all is temporarily well until it cycles off and tries to turn on again later, then the 50/50 odds kick in once again.

I originally suspected that it was just the thermostat getting wonky, and after changing its batteries, I tried a little amateur diagnosis using the “Safe-T-Switch” installed in the condensate drain pipe. If the A/C is “ON” but the compressor isn’t working, and I manually trigger the float on the “Safe-T-Switch” to turn it off then on, that will have exactly the same effect as using the thermostat controls to turn off the A/C and turn it back on. IOW, sometimes using the “Safe-T-Switch” to cycle it off/on will cause the compressor to finally kick in, and sometimes it won’t, just like with the thermostat.

So, in my non A/C-expert mind, this eliminates the thermostat as the point of failure, since the same intermittent behavior of the compressor is exhibited when the thermostat isn’t even used to cycle it off and on.

Any ideas, suggestions, observations? I can do pretty much anything that doesn’t require specialized tools/equipment. If it comes to that, I’ll buck-up and call a pro, but if it might be something simple I can check for and fix myself, I’d rather do that.

Thanks!

(PS: Here’s another thread with someone having a different A/C problem. Since I plan on shamelessly promoting my thread there, I thought I’d share the favor here. :wink: )

2

Is that big ol’ capacitor outside in the compressor shot?

For G-d’s sake, call a guy. That cost me $175.00

3

Ok…

The outdoor unit gets its signal from inside. The first question is: Is the outdoor getting a 24V “directive” to come on or not?

So…If you have a screwdriver take the panel on the outdoor unit and look at the contactor.

Does it look like this?

If the contactor is drawn in (and it might be humming) then the problem is outside and we can address that.

If it’s not drawn in, then the outdoor hasn’t gotten the memo you want cooling so its doing nothing.

Do that first and we can give you better direction.

4

With no better information, I’m seriously thinking its the safety switch.

But that’s a swag. Get us better info (and you’ve given us a great start) and we’ll give you better answers.

5

It could be the cap in the compressor. When the compressor decides not to run, go outside and see if you can hear the unit humming like it’s trying to start. I just learned in another thread that some units use a single cap to start the compressor and blower on the unit so if the fan isn’t running also and you hear the humming, that’s the likely cause. You could also trying spinning the fan up by hand by putting a screwdriver or stick down through the grates and giving it a little nudge…This is all if the unit is in it’s ‘decided not to run’ mode and you hear a humming sound.

The reason the blower inside your house is running ‘forever’ is because your house never comes down to temp so the t-stat never stops calling for cold air. It thinks the AC is on. That all makes perfect sense.

A few other thoughts.
It could be the relay on the compressor.

It could be the circuit board on your furnace. If you want to test that, pull off the panels on your furnace to find the circuit board and tell the t-stat to call for cold air. When it ‘decides not to run’ take a look at the circuit board and see if there’s any blinking lights. There should be a legend somewhere on one of the panels. If you have to take the bottom panel off, you’ll have to hold the safety switch down to get everything to engage.

These are just some quick, off the top of my head thoughts.
ETA Raindog’s ‘humming’ and my ‘humming’ are two different things. I was referring to the humming of motors due to a bad cap. I believe raindog was referring to the humming of the relay. That’ll be a different sound. Higher pitched and if you put your hand on the (plastic) case you’ll be able to feel it vibrate.

6

voltaire, do you have some familiarity with electricity and relays?
If not, I don’t think you ought to go poking around with a screwdriver.
You could, like, die.

:slight_smile:

7

He probably wouldn’t die. It wouldn’t be any worse than a severe electrical shock.

And of course, he’d probably soil his armor.

8

Central AC Compressors run on 220V and 20-30amp. Yes, that can easily kill you.

9

Thanks guys, those are just the kind of “things to look for” that I wanted.

Might take me a while to implement, since the outside unit is on the roof of my building, so I have to procure the key to gain access. I’m also a bit sketchy about having to inspect it while it has power going to it, so I’ll take all precautions, which will also include waiting until I can have a buddy around to make sure I don’t end up electrocuted and baking in the sun alone on the roof.

Thanks again!

(ETA: Heh, yeah, I wrote the above before seeing the “warning” posts. Yeah, I have decent general electrical experience, and fully realize the dangers if I should go poking the wrong thing. I’ll be careful, promise!)

10

Pretty unlikely in this scenario.

Not to say that an electrical shock is not unpleasant, and potentially dangerous, but if voltaire has any gumption at all the act of removing a panel and observing a contactor is not going to put him/her in imminent danger.

11

The 24V contactor is independent of the 220V feeding the unit, so feel to pull the disconnect. If the unit inside is calling the contactor will still be drawn in.

But be aware:

Theres still 24V present from the indoor unit.

The capacitor may be holding a charge.

So don’t stick anything in it; fingers, screwdrivers etc.

12

Agreed. If he puts his fingers in the wrong place he could be history.

13

In a residential air conditioner, you’d have to simultaneously grab both hot wires to feel 220.
Touching a single one is no different then accidentally sticking your finger in a light socket. Which, while not pleasant, isn’t likely to kill you.

Don’t forget, If you stick your finger in that light socket, your getting milli-amps through your body. It doesn’t matter if there’s a 10 amp fuse or a 30 amp fuse on the circuit since you’re not going to pull anywhere near that much. Your body is a good resistor.

14

Inside a house if you stick your finger in a light socket you donot get a high current flow because you are not well grounded, that is unless your finger also hits the side and center of the light socket. And then the current flow is through the tip of the finger to the side of the finger.

But out side it is easer to be well grounded, and a current flow from the left arm to the right leg could kill you.

Also if you think 120 is not a killer why is a GFIC required around water sources of a kitchen or a bathroom. 120 VAC can damage or kill.

I got cross wise of a igniter on a boiler once 10,000 volts. Hurt like hell but no perminate damage. So for electricity to kill the conditions do havve to be right, but why take chances.

I also know a case of a guy in a cieling space of a sears store. There was a 4X4 box with no cover and one loose wire. He was on his hands and knees when he hit the wire with his head. They found his body a few day later.

15

So you’ve made your point, you’ve added your warning.
Electricity can kill. So can a lot of other things. No one ever doubted that.

Be careful, respect the electricity, learn how to use a multimeter, pay attention to what you’re doing, etc etc etc…

Let’s get back to working on the AC.

As for why take the chance. Because people like to fix their own stuff, people like working around their house. People like to diagnose and replace $50 parts instead of spending $500 to have someone else do it. For me, it’s fun and I enjoy it (I fix coolers and freezers at work all the time).

That’s all I’m gonna say about it. When we’re ready to jump back into fixing the AC, I’ll be here.

16

Hey, no matter how unlikely, if we get somebody kilt, the Mods are going to be really, really annoyed.

Just sayin’.

:slight_smile:

I’ll take notes on your advice; you know your stuff.

17

I try, but I only dabble in it and I’m totally self taught. I’m just good and thinking through problems and rationally troubleshooting them. Be it an HVACR issue or a broken commercial orange juice machine. But I’ve always been mechanically inclined and never really been scared to take things apart and figure out how they work or why they aren’t working (which usually involves running them with panels off and safety switches bypassed).

BTW Raindog is an actual HVAC guy, so anytime he’s around, always go with his advice over mine. I’ve actually learned quite a bit from him just on these boards on used in my IRL job to diagnose my own coolers.

18

I am actually having a similar problem right now. The AC unit is a 6-year-old Trane, and it just started doing something like the OP’s unit: the compressor (and the house fan that blows the hopefully cold air through the ducts) stops working. When I turn it off and let it sit for a while (hour, two hours), then turn it back on it starts and works.

I just called the HVAC guy to come over, he said something is probably causing it to overheat, triggering the safety override, and when it cools down it works again. But what is causing it to overheat?

19

It’s odd that the house fan (main blower) is turning off as well. That would (to me anyways) imply that this is a problem that either the t-stat or furnace is controlling and not something originating at the compressor. I don’t believe the compressor can send a signal back into the house to say “I’m broken, you guys can stop running”. T-stats and furnace circuity usually won’t let the compressor turn on immediately after it’s been turned off, but I think the lock out time is something like 5 or 10 minutes, not several hours.

*Anyways, if the blower in your house is turning off as one of the symptoms, I’d be surprised if the compressor had anything to do with the problem. * Electrically or due to other sensors, the compressor will turn off on it’s own if/when the inside blower shuts down (or for other reasons can’t move air) but if the compressor turns off because it’s broken, the blower won’t turn off as a symptom (does that make sense).

20

Now that I think about it, it could also just simply be a broken fan motor.

I just can’t think of a scenario off the top of my head where an overheating compressor would make the inside blower motor shut down as well. Like I said earlier, to the best of my knowledge, it’s just one way communication to the compressor (I think). If I had to guess (and this is just a guess) the HVAC guy you spoke to didn’t hear you say that the furnace fan was turning off, the same way I didn’t see that you wrote it the first time I replied (you’ll notice the edit in my reply where I got rid of a bunch of stuff).

The odd thing is that it takes hours for it to come back on which sounds like an over heating compressor (or something really wonky with a t-stat or circuit board). I assume you’re trying to force it to turn back on by lowering the t-stat temp and nothing happens. But again, if the house fan isn’t coming on, I have to assume it’s a problem inside somewhere.

If your t-stat has batteries, you should change them before the HVAC guy gets there. That’s the only thing I can think of.

A bad blower motor wouldn’t be reliable enough for you to nail down a pattern like this.
A bad circuit board…I just don’t see that being the issue.