What causes an roof-top A/C to stop for a while

[Tinstar]

My brother's roof-top ducted A/C unit works good,,,, sometimes. He said it will work for several cycles, switching off and on, then it will start heating up and the A/C will not come on. Sometimes he can switch the thermostat off and on a few times then it will restart and run fine a few more cycles. Other times, it will eventually just start back up but the temperature may have climbed 10 or 15 degrees before it comes back on. He took the caps to an electrical supply and they are both good.

It sounds like something is overheating to me. I am wondering if the fan motor overheats, will that cause the unit to not come on at all,,,, until it cools off enough. He said he can't hear anything like the fan trying to start and stalling. I guess it could also be a circuit board.

Any help with ideas is appreciated.

[time2roll]

At this point I think you need to get up inside the control box and see if the relay is actually sending power the compressor/fan. Need to determine if the thermostat is bad, relay box is not functioning, compressor issue or even a source power issue.

[catldog]

If he's knowledgeable enough, have him remove the tstat and jump out the control wires if it doesn't shut off then the tstat is bad, if it still shuts off could be a control board or one of the aforementioned problems. jg

[doughboy]

mine does the same
it will run fine for hrs.
the fan kicks on, then a few seconds later the compressor, which is how
it's suppose to do.
then for some reason the blower fan kicks on, the compressor doesn't when it's suppose to, the blower shuts off. this goes on for, maybe hrs or till i get up and change thermostat threw it's settings, off, heat, cool, auto, back to cool. then it kicks on and my run fine for the rest of the day and night or maybe not

[Tinstar]

Joatha wrote:
A couple of more questions:
1. When it dies, is the thermostat still on? If its on, then its not the t-stat (or less likely anyhow).

2. When it dies, does he have 120V at the AC unit? He should be able to to drop the bottom and check for voltage inside the unit.

Yes the thermostat is still on. It dies after cycling the compressor and fan several times. It's pretty random and may run several hours before it shuts down then won't come back on like it should. He may have to switch it off and on a few times and then it will come back on and soemtimes it won't OR if it just sits for an hour or so (getting hotter and hotter inside) then it will come on by itself.

I would have to ask him about 120 volts power at the unit. I told him to check all wiring at the a/c unit and check the 120 breaker in the distribution panel.

[Joatha]

A couple of more questions:
1. When it dies, is the thermostat still on? If its on, then its not the t-stat (or less likely anyhow).

2. When it dies, does he have 120V at the AC unit? He should be able to to drop the bottom and check for voltage inside the unit.

[doxiemom11]

Fan should be set on high to help prevent freeze up.

[Tinstar]

A little follow-up and to answer a couple of questions.

I found out it is a Duo-Therm Briskair 13,500. I don't know what thermostat he has. It is a wall thermostat and there are no ceiling controls. The problem is not due to freezing. It's been off for two weeks and he's been out several times to run it and it will not come on. He has it plugged into 30 amp shore power so I would say it's not low voltage. Neither the compressor nor the fan will come on after it quits. He's cleaned up the thermostat wires. I told him to climb up and clean-up any wiring up there. I also told him to make sure his 12-volt battery is fully charged.

Any other ideas would be helpful.

Thanks

[ScottG]

It's probably just freezing up.
Have him clean the filter and run the fan on HI.
That may be all he needs to do.

[wa8yxm]

Question .. When you say it "Shuts off" do you mean COMPLETELY both blower and compressor.. Or just the compressor?

The reason: Both motors have thermal cut-outs on them self-resetting Thermostat type switches that if the motor overheats.. CLICK and it shuts down.. Eventually... as it cools... CLICK and it starts back up.

These switches however operate INDEPENDENT of each other. Each motor has one.

On many, most in fact. A/C's in addition to the over temp switch in the motors there is a "FROST" sensor stuck in the evaporator coils.

This will shut down THE COMPRESSOR if it detects icing of the condenser.. Others have already explained what to do.




Finally your A/C uses TWO power sources if it has a wall thermostat or a remote control (only one if it has "Ceiling controls")

12 volt is used for "Remote" control (Wired or wireless) 120 volt for Operation power (Local controls use human power only)

ONe issue I had with MY RV, when it was new. Sometimes the front A/C worked but not the rear. Sometimes the opposite. Sometimes both and one time neither.....

TUrns out I had a few screws loose
The screws in the 120 volt power distribution panel that hold the wires.. All 3 colors
Thw screws in the breaker wire clamps (Black)
The white bus bar and the green/bare bus bar all had loose screws.

(Now some folks still claim I have a few screws loose but I don't know what ones they mean. I have checked all the screws in the RV and they are nice and tight...).

[dougrainer]

If the fan motor overheats, that will not stop the compressor from running. If the compressor overheats, that will not stop the Fan from running. Without a Brand/Model number of AC and RV and type tstat, nobody can give you any help at all. Doug

[rgatijnet1]

Low voltage can also cause it to overheat.

[Bruce_Brown]

Sounds like it could be freezing up. Once it thaws out it starts back up again.

Make sure everything is clean with good air flow.