Central Air conditioner troublshooting

[smc]

Central Air conditioner troublshooting

Moved into a 2011 home in December, just now testing out the central air conditioner.

It will not turn on, neither the fan or the compressor. The setting the thermostat to cool does activate the switch on the a/c unit. I have taken the panel off to verify. The a/c is getting 120V from two wires so I guess 240 total (or maybe I am measuring the voltage in the wrong spot). There is a very faint hum regardless if the unit is fuse for the a/c is on or off.

Thoughts? The unit is less than three years old. The capacitor looks fine.

Thanks

[Darell_n]

Your furnace will energize the contactor in the outdoor condensing unit, even with the 240v disconnected. (this is the humming sound when calling for cooling). I would check for voltage drop across the contact with it pulled in (should be 0 volts per phase) After that I would need a schematic to see the safeties in the circuit. You may be simply out of gas.

[jeffh]

I had a similar problem recently that was the result of a faulty relay on my board. Difference is my fan on my outdoor unit would turn on, but nothing else. So, based on that, I think your problem rests with something earlier on in the sequence. I'd check your tstat(thermostat) and its wires/connections, make sure none of the wires have loosened. I am pretty sure there is a way to jumper your tstat controls at the board's 'tstat connectors' to rule out a faulty tstat.

[smc]

Originally posted by jeffh
I had a similar problem recently that was the result of a faulty relay on my board. Difference is my fan on my outdoor unit would turn on, but nothing else. So, based on that, I think your problem rests with something earlier on in the sequence. I'd check your tstat(thermostat) and its wires/connections, make sure none of the wires have loosened. I am pretty sure there is a way to jumper your tstat controls at the board's 'tstat connectors' to rule out a faulty tstat.

It definitely isn't an issue with the thermostat. When I sent the thermostat to cool, the contactor closes. I'll check the voltage across the contractor tonight to make sure it isn't broken.

[Darell_n]

Post a pic of the wiring diagram in the outdoor unit.

[Maxt]

Does it have an outdoor disconnect switch and is it on?

[smc]

I'll post a pic tomorrow.

It does have an outside disconnect and it is on. I have measured the power going to the a/c unit with a voltmeter.

[smc]

Not sure if these are useful but I'll post anyway

[rage2]

So my A/C has been inconsistent every spring. The only pattern I can find is that the outside air isn't hot, basically ~15C and under.

Is there a outside temp sensor that disables the A/C at a set temp to reduce the chance of the coils freezing? Or are my coils actually freezing?

[Darell_n]

Originally posted by rage2
So my A/C has been inconsistent every spring. The only pattern I can find is that the outside air isn't hot, basically ~15C and under.

Is there a outside temp sensor that disables the A/C at a set temp to reduce the chance of the coils freezing? Or are my coils actually freezing?

The low ambient control will prevent the compressor from starting and damaging itself when it's too cold outside. These are optional on residential units and are typically at 55*F (12.7*C) for commercial units without crankcase heaters. The indoor evaporator coil may also have a freeze stat to prevent ice formation. Low air flow (dirty filter or coil) or low refrigerant charge are the common causes.

[Maxt]

Put voltmeter on yellow and black at the top of the contactor (21)-(28) should be 220-240 volts ac
Blue and brown are the control wires should be in the range of 24 volts when thermostat is asking for cooling.
Red and black at bottom should have 220-240 ac when contactor is energized( blue and brown have 24 ac.

[Darell_n]

SMC, your photo has the contactor not pulled in and is the de-energized state. There should be no humming or call for cooling. If you force on the A/C the contactor should pull in and you should measure 24Vac on the small blue and brown wires.

A quick test you can try with the outdoor unit powered up is to quickly press in the contactor with a plastic pen. The compressor and condenser fan motor should run as long as the contactor is held in. If they both run, then you have a control problem.

[Maxt]

I should ask if anyone has been into the main panel at some time? Its possible to stab the 2 pole breaker into the same bus so you end up with the same phase of 120 on both poles of the breaker instead of 240 from different phases.

[smc]

Originally posted by Darell_n
SMC, your photo has the contactor not pulled in and is the de-energized state. There should be no humming or call for cooling. If you force on the A/C the contactor should pull in and you should measure 24Vac on the small blue and brown wires.

A quick test you can try with the outdoor unit powered up is to quickly press in the contactor with a plastic pen. The compressor and condenser fan motor should run as long as the contactor is held in. If they both run, then you have a control problem.

When the photo was taken the A/C signal from the thermostat was off. There was no humming. When the thermostat is set to cool the contactor does pull in, but nothing runs

If I push hard on the contractor pin I get a louder sound, but still nothing starts.

[smc]

Originally posted by Maxt
I should ask if anyone has been into the main panel at some time? Its possible to stab the 2 pole breaker into the same bus so you end up with the same phase of 120 on both poles of the breaker instead of 240 from different phases.

Thanks Maxt!!!!

The bus was the problem. It is now working. It was drawing only 120V before.

[Crazyjoker77]

Originally posted by smc


Thanks Maxt!!!!

The bus was the problem. It is now working. It was drawing only 120V before.

its actually very easy. put your voltmeter on terminals 11 and 23 you should see ~240v means everything is fine breaker side.

If it reads 0v between 11 and 23 then use your meter and measure each one individually to ground (that lug with the green wire) if both have 120V to ground and nothing between the two terminals the breaker is incorrectly installed in the panel.

[rage2]

Originally posted by Darell_n
The low ambient control will prevent the compressor from starting and damaging itself when it's too cold outside. These are optional on residential units and are typically at 55*F (12.7*C) for commercial units without crankcase heaters.

Thanks man, means I'm not going crazy haha. Yesterday was one of those days where it wasn't hot out, but the sun was global warming my whole house. A/C wasn't going and house hit 27C. Brutal.

[benyl]

Originally posted by rage2

Thanks man, means I'm not going crazy haha. Yesterday was one of those days where it wasn't hot out, but the sun was global warming my whole house. A/C wasn't going and house hit 27C. Brutal.

Open the windows?

I have 12 SW facing windows and the sun will heat our house to 30C when it is -10 outside. My A/C still has the cover on it. We just open the window. haha (yes, I am cheap).

[Jeff TYPE R]

Originally posted by rage2
the sun was global warming my whole house.

[hedge]

For some reason mine is termpermental the first time in the season. The house fan will come on but the outside unit will not. I'll try it again later and it will then start up. Only seems to be the first time in the season and then it's fine the rest of the summer.

I've had 3 different companies in to check it and they can't find anything wrong and of course it works fine when they are there. Frustrating.