AC issues at Mom's house. Low voltage problems??

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Old 07-05-13, 04:00 PM
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AC issues at Mom's house. Low voltage problems??

I have been reading/watching videos on line all day trying to trouble shoot Mom's 2 ac systems (had 1st one when she moved in 17 years ago or so I believe) with the 1st floor. 2 years ago she had a 2nd system installed with ductwork (completely independent of 1st system) for the 2nd floor. That one is working again.

Now on to the issue...
T stat calls for cold and the fan comes on with no cold air. Breakers are all "on" and there is 240 at the contactor. If I push the contactor on with a screwdriver I get a very small arc and both the compressor and fan turn on (sound normal) but nothing when I release it.

I had mom turn the temp from 85 (T stat reading 83) to 78 when I was out listening to it and didn't hear anything. I'm not getting any significant reading from the 24v system (about 50mV shows up sometimes, but that may just be a fluke on the meter because it doesn't stay for long). I was testing it to the ground wire in there as well as to some of the other 24v wires, no luck.

I'm wondering if it's a bad contactor (if so any idea where I could get one on a Saturday in Philly) or if it's the control board (or something else).

It's a Bryant unit that is 18 years old (has run without issues in the past, and none of the wiring (or T stat) have been changed around since it was last run. Has not been turned on this year.

Thoughts?
 
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Old 07-05-13, 04:30 PM
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You need to check the wiring that comes to the compressor unit for 24 VAC. That will tell you if it's a problem in the house or in the compressor. Open the unit and check at the wire nuts where the wiring comes in.

If you have 24 VAC there and not on the contactor coil you will most likely need to call an HVAC tech to check the charge in the system.
 
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Old 07-05-13, 05:14 PM
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The contactor has a high voltage (240V) side, and a low voltage (24V) side. The low voltage side gets it's power from the 24V transformer passing through the thermostat. When the thermostat calls for cooling, it's contacts close, passing the voltage to the low voltage side of the contactor.

Here's what I would suggest. First, turn off the power to the system. Remove the two wires going to the low voltage side of the contactor and make sure that they don't touch each other or anything else. Turn the power on, and with the thermostat calling for cooling, check to see if you have ~24V between the two wires. If you do, then the contactor is likely bad (shorted or grounded coil). If there is no voltage between the wires, then you'll have to do some trouble shooting. Among the possibilities are a bad 24V transformer, a broken or shorted wire, or a bad thermostat (batteries OK?).
 
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Old 07-06-13, 07:38 AM
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I can't seem to get any 24v from anything coming off the brown coil that is wire tied to the insulated line.

Odd question that I feel dumb asking, mom has a party tonight and wants the AC on, can the contactor switch be taped in (obviously turning the power off first) and run like that (presumably it will stay on the whole time) or does this present a safety hazard?)

Thanks!
 
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Old 07-06-13, 08:12 AM
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NO, the outside contactor has to work hand in hand with the inside blower. When the inside blower stops, the outside unit has to stop too. You do have an ODD question, but here may be an ODD answer for you: Turn your inside blower to ON position (assume the ON blower speed has the same speed as AUTO) so to keep the blower run all the time. then monitor the room temperature and turn it on and off (both inside and outside at the same time) yourself. I am not 100% sure this will work, but , for one night, you can try. (or just keep the inside blower run all the time, only stop/start the outside unit.)
 
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Old 07-06-13, 08:49 AM
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Are you saying that you're not seeing 24VAC at the wires that connect to the low voltage side of the contactor (with the wires disconnected)? If so, then it's one of two things: either a problem with something inside the house (bad 24V transformer, broken wire, etc.) or something more serious at the compressor. Many units have a pressure switch that disables the compressor if the unit is very low on refrigerant.

I would check the 24V transformer first, making sure that there is voltage coming out of it. Next, I would check the low voltage wires coming out of the house, where they go into the compressor unit. If you have 24V coming into the compressor unit, but nothing at the contactor (with the wires disconnected from the contactor), then the pressure switch has likely opened indicating the unit may be low on refrigerant. If you have no 24V coming into the compressor unit, then you likely have a break in the wiring somewhere. I've ruled out the thermostat since you said that the air handler fan comes on when the T-stat calls for cooling.
 
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Old 07-06-13, 09:36 AM
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Bob,

It seems like there is nothing coming from the house to the outside unit (from the air handler) but I will continue playing with the meter and see if I can't confirm that. I may try to rig it up for the night leaving the fan "on" and using the breaker for the condenser, then just leave it off and let Mom's HVAC guy go after it this week.

I'll take a look back and make sure it's not a broken/chewed wire.

Thanks for your help!
 
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Old 07-06-13, 10:41 AM
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Perhaps this will help...

I'm getting a reading of 0v in the air handler on both the Y and the Common.

I have an 18-2 wire going out to the condenser with red and white (red to Y, white to common). It looks like there is also 18-4 coming from the Tstat: white to common, yellow to Y, red to R, and green to G. I am getting a reading of 27-28v to ground on the R and G, even though the stat is calling for cold air...
 
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Old 07-06-13, 12:09 PM
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Adam,

G is your air handler (indoor fan), so it should read 24V (from common or ground) when the T-stat is calling for heating or cooling, so that is normal. However, when the T-stat is calling for cooling, you should be seeing 24V at the Y terminal. I would suggest going down to the thermostat and check to see if you have 24V on the Y terminal of the T-stat when the T-stat is calling for cooling. If you don't, then I would suspect that the thermostat is bad. One way to check to see if this is the problem would be to try momentarily connecting a jumper wire from the G terminal to the Y terminal. If the outside unit comes on, you've identified the problem (bad thermostat).

Good Luck!!
 
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