HVAC help needed please

Paul

Paul

Dr 'Dre
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Joined
Feb 10, 2008
Location
Kings Mountain
My daughter called today and said her ac unit was blowing hot. Went by after work and checked it out.
The facts,
Air handler and gas furnace are under the house, Goodman compressor is outside.
Set the thermostat to a/c, air handler fan came on, compressor did not.
Checked voltage @ contactor in compressor. 242 volts ac.
Checked red and white wires from thermostat. 4 volts ac.
Checked red and white wires at the thermostat. 4 volts ac.
Shouldn't it be 24 volts dc?
Thermostat bad?
TIA!
'Dre...
 
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Could be a bad transformer. It is in the air handler. Also try the spin start on the compressor blades and see if that makes the unit kick in. I am not a heat man but I stayed at a Holiday inn express last night.
 
The hvac unit supplies 24v to the thermostat I think, and the thermostat switches that back to the HVAC unit to engage a heating or cooling contactor. I think the red is the 24v feed from the HVAC unit, and the white is the heating control wire back to the HVAC unit? Don't quote me on that.

If you're not getting 24v, that may mean the HVAC unit has one of the pressure switches tripped for protection. I can't remember how all this works. Time to Google, I'm curious now. (EDIT: It doesn't work like that, I just found out. See post below.)
 
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I'm no HVAC guy either, but I'm an ex- electronics design consultant and ex- professional electronics tech and can read a schematic with the best of them.

So here's what I've found out:

First and foremost, there's no output rectifiers on the control supply transformers from the HVAC unit, so they're 24VAC and not 24VDC.

You have a A/C unit and a furnace, so you'll likely have a 4 or 5 wire control. If it's a 4 wire, there will be one transformer to supply contactor coil power for the heat, fan, and A/C contactors. If it's a 5 wire, it will have a transformer for heat, and another transformer for the A/C and fan.

Either way, the fan is working, so you're getting control power from a transformer. So if the thermostat is working, you should see 24VAC between the yellow wire and the low (ground) side of the A/C contactor coil (the compressor contactor coil I believe) when the thermostat turns the A/C on.

If you want to get fancy and check the thermostat directly, and you're using a digital multimeter, you should see 24V between the power wire (the red wire for a 4-wire thermostat or the blue wire for a 5-wire thermostat) and the yellow wire when the A/C is off. You should see very close to 0VAC between the red (or blue) and the yellow wire when the A/C output is turned on, because both wires will be at 24VAC (so 0 volts differential).
The reason you have to get fancy is that there is not really a ground reference at the thermostat, or at least there likely won't be. There's not one on my thermostat, I know that much. Therefore you can only check voltage between an output and the supply power, which needs to be interpreted backwards from checking voltage between an output and ground.

Green is the fan contactor output from the thermostat, apparently.
 
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Correct you are on the 24 volt being ac not dc.
The Tesla v Edison thing.
Checked the power with a Fluke digital. Would me having the meter set on dc skew my readings?
And it's a 5 wire thermostat.
 
Yes, having the meter on DC would totally screw up proper readings, and your interpretation.

Low voltage control power being AC in houses isn't the Tesla vs. Edison thing, it's usually just because transformers are cheap and reliable and it takes extra electronic components to turn AC into DC after changing voltage with a step-down transformer. Extra electronic components are extra money and can fail. AC is just cheap and easy for low voltage control. Relays don't care what they're switching.

What are the 5 wire colors, and the corresponding terminal names on the thermostat?

Again, testing the function of the yellow wire is the important part, because you're already getting power to the thermostat if the fan is turning on. I think you're very close to ruling out the thermostat as the problem.
 
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Id check a few other things as well.

You have high voltage. That's good.
as others said, on low voltage wiring at outdoor unit, check red to ground. Should be around 24 volts. If the stat is calling for cooling, you should also have 24 volts between yellow and ground.

If you have voltage on both, push in the contactor and see if it runs. Might be a bad or stuck contactor.

If you don't have voltage on yellow, i would check for a condensate float switch as well. There are not typical for a unit under a house but it may have one. Some folks break yellow to cut off AC only if there is a condensate issue.

You didn't mention this but you didn't hear the compressor humming did you? Was the outdoor unit fan running or was it all dead?
 
shawn said:
Capacitor?
Click to expand...

Could be. If Paul has a meter that can read micro-farads, that would be the next step. If the capacitor is swollen odds are its bad. If its not swollen, pull the wires off and take a screwdriver and short the posts together then read the capacitance. It will be a dual capacitor so it will have two ratings (35 + 5, 40 + 5, etc.)
 
Where's daughter live?
 
Ranlo.
 
Doesn't sound like this is the issue but just in case. If it has a condensate pump make sure it is working. I've replaced mine three times already. My unit will not run at all if the pump isn't working.

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I had a capacitor go bad on mine and it had the same symptoms as you described. Replaced it and it was good.

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I know if u put your hand over the fan and it is blowing out hot air that is good, i would check capacitor and coolant. Sometimes when mine is low on coolant it acts retarded like that.


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If you push the contactor in does the condinser start? If the contractor is pulled in do you have the same voltage on both sides sometimes bugs get between the contacts and won't pull all the way in.

If when you push it in it starts you have either a bad transformer or tstat but it's very hard to diagnose a problem with out seeing with your own eyes. Good luck hope you get her fixed up
 
Update. Pushed the contatactor in and the unit ran fine. Found a bad splice in the tranformer 24v wire to the compressor.
Trouble shooting 101 i guess.
Sometimes i over think shit waaay too much.
 
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