Heat Pumps and Electric Heating - No AC from heat pump

I have no cool air blowing from my heat pump to thermostat is set to cool. The indoor blower works fine. So does heat. If I jumber the R and O wire at the stat, where do I test for 24vac voltage at the defrost board low voltage terminal strip in the outdoor condenser? Across R and O, O and Common or O and ground ?

The convention for most tstats is O is your reversing valve with C your common/neutral. So check O and Common. First check that at the tstat and then at the outdoor unit.

Let us know what you find.

Nearly all reversing valve are energized for cooling. This insures that should the valve solenoid coil fail you will at least have heat.

Rheem / Ruud are backward, they want to make sure you say cool in the dead of winter.

Go right to the valve. Kill power to the outdoor unit and put tstat in cooling. This should pull in contactor and energize rev valve, without the comp and fan having power to run.

Pull the connector plug off of the valve solenoid and read voltage. 24 volts all is good to that point. Switch to ohms and read the actual coil. Open means coil is bad.

Nearly all reversing valve are energized for cooling. This insures that should the valve solenoid coil fail you will at least have heat.

Rheem / Ruud are backward, they want to make sure you say cool in the dead of winter.

Go right to the valve. Kill power to the outdoor unit and put tstat in cooling. This should pull in contactor and energize rev valve, without the comp and fan having power to run.

Pull the connector plug off of the valve solenoid and read voltage. 24 volts all is good to that point. Switch to ohms and read the actual coil. Open means coil is bad.


I have 24 volts from O to Common and Y to Common at low voltage strip at defrost board. 24 volts coming out of defrost board at O - out and Y - out. 24 volts at the end of wire that plugs into reversing valve solenoid. Reversing valve solenoid reads 17.8 ohms. I can hear the contactor and solenoid valve click when I jumper R to O and R to Y at thermostat with disconnect pulled at condenser. Two seconds after I put in the disconnect ,the outdoor unit stops and the low voltage fuse trips on the transformer on the air handler in the attic .

I have 24 volts from O to Common and Y to Common at low voltage strip at defrost board. 24 volts coming out of defrost board at O - out and Y - out. 24 volts at the end of wire that plugs into reversing valve solenoid. Reversing valve solenoid reads 17.8 ohms. I can hear the contactor and solenoid valve click when I jumper R to O and R to Y at thermostat with disconnect pulled at condenser. Two seconds after I put in the disconnect ,the outdoor unit stops and the low voltage fuse trips on the transformer on the air handler in the attic .



Anyone know where I should go from here?

Thinking the ohms for the rev coil is very low. Maybe drwaing too much current when holding under load. Or there is another short when power is on.

If you can do this safely... Try unplugging coil. Start unit in a/c if fuse doesn't pop, reach in and plug in coil. Popped fuse, bad coil.

After reading his post several times, and the fact he says he gets the click in the reversing valve solenoid, with the disconnect pulled, and the fuse does not blow then. But then only after he plugs in the disconnect, to naturally activate the contactor, does he then have things trip, including the low voltage fuse. But not until he plugs in that disconnect. Therefore, I think he may have a short in the contactor solenoid which also backfeeds some of the 120 current back up the low voltage wire back to the low voltage fuse -maybe. Something close to that anyway.

Agree EC. Could be the contactor.

Where I'm heading is that I think the rev coil should be over 200 ohms. I looked for a spare in the garage but could not find. Maybe in the shed but raining.

I remember troubleshooting a veg steamer and the factory guy said it was a coil. Anything less tham 400 ohms (in this case - 120v coil) was bad. Change it.

Agree EC. Could be the contactor.

Where I'm heading is that I think the rev coil should be over 200 ohms. I looked for a spare in the garage but could not find. Maybe in the shed but raining.

I remember troubleshooting a veg steamer and the factory guy said it was a coil. Anything less tham 400 ohms (in this case - 120v coil) was bad. Change it.

Thanks for the replies. The contactor is brand new. Well almost. I changed it about 9 months ago. The old one was over 9 years old and the contacts were worn down to bare steel. The new contactor ohms out at 11.2.

From what I've read online, the solenoid for the reversing valve should be between 10-60 ohms.

Agree EC. Could be the contactor.

Where I'm heading is that I think the rev coil should be over 200 ohms. I looked for a spare in the garage but could not find. Maybe in the shed but raining.

I remember troubleshooting a veg steamer and the factory guy said it was a coil. Anything less tham 400 ohms (in this case - 120v coil) was bad. Change it.

I know what you mean about the ohms. That number is as low as some heating elements get.

But sometimes one can be surprised. Just like the ignitor I found that worked perfectly good at 178 ohms this winter - which is a lot higher than normal - and the duplex neighbor's identical one was only 34 ohms.

The fact sbaraik said it clicked(without blowing the fuse), got me thinking that it just may be working, regardless of the number. Did you happen to read that thread on the A/C forum, where the guy had his reversing valve misdiagnosed, and found out what the real cause was? Read that one, unless you have already.

Thinking the ohms for the rev coil is very low. Maybe drwaing too much current when holding under load. Or there is another short when power is on.

If you can do this safely... Try unplugging coil. Start unit in a/c if fuse doesn't pop, reach in and plug in coil. Popped fuse, bad coil.

I'm not that brave especially since the fan has to be removed and sitting on top of a metal garbage in order for me to gain access to the coil. I suppose I could disconnect the fan.

What about plugging in the coil where it connects to the defrost board?

Thanks for the replies. The contactor is brand new. Well almost. I changed it about 9 months ago. The old one was over 9 years old and the contacts were worn down to bare steel. The new contactor ohms out at 11.2.

From what I've read online, the solenoid for the reversing valve should be between 10-60 ohms.

Hmmm. You posted as I posted. I can't recall, as it has been years, what voltage drives the reversing valve, as this is most important to know now: 24 volts?, or does 24 volts switch closed 120 volts that actually drives it? Jarredsdad? Anyone?

Need to know this based on the fact that your reversing valve clicks, yet does not blow the fuse when the disconnect is out, but does it when the higher voltage disconnect is plugged back in. This behavior is an important key in one trying to come up with logical answers.

Hmmm. You posted as I posted. I can't recall, as it has been years, what voltage drives the reversing valve, as this is most important to know now: 24 volts?, or does 24 volts switch closed 120 volts that actually drives it? Jarredsdad? Anyone?

Need to know this based on the fact that your reversing valve clicks, yet does not blow the fuse when the disconnect is out, but does it when the higher voltage disconnect is plugged back in. This behavior is an important key in one trying to come up with logical answers.

It is 24 vac. Also, I may not have mentioned this previously, but the system works fine in heating mode.

It is 24 vac. Also, I may not have mentioned this previously, but the system works fine in heating mode.

Unless it is a Rheem or Ruud your reversing valve isn't actuated in heating mode, just cooling mode. So that explains why you're good in heating mode, but doesn't get us much closer to the problem since we already suspect it is near the reversing valve.

Unless it is a Rheem or Ruud your reversing valve isn't actuated in heating mode, just cooling mode. So that explains why you're good in heating mode, but doesn't get us much closer to the problem since we already suspect it is near the reversing valve.


Correct. But doesn't this rule out a bad contactor?

Okay. I got your answer; 24 volts.

This is where I was headed with my logic. Since it actuates on 24 volts and you say it clicks even with the disconnect unplugged - but the 24 volt fuse only trips out when the disconnect is in - well, the high voltage disconnect does nothing as far as that reversing valve goes.

That is why I thought it could be something else. I took a guess at the contactor since that energizes on that higher voltage when the disconnect is plugged back in. And also the 24 volts is at the contactor as well.

But if the heat works, that means the contactor is working, if your heat is coming from the compressor and not from auxilary electric strip.

If you run it in heat mode, is your compressor running?

You could try to bypass thermostat cooling, by jumping R to Y and see what happens then. (But I am not entirely sure about this, on a heat pump.) Others may weigh in. I'm trying to figure all that can be different by simply having it in cooling mode.

We know, for one thing, the stat switches to different contacts. Is this the same stat you have had?, and for how long has it operated okay in the past in cooling mode? And we know the reversing valve has to change. And we know that the blower speed gets faster (but this probably is not any source of the blown fuse since you say the blower still works in cooling mode.) I can't remember if you said if the compressor and fan out there come on for the few seconds before the fuse blows or not. Can't reread posts at the moment as I have to go do a rental maintenace job. I'll be back shortly.

How many low voltage wires go to that reversing valve?

Ec, everything is going to be 2 wire 24 volts.

Yes, disconnect rev at the board.

Since we have not discussed the board, do two things.

Disconnect the rev wires and disconnect the "O" tstat wire.

Energize system in cooling. If the fuse blows the stat is now in question.

No blown fuse unit is actually in heat mode. While it's running, reconnect tstat wire to "O". Blown fuse brings the board to the front.

No blown fuse, reconnect rev wires to board. Blown fuse means the valve coil.

If no blown fuse it's time to look for a short in a wire somewhere.

Ec, everything is going to be 2 wire 24 volts.

Yes, disconnect rev at the board.

Since we have not discussed the board, do two things.

Disconnect the rev wires and disconnect the "O" tstat wire.

Energize system in cooling. If the fuse blows the stat is now in question.

No blown fuse unit is actually in heat mode. While it's running, reconnect tstat wire to "O". Blown fuse brings the board to the front.

No blown fuse, reconnect rev wires to board. Blown fuse means the valve coil.

If no blown fuse it's time to look for a short in a wire somewhere.


**UPDATE***

Still not fixed. We had a cold spell here in MD the past week so the urgency in finding a solution has wained a bit. This is what I did on Monday (63 deg for the high temp for the day)when I was off from work:


Removed thermostat from wall. Jumpered R to O, R to G and R to Y. AC ran fine for 20 minutes. I disconnected the jumpers and left it alone for a few days. This test was in the morning and and no sun had been shining on the condenser unit.


Last night when I got home from work around 7pm, I connected R to O using a 2 amp fused jumper. I went through about 4 fuses. The fluse blew everytime I made the connection between R an O. The sun had been shining on outside unit all day and outside temp was 75 deg.

This morning around 8am and about 65 deg outside I jumpered R to O. Fuse did not blow and I heard the solenoid click. Jumpered R to G and R to Y. Heard contactor click but condenser did not start. All 3 circuits had a 2amp fuse installed and none of teh fuses blew.

[QUOTE=sbaraik;1568242This morning around 8am and about 65 deg outside I jumpered R to O. Fuse did not blow and I heard the solenoid click. Jumpered R to G and R to Y. Heard contactor click but condenser did not start. All 3 circuits had a 2amp fuse installed and none of teh fuses blew.[/QUOTE]

You did not try to test for volts then, out by the contactor?

What kind of fuses are you using? If they are not slo blos, the inrush current of the magnetic coil on the RV could be blowing the fuses.

What kind of fuses are you using? If they are not slo blos, the inrush current of the magnetic coil on the RV could be blowing the fuses.

AGC 5mmx20mm fast acting 250v. Should I use a slow blow 3amp? Is the 3.5 amp on the transformer fast or slow blow. I thought slow blow were only used on circuits that contained motors? I didn't think the low voltage circuits carried more than about 1 amp.

You did not try to test for volts then, out by the contactor?

Correct. It will have to wait until the weekend.