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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22519
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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My heat pump will not come on. Outside unit nor the attic unit.

This answer was rated:

My heat pump will not come on. Outside unit nor the attic unit. I crossed the G wire and the R wire together and the attic fan doesnt come on either.

was it running recently?


Customer: replied 4 years ago.

It was running about 1 month ago. We turned it off and it wont come back on

Thank you,

First thing is that this is a sign of no power to the thermostat.

before proceeding -

do you have a volt meter
do you have access to the air handler in attic


Customer: replied 4 years ago.

Yes I have both


If you follow the thermostat wires into the air handler....they will attach like they do at stat...R,G,Y etc....

we need to set meter for ac volts...and see if R to C (R to cabinet metal works also) has 24v.

Then we can pick a direction.

Customer: replied 4 years ago.

I am unclear. Are you saying to test the R wire to the C wire or are you saying to test the R wire to the metal housing of the air handler to measure for 24 v? Also just so you know the thermostat on the wall does not use C wire. I was told this is only for power to the thermostat and mine runes off battery power only.

C is a common terminal inside the air handler. It is like a neutral to 120v.

The metal cabinet or C terminal fill the same role for the test we want to do.

So in short...find the R terminal inside the air handler...and a C terminal OR just the cabinet.

Customer: replied 4 years ago.

continuity is all I get. No voltage


do we have main power to the unit? Is power cut off by a door safety switch when door removed?

If you have make/model of air handler I can assist further

Customer: replied 4 years ago.

power is to the unit and it is a Goodman Manufaturing Corp Model no. a24-10

Thank you.

Are you familiar with what the transformer is? and doo you see a yellow blade fuse on a circuit bord?

Customer: replied 4 years ago.

I do know where the transformer is. Also I do not see a blade fuse anywhere.

We need to see if the transformer has an output of 24v.

Then we caan see if transformer is bad....which is generally caused by a short in wiring outside.

Customer: replied 4 years ago.

Ok. How can we do that

On one side of transformer you have 209/230v supply.

The other side is the low voltage output of 24v which gets sent to the thermostat.

Now we want to check the 2 low voltage wires to see if 24v is actully coming out of the transformer.

Customer: replied 4 years ago.

So just touch the volt meter to both sides of the transformer correct? And I should have a 24v reading?

that is correct..make sure you are on the 24v side
Customer: replied 4 years ago.

Ok I checked the 24v side and there is no voltage coming out.


now there is one last you have 220v supply to the transformer?

Customer: replied 4 years ago.

yes I had also checked the other side while i was up there and I did get a reading for 220

Ok, then you have a bad 208/230v to 24v transformer.

Something either went wrong with the transformer or there is a short that took out the transformer.

Once the transformer is replaced and wiring all checked for damaage you should be back to normal.


Customer: replied 4 years ago.

OK. I was thinking thats what you were going to say lol.. I will replace it tomorrow once the supply house opens. Thank You for your help on this issue.

You are very welcome. I am available for 30 days after answer is rated so feel to return for follow up


BillyHvac and 3 other HVAC Specialists are ready to help you
Customer: replied 4 years ago.
Greetings Billy, I am still having issues with the A/C. I replaced the transformer. The fan did come on. After the fan in the air handler came on and I ran it for about 10 min. I turned the A/C to cool. I went outside and noticed the outside fan was not turning. After about 10 min in cool mode I noticed the air handler fan stopped working. I went back in the attic and checked the transformer again and it had the 220 volts coming in but it was not measuring any voltage on the 24 volt side. I also smelled burning smell from the transformer. It was very hot to touch. So apparently I blew the new transformer. Is there something I can check to see why it is blowing?
Have you inspected all the thermostat wire from the indoor unit to the outdoor unit?

Any scrapes, nicks or pressed against metal?

Also we want to put a cheap blade fuse in the wire from transformer (24v) this will save transformers and instead blow the fuse.

we can make one like this with a blade fuse

This can be the outdoor contactor or the reversing valve also.

You can also buy a tranformer that has a built in trip breaker switch...this one only needs to be reset instead of replaced.

Customer: replied 4 years ago.
I did not notice any nicks in the wire however I will go back and do a thorough check on all wires. Would there be anything else I can check on tonight since the parts store is closed? If I wanted to make the blade fuse, what particular fuse rating should I use? Tomorrow I will check on the transformer with built in breaker when the store opens.
Customer: replied 4 years ago.
ok Great! I will inspect all wires now. And tomorrow I will either place an inline blade fuse or the transformer with built in switch. If I do not see any nicks in the wire is there anything else I can check on tonight?
Do you know how the contactor pulls down when energized?

Customer: replied 4 years ago.
No I do not know this part
The contactor is where the wires end up at from the is in the outdoor unit and is responsible for closing and turning on the 220v compressor and motor...but it is the 24v from the Y terminal that closes this switch.

If the wires all look good then we have to look at the contactor.

Customer: replied 4 years ago.
OK I will go outside and check this out.
Please be aware that if the disconnect is on at the outdoor unit there is 220v present

Customer: replied 4 years ago.
I looked outside and the wires look good in the condenser unit. I also pressed down on the contactor with the power on and the fan motor did start. Can I assume there is a short in the Y? or perhaps another possible issue?
The contactor could have a bad coil (the part that cause magnetism)

This is a rudimentery test....but you can turn on indoor blower (if transformer good ) and push the contactor in and wedge it closed. This provides cooling BUT it will not shut off.

did you say it does not blow in heat and this is a heat pump?

Customer: replied 4 years ago.
Well I cannot turn on the blower till tomorrow since I have blown the transformer and need to replace it. I have not tested the heat. If the contactor is bad, is it a possibility that is what caused the transformer in the air handler to blow?
That is correct.

we have only 2 possibilities IF the wiring is all good and no bare spots...that would be the contactor or reverse valve.

The reverse valve vale only works in if you ran heat ok...but it blows in cooling, this would lead me to the reverse valve. This would be found following the O wire from thermostat to outdoor unit.

Customer: replied 4 years ago.
OK. I will inspect all the wire tonight, replace the blown transformer tomorrow with either installing a transformer with a breaker or installing an inline 3 or 5 amp inline fuse. Once all that is in place I will try the heat first, then I will try cool. If the transformer blows in cool I will check the reverse valve.
And remeber I am available for that 30 day time so feel free to return as needed.


Customer: replied 4 years ago.
ok thank you for all you have done!

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