Folks,

The Thermostat for my upstairs AC went blank.
I found this web site and found some very useful tips.

Here's what I found so far by following the tips on this message board:

1. I looked for the 24 Volts and there was no voltage.
2. The thermostat is a simple one (not programmable) and has not battery.
3. Then I found that the 3 Amp. Fuse (like an auto fuse) in the furnace / AC unit was blown.
4. I replaced the fuse and turned the power to the furnace and AC back on and was thrilled to see that the thermostat display came back on.
5. However, when I came back from work, the display was blank again.

I'm assuming that the fuse must have blown again when the AC kicked in (the AC did not kick in when I replaced the fuse in the morning since it was cooler that the set temperature).

What should I do next ?

I have a new home (only 1 1/2 years old). The AC has a 5 year warranty. However, the labor is only warranted for the 1st year (my luck!! ).

You are first going to have to find out why you are blowing fuses. It is caused by a short in the low voltage side. Are the 2 small wires from the furnace to the outside unit shorted? Do you have a shoted wire at the stat? Check all the low voltage wires. Could also be the coil in the contactor at outside unit is bad, but if this has just started since changing the stat I would suspect problem in that area.

Thank you esquire1.

I think I was not clear about the Thermostat. I did not change the Thermostat. As suggested by you, I will check if the two wires to the outside unit are shorted. How do I identify these wires though ? Is there a standard color code ?

Also, as I mentioned in my previous post, the equipment itself is warranted for 5 years. I'm past the 1 year warranty for the labor though. Not sure how far I should meddle with the system before I call service.

There will only be 2 low voltage wires leaving the air handler going to the condensing unit. Could be any color.(probably inside brown insulated covering)
My thoughts are those wires or the coil on the contactor inside the contactor. As long as there is no call for cool everything will be fine. Once there is a call from the stat it blows. Good luck

Thanks for your continued help, Doug.
You were right on the money about the short.
The two wires (inside a brown insulated covering) showed a short from the furnace end.
I opened the condensing unit and unplugged the wire at the condensing unit.
The short was not there anymore from the furnace end. That tells me that the wire is fine.
Then I checked between the two points at the condensing unit to which these wires were attached and sure enough, there is was short at the unit.

So it looks like the unit to which the wires are attached is bad.
What is this unit called (looks like a small transformer) ?
How much would it typically cost ?
Since my equipment is still under warranty (5 years), does it make sense to try and replace this unit or should I just call for service and bear the labor costs (since my labor warranty ran out after 1 year).

I guess I'll also void my warranty if I try to replace any parts on my own.

SLM, chech with the installing contractor, if this is a Goodman unit , I believe they have a eight year warranty on all parts, you may have to pay labor charges though, Carrier offers a five year warranty on all parts. The coil on the contactor is what is probably burnt out, this is where the wires from the thermostat connect. Be sure to disconnect power first . Mike

SLM, chech with the installing contractor, if this is a Goodman unit , I believe they have a eight year warranty on all parts, you may have to pay labor charges tho, Carrier offers a five year warranty on all parts. The coil on the contactor is what is probably burnt out, this is where the wires from the thermostat connect. Be sure to disconnect power first . Mike

Goodman now offers 10 year parts and one year labor, some of the newer stuff has lifetime heat exchangers.

Thanks for your continued help, Doug.
You were right on the money about the short.
The two wires (inside a brown insulated covering) showed a short from the furnace end.
I opened the condensing unit and unplugged the wire at the condensing unit.
The short was not there anymore from the furnace end. That tells me that the wire is fine.
Then I checked between the two points at the condensing unit to which these wires were attached and sure enough, there is was short at the unit.

So it looks like the unit to which the wires are attached is bad.
What is this unit called (looks like a small transformer) ?
How much would it typically cost ?
Since my equipment is still under warranty (5 years), does it make sense to try and replace this unit or should I just call for service I guess I'll also void my warranty if I try to replace any parts on my own.
And bear the labor costs (since my labor warranty ran out after 1 year).

It is called a contactor. It is something you can do yourself. Contactor itself doesn't cost much Less than $15. Can be picked up from HVAC house. If you change it yourself, disconnect the power to unit first tehn just changer it wire for wire to avoid confusion. Labor would cost much more than replacing yourself.

Thanks for your continued help, Doug.
You were right on the money about the short.
The two wires (inside a brown insulated covering) showed a short from the furnace end.
I opened the condensing unit and unplugged the wire at the condensing unit.
The short was not there anymore from the furnace end. That tells me that the wire is fine.
Then I checked between the two points at the condensing unit to which these wires were attached and sure enough, there is was short at the unit.

So it looks like the unit to which the wires are attached is bad.
What is this unit called (looks like a small transformer) ?
How much would it typically cost ?
Since my equipment is still under warranty (5 years), does it make sense to try and replace this unit or should I just call for service I guess I'll also void my warranty if I try to replace any parts on my own.
And bear the labor costs (since my labor warranty ran out after 1 year).

It is called a contactor. It is something you can do yourself. Contactor itself doesn't cost much Less than $15. Can be picked up from HVAC house. If you change it yourself, disconnect the power to unit first then just changer it wire for wire to avoid confusion. Labor would cost much more than replacing yourself.

Thank you Doug and all for all your help so far.
I am a little late with this post since I was shopping and trying to locate a contactor.
Turns out it is a 1 pole contactor; GE part # CR453CB3HBAFG B13603-21.
I could not find the original GE part. Instead, I found and bought a generic one.
There are some differences in the specifications though and I'm giving them below.

Original from GE: Generic from Square D:

VAC: 240/277 VAC: 277
FLA: 25 FLA: 30
LRA: 150 LRA: 150
RES: 35 RES: 40

Do these differences matter ?
The HVAC Shop gentleman that sold it to me said I that most contractors buy the generic contactor and are able to use it with some wiring changes. He couldn't tell what change though and couldn't be sure.

What kind of wiring changes do I need to do ? From what I can see, I can use it as is.
The terminals look the same to me; the two terminals connected to the coil for the wires coming from the furnace, two terminals for the 240 volts input and the two terminals that go to the Condenser unit and the big capacitor in parallel (connected with black and red wires).

Can you please tell me if the slight difference in specifications matter ?
Can I replace it wire for wire or will I need to make any changes ?

Thanks for your help in advance,
slm.

It's fine. Just change it wire for wire and your're set to go. Good Luck

It's fine. Change it wire for wire. Good Luck

And I might add one thing , since your old contactor was a single pole , is your new one a single pole or two pole? if it is a two pole you need to put a jumper across one pole to provide for the heating of the crankcase in the compressor. If this is overlooked it could shorten the life of the compressor.

And i might add one thing , since your old contactor was a single pole , is your new one a single pole or two pole?, if it is a two pole you need to put a jumper across one pole to provide for the heating of the crankcase in the compressor. If this is overlooked it could shorten the life of the compressor.

Thanks Mike.
The first shop did not have a 1 pole contactor. It is this shopkeeper though that explained to me that the clicking spring on the contactor is counted as a pole. Mine had only 1 pole while he had only contactors with 2 of poles. He pointed me to another shop and I was able to get a 1 pole contactor in that shop. So I think I am good if I do a wire for wire replace.

I'll let the forum know what I find out tomorrow when I do this.

Thanks again for all your help.
slm.

I replaced the contactor wire for wire with the generic one that I bought and my AC started working like a charm!! It is a great feeling of achievement to fix the AC myself and I owe it to all of you who helped me on this forum. I would have got no further than locating a blown fuse had it not been for the help from this forum. This is the best and the friendliest forum that I have come across and I am glad that I found this group.

For the benefit of other looking for information, I wanted to mention that the resistance across the coil on the contactor should be measured using a digital multimeter. An analog multimeter (with 1K range scale) shows a short across the coil of for a new contactor too since the resistance across the coil is very small (only 10 ohms in my case).

Thanks again,
slm.