Any HVAC brothers (or sisters) out there?

[earnold25]

Any brothers (or sisters) out there who can help me with an HVAC problem / question?

1st problem has to do with my outdoor AC condenser blowing the fuse on my furnace's control board (not my circuit breaker) every time I try to turn it on.

2nd problem and much less important is my new digital thermostat isn't receiving power from the furnace (only its internal batteries).

Thanks!

 

[JRL]

The fuse should be a 3 amp right? You might have a short in the two wire running out to the condensing unit. Also if you just noticed this after you hooked up you new digital thermostate you might not have that wired right. You will not receive power from the furnace as long as that fuse is blown. I blew a fuse yesterday at work when I shorted out the two wire running from the control board to the outdoor unit. It is hard to say without looking at it but maybe that will give you a start on what to look for. Let me know if you have any other questions.

 

[earnold25]

correct, 3 amp fuse. automobile style.

and that was my next guess, perhaps an animal chewed through that wire running to the condensing unit? how would I test that?

and I changed the thermostat after this issue started, because I'm an idiot and thought my thermostat might have gone bad. I'm about 95% sure its hooked up correct, as I can start the heat and blower from it. I just believe I need a 5th wire to receive power from the furnace to the thermostat (a common to complete the circuit from the thermostat hot?) When I try turning on the A/C from the thermostat, the fuse blows. Could I bypass the thermostat by jumping the hot portion of that two wire running the the compresser to the Red wire terminal on the board?

 

[JRL]

The best way to check the two wire is to check for continuity to see if it is shorting out on itself. Did you look at the wire at the condensing unit itself? Does it look chewed up? I have also seen it short out right where it comes out of the house to the unit. It might be noticiable and might not. You should only need four wires for your thermostate. Unless you are running additional devices. You can bypass the thermostate on the cuircut board and also with the wires coming out of the wall behind the thermostate. But if you keep blowing fuses then you might just have a short in the two wire. Hard to say without being there and testing it with a meter. I would not just start jumping wires on the board.

 

[earnold25]

well part of the problem is the wire runs under a deck, most of which I can't see too well. I can check the wire where it basically reaches the compresser (it should terminate near the breaker on the compresser itself, right)

And how exactly would I check to see if its shorting out on itself? I have a multimeter, but not sure what to do.

 

[JRL]

K well the two wire should run into the condensing unit itself. There should be a cover you can remove that will expose the capacitor, the contactor and all that fun stuff. You would need to unhook that two wire from the condensing unit. Should be a white and a red. and from where it leaves the control board in the furnace. Make sure the power is off and that the wires are not touching. On your meter there should be a symbol that looks kinda like dot with some half cirlces around it. If you have the right setting when you touch your probes together you should get a beep and when you don't touch them together it should stop. So if you touch your two wire with the probes one on red and one on white, and you get a beep you have a short. Make sure that neither of the ends are connected. Does that make sense. Things are kinda hard to explain via typing. I will check back later I have to get to work. Good luck hope it helped a little.

 

[earnold25]

I understand. I'll give that a try when I get home as well. Thanks again. Hopefully its nothing more than a short.

 

[Gummi Bear]

Did you hook up the Common wire to the T-stat when you installed it? (it's usually blue)

Many t-stats don't need it, they're just a switch, and there's no need for a common.

Check on this, and let me know.

 

[earnold25]

no, the blue was unused on my old thermostat. on the new thermostat, the red / power wire slot is jumpered to the common slot.

 

[Gummi Bear]

'typical' residential t-stat wiring:

Red - Power
White - Heat
Green - Fan
Blue - Common
Yellow - Cooling

An 8 wire heat pump is slightly different, and I can't remember the color codes for it, it's been too many years since I hooked one up.

Anyway - I hope this helps.

The other thing I'd check - is the coil on the contactor. If it shorts out, or takes a poo, it'll cause mayhem like this. Disconnect the control wires, and turn off the breaker before testing the continuity of the coil.

Also - visually inspect the coil. See if a bug crawled into the contacts. This is a common thing to have happen. BE careful, remember - the capacitor still has potential electrical energy, and will probably make you pee your pants if you ground it out with your finger.:rolling:

Good luck

 

[earnold25]

thanks. it's not a heat pump, and I matched all those wires to the correct color in the furnace as well, so it appears to be hooked up correctly.

so the contactor coil would be in the box that house the circuit breaker that's attached to the condenser (basically the outdoor box near the condenser, not inside the furnace?)

Is there a way to fully discharge the capacitor before I go near it? I'm awfully clumsy, haha

 

[JRL]

The contactor should be in the panel that you take off on the condensing unit itself. At least that is how I have seen all of them. Have you had any luck with that damn thing yet??

 

[Gummi Bear]

On the condensing unit, you'll pull the panel above where the power feed from the house enters.

The contactor looks like this:

They're relatively inexpensive, and often times the source of trouble. They live in a harsh environment, and sometimes they just poop out.

 

[earnold25]

The contactor should be in the panel that you take off on the condensing unit itself. At least that is how I have seen all of them. Have you had any luck with that damn thing yet??

well yesterday I did a continuity check to see if the contactor control wires (running from my thermostat / furnace to the compressor) might have had a short (from an animal chewing, etc.) I did this from the furnace side and left them connected to the contactor. Continuity check passed, so I believe there are no shorts there.

Next someone told me to take the two wires off the contactor and then call for A/C and see if the fuse (on the furnace control board) pops again. If it doesn't, its likely the contactor. Is this what you were telling me Gummi?

And can I get a contactor at Home Depot or Lowe's? Basically I just have to find a 24v one, or does it have to be specific to my model compressor?

 

[JRL]

Any general contactor should work. 24 V control with 240 passing through it. If you can get maybe a model number but it should be general. I don't know if you can get it at home depot of not.

 

[earnold25]

thanks. i looked at home depot and lowe's webpage and they don't sell contactors. I'd have to find an HVAC store around here. (which I'm sure will be closed on the weekend!)

I'll see if i can get a model number for ya jeeper. Of course the control panel is nearly inaccessible and i'm gonna be crawling halfway under my deck. I pity you HVAC folks who do this day in and day out.

 

[earnold25]

Jeeper, thanks so much for sending me the contactor. unfortunately it looks like i have further troubles.

I replaced the contactor, fuse still blows on the furnace's board when I call for AC. When I press the button on the contactor, the compressor starts up, and the copper pipe running to the furnace is cold to the touch.

Any other ideas?

 

[Boppa-Wasch]

Good luck!

 

[JRL]

Jeeper, thanks so much for sending me the contactor. unfortunately it looks like i have further troubles.

I replaced the contactor, fuse still blows on the furnace's board when I call for AC. When I press the button on the contactor, the compressor starts up, and the copper pipe running to the furnace is cold to the touch.

Any other ideas?

Well you know it is not anything to do with the compressor or anything with that. Sounds like it is in the 24v side. Thermostate, transformer, lowside of your control board. Low voltage wiring. It is hard to say without trouble shooting everything. Dang I hoped that it was just the contactor. You can back track from the contactor and see if it still blows. Its hard to say without seeing it.

 

[earnold25]

you and me both!

could i test the voltage going to the contactor with a multimeter to see if its reading 24v? If it's not reading right around 24, is that an idicator that the transformer isn't putting out enough juice? (even though I can call the blower and heat?)

thanks!