[95ponymobile]

My unit was installed 2 weeks ago and hasn't worked correctly since. If the A/C or fan is turned on the board fuse blows. Just installed new thermostat wire from thermostat to unit since my installer was confident that's what it was. No good. With green wire from thermostat disconnected at unit fuse doesn't blow. They have already put a board in the unit. That was a guess I'm sure now. Any ideas?

 

[Doc Holliday]

There's a short to ground somewhere.

 

[95ponymobile]

That's usually why fuses blow. Any ideas what may be shorted is were Im going. All wiring I can see looks good. Could it be a bad component...if so what would be a good place to start?

 

[old_squid]

Sounds like you need a new installer if they can't figure out something as simple as why a low voltage circuit is blowing fuses. There is a short somewhere (more than likely) and the only way to find it is to isolate all the different paths (with the power OFF) and ohm out each until the one is found that has a path to ground.

The only other way I can think of that would cause this is if the AC has it's own low voltage power source. That is if you have a split system with an AC and furnace. If it's a package unit (all in one outside) then this shouldn't be the problem.

 

[Doc Holliday]

Good luck.

 

[Doc Holliday]

Sounds like you need a new installer if they can't figure out something as simple as why a low voltage circuit is blowing fuses. There is a short somewhere (more than likely) and the only way to find it is to isolate all the different paths (with the power OFF) and ohm out each until the one is found that has a path to ground.

The only other way I can think of that would cause this is if the AC has it's own low voltage power source. That is if you have a split system with an AC and furnace. If it's a package unit (all in one outside) then this shouldn't be the problem.

Green wire, fan, inside unit, not concerning the a/c at all.

 

[beenthere]

Could be a short in the thermostat.

Jump R to G at the board. if the fuse doesn't blow its probably your thermostat.

 

[95ponymobile]

If the thermostat was shorted it would still blow with the green wire at unit disconnected.

 

[95ponymobile]

Sounds like you need a new installer if they can't figure out something as simple as why a low voltage circuit is blowing fuses. There is a short somewhere (more than likely) and the only way to find it is to isolate all the different paths (with the power OFF) and ohm out each until the one is found that has a path to ground.

The only other way I can think of that would cause this is if the AC has it's own low voltage power source. That is if you have a split system with an AC and furnace. If it's a package unit (all in one outside) then this shouldn't be the problem.

They suck. Its a package unit. If the green is a input to the board where is the output?

 

[old_squid]

Just a wild guess here, but someone didn't think G on the low voltage terminals on the outdoor unit meant ground/common did they? :whistling2: Cause it doesn't. :wink:

With it being a package unit, the short is in the fan circuit (G) somewhere is a guess from a 1000 miles away.

 

[rootboy]

They suck. Its a package unit. If the green is a input to the board where is the output?

Do us a favor and take a picture of your thermostat connections, your unit's connections, and give us a link to a manual for the HVAC unit and the thermostat showing how they are supposed to be wired up.

Essentially your problem is that there is a low impedance path between one side of your transformer (presumably the red wire) and the other side of the transformer (connected to your green wire).

Now assuming that the green wire isn't hooked up to the other side of the transformer (if it is, then there's your short), proceed with the following instructions.

Do you have a voltmeter? With the green wire disconnected at both ends, and the unit shut off (insert safety blather here, just don't kill yourself), measure the resistance from each of the low voltage terminals to ground. If the resistance is low, say 0 - 50 ohms, then your control transformer is bonded to ground. This means that grounding out one of your thermostat wires (other than the white wire) will blow a fuse. Otherwise it won't.

It's a fifty-fifty chance that your unit does not have a grounded secondary (the 24 VAC side) and if it doesn't, then even if the green wire was "grounded" it wouldn't short out the transformer.

If it is grounded, and only if it is grounded, then keep the unit turned off, and measure the resistance of the green wire to the frame of your unit (which will be common to the transformer if it is shorted to ground) and measure the resistance. If it is in the tens of ohms (or lower), then it is indeed shorted. If you are lucky, you will have a spare wire that you can use instead of your green wire.

Now if your transformer isn't grounded to the frame of the HVAC, or if your resistance reading is well above 0 - 50 ohms (as in the megohms), then the "short" is either in the thermostat where the red wire and the green wires are connected, or the short is in the HVAC unit on the wire that the green wire hooks up to (presumably the fan).

The chances of it being shorted in the thermostat are slim, so I would start with the unit itself, so turn off the HVAC unit, connect the fan wire and the red together at the unit (just put a short jumper between the red and green terminals) leaving the green wire disconnected at both ends.

Now turn the unit on. If the short is in the fan relay, then you will blow yet another fuse, if not (and the HVAC fan runs when you turn the unit back on), then the problem is in the thermostat.

 

[beenthere]

If the thermostat was shorted it would still blow with the green wire at unit disconnected.

Not if its a resistive short.

 

[NiNe O]

disconnect everything on control board and start plugging stuff back in one by one until the fuse pops. Now you know what circuit, unplug everything in that circuit. Then, start pugging stuff back in one connection at a time until a fuse pops, thats your culprit. I had one yesterday on an old low limit for an AHU, the short was through the bakelite. from contact to casing. New one for me.

 

[95ponymobile]

Update. I removed the unit panels this morning and started checking after studying the wiring last night. Found the problem in 5 minutes. Green wire from board to economizer plug in blower housing shorted to ground somewhere in between. Since it's not used I just disconnected it from the board and taped. Found it with my DVOM.set to ohms. Probed the green thermostat wire and touched to ground. 0.6 ohms. Checked green wide leading to eco plug and also had continuity to ground. Unplugged from board and checked gr thermostat wire again. No continuity. Powered up unit turned fan on fixed. Maybe they should give me a job lol. I could be a auto tech and hvac tech lol. Thanks for help and good suggestions. :thumbup: