AC trying to start up with furnace running


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Old 10-05-09, 03:18 PM
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AC trying to start up with furnace running

The entire time the power switch to the furnace is "on" the AC keeps trying to start up. I have the thermostat set a 62. Even after the house has reached the set temp and the fan has gone off the AC is still trying to kick in. It sounds terrible, like it's shorting out.

I only ran the AC once for about 4 hours this summer - other than that the power switch has been in the "off" position since spring.

I have a 7 year old Amana high effiency furnace and AC.

I thought it might be the circuit board because I was having problems with the fan not turning off (I posted about that yesterday) but today the fan worked fine (???).

I checked the thermostat wires - of what I can see everything seems to look as it should.

Any thoughts.
 
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Old 10-05-09, 07:18 PM
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A/C & Furnace

Is the outdoor unit coming on? Is your thermostat manual or automatic changeover from heat to cool?
 
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Old 10-05-09, 07:36 PM
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Now the lights are dimming when AC tries to come on.

The AC is not able to fully come on. It sounds like the compressor is struggling.

I turned the furnace back on just now and now all the lights are flickering badly and because of the AC trying to come on.

I think there's something major going on - kind of scarey!

The thermostat contols AC or Furnance just by the temperature settings. Presently I have the settings at 62 and 80 (the max). So, I guess that makes it automatic.
 

Last edited by themadcatter; 10-05-09 at 08:16 PM.
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Old 10-06-09, 03:54 AM
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Heat & A/C

The first thing I'd do would be to remove the thermostat & connect the wires from terminals R & W together. If you still get the heat & A/C trying to run at the same time, the problem is either in the wiring or more likely the furnace.
 
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Old 10-06-09, 08:21 AM
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Doesn't that stat have a systems control switch for you to choose to put in: off, cool, heat or auto? If so, put it to heat only. Regardless, read all the stat setting options, and see if dipswitches inside are all correct, and see if you can change the heat occupied setting and cool occupied setting to be separated by maybe a few degrees, if you find you are able to do so.

Til you solve the problem, turn off the outside unit's 240 volt power supply in the main panel box, or pull the outside disconnect that is mounted outside near the unit out there. Then your a/c cannot run. At worst, the blower in the house will run all the time (if something in the circuitry is stuck/shorted), but there is nothing wrong with that, from an operational perspective. It can just waste electric/shorten blower motor life.
 
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Old 10-06-09, 08:44 AM
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I had pulled the outside disconnect (I always do that when not using the AC). Not knowing what the problem was I put it back in last night and that's when the lights starting flickering and dimming. With it disconnected the AC sounds like it's trying to start up - a very disconcerting sound - I have to wonder if it isn't damaging the unit.

I don't know where the main power supply is. And to my surprise I do not see a dedicated circuit breaker for the furnace or AC in my breaker box.

The thermostate has "on-off", "auto-on" fan, "mode" and another that works with "mode". Then you tap up or down or hold down the "heat" or "cool" buttons to set the temp. There's nothing to separate AC from HEAT except the temperature settings.

I am wondering about something...I have a 4-wire thermostat and a two-stage high effiency furnace. I read you can't use a 4-wire together with the 2-stage... but it's been okay for the last 6 years. I think the installer told me he made an accomodating adjustment inside the unit. Anyway...could this have anything to do with anything?
 
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Old 10-06-09, 09:22 AM
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Originally Posted by themadcatter
I had pulled the outside disconnect (I always do that when not using the AC). Not knowing what the problem was I put it back in last night and that's when the lights starting flickering and dimming. With it disconnected the AC sounds like it's trying to start up - a very disconcerting sound - I have to wonder if it isn't damaging the unit.
It is impossible for the compressor to even try to run one iota with the disconnect disconnected. All that could possibly happen out here is some buzz from say the 24 volt wires trying to close the contactor, and the contactor is what is humming. You could disconnect the 2 low voltage wires. But first test and rule out some fluke where somehow 120 volt or a little less (anything much higher than 26.5 volts, is traveling those low volt wires!)

I don't know where the main power supply is. And to my surprise I do not see a dedicated circuit breaker for the furnace or AC in my breaker box.
The one for the furnace is only a single 15 amp breaker. If you have a breaker out at the unit, I can't say if you'd have another 240 breaker in your panel box. If there is only a disconnect outside, you have to have a 240 breaker somewhere. And there are electrical codes about having mechanical equipment like this so breakers are additionally required if appliance out of veiw of main panel. IF a/c was a retrofit - try looking for a sub panel box between panel box and outside unit.

The thermostate has "on-off", "auto-on" fan, "mode" and another that works with "mode". Then you tap up or down or hold down the "heat" or "cool" buttons to set the temp. There's nothing to separate AC from HEAT except the temperature settings.
Okay. I just had to ask. But what is this other one that works with "mode"?

I am wondering about something...I have a 4-wire thermostat and a two-stage high effiency furnace. I read you can't use a 4-wire together with the 2-stage... but it's been okay for the last 6 years. I think the installer told me he made an accomodating adjustment inside the unit. Anyway...could this have anything to do with anything?
Don't know. Jay is up on all these finer technicalities. Since it has been fine though for the previous 6, you'd think problem is not related.
 
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Old 10-06-09, 09:52 AM
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I am not at home at the moment to look at the thermostat, etc. But I recall seeing another breaker box with a similar looking "plug" (I don't know what it's called) next to all the meters. Could that be the 240 breaker you're referring too? If so, should I remove it too?

The sound I am hearing is not a buzz - it's more like what I might expect to hear at a power station. It's irratic, loud, almost a popping crackling sound. I can't describe it.

I wish I knew something about electrical wiring - it all seems so daunting.
 
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Old 10-06-09, 10:44 AM
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Well, you really need to know something about testing electrical, for this. For one thing, you have to confirm that the power has been shut off out there at the unit. By just disconnecting stuff without positive proof that you are disconnecting the right thing, or that like I said in other post, that for some reason higher voltage is not entering the low voltage wire, is no good. I always verify such stuff with my test meter to be sure, if in any doubt.

But we know that there will be something to either disconnect or unswitch, to stop the 240 current from getting to that unit.

And if need be, because this is separate from the 240 disconnect, is you should be able to unhook the low voltage wire even off your control board inside the furnace. If you unhook the thin wire plugged onto the Y connection on the board should stop that (or it might be labeled as A/C on the board where that wire plugs in), unless some other wire with a bare spot were touching that Y wire in a bare spot.

The reason I mention to unplug it from the control board, rather than from the low voltage wiring terminal bar that might have 6 or more terminals with abbreviated letters, is you are more likely to stop a phantom voltage situation in case something were shorting to the Y terminal's connection/motherboard solder run, on the board.

If you unplug the wire from Y (or labeled as A/C)there, shut off the furnace power switch first incase higher voltage is leaking into that wire. Use rubber insulated needlenose to pull it off the spade, and make sure the wire can't contact anything once disconnected, before you turn the furnace switch or 15 amp breaker back on.
 
 

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