Honeywell RTH8580WF Blank Screen


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Old 05-03-15, 01:46 PM
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Honeywell RTH8580WF Blank Screen

I've just installed a Honeywell RTH8580WF thermostat replacing an old mercury bulb model. Behind the old model was an unused blue wire, and after testing with a multimeter between the blue wire and the red wire that was installed in the "R" junction of the old thermostat, I got a voltage of around 28 VAC (a little high, but there wasn't a load on it at the time). The heater/air handler is a recent unit and who ever installed it ran the extra wire (by extra I mean a 5 wire bundle) and it appears to be hooked up to the furnace/air handler. It is hard to VERIFY this for certain as the furnace is in the attic space of the garage and I don't have a ladder tall enough to inspect it.

After following the quick start guide that came with the thermostat, I am not getting anything displayed on the thermostat. Totally blank screen.

I have replaced the model once already today and have gotten identical results. I have flipped both the power switch and circuit breakers that go to the air handler/AC system, and I am still reading 28 volts AC on the line when I remove the thermostat from the back plate between "R" and "C" (the blue wire) when the circuit breakers and switch are powered (nothing other wise, of course).

Any help would be appreciated.
 
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Old 05-03-15, 01:51 PM
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Welcome to the forums.

You read an unloaded (no connection) 24vac.
Go to the air handler/furnace and confirm the blue wire is actually connect to C.

If yes, measure your voltage there, at the air handler, between R and C with the thermostat connected.
 
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Old 05-03-15, 02:27 PM
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Unfortunately, I don't have a ladder tall enough to get to the air handler/furnace. The best measurements I can do with out an adequate ladder:

Open "R" to "C" at wall plate terminals: 28.40 VAC
Thermostat attached, other wise same as above: ~2-3 VAC that varies

Weird thing with the second one, I read a little over 3 volts AC when I just touch the red probe to "R" and black probe is just dangling in the air. I do NOT have this effect occur when thermostat is detached from the wall plate and I'm just reading the wires open.
 
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Old 05-03-15, 03:23 PM
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Is there any other connection that the blue wire could have with the furnace/air-handler board and read 24-28 volts AC between it and a "R" (red) wire?
 
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Old 05-03-15, 04:10 PM
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Any connection slightly to ground on the blue wire will allow you to read 24vac between C and R.
I couldn't tell you what it's connected to.
As a guess.... an old humidifier.
 
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Old 05-03-15, 04:21 PM
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If this blue wire is connected to "C" on the furnace, would there be any reason why the thermostat wasn't powering up? This is a very old house, I doubt there has ever been a humidifier. The furnace and AC are probably 10 years old, and the house had floor heaters previously.
 
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Old 05-03-15, 04:26 PM
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If this blue wire is connected to "C" on the furnace, would there be any reason why the thermostat wasn't powering up?
That's the problem. The blue wire is not connected to the C terminal. Without physically looking...we are just guessing here.
 
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Old 05-03-15, 04:37 PM
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Well, I'll see if I can find a way up to the furnace then.
 
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Old 05-03-15, 04:47 PM
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While your there don't forget to check the filters as well.
Just Saying!
Geo
 
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Old 05-03-15, 10:30 PM
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Ok. So I got it to work.

The blue wire was not hooked in to anything. Completely free. It was wrapped a few times around the wire insulation, so I guess the 28.4 VAC was an induction effect/cross talk from a nearby wire. This would probably also explain why it dropped to 3 or less VAC when a load (i.e. the thermostat) was put on the line.

There was a separate set of wires going out to the condenser coil (hooked into the "C" and "Y" terminals in the furnace). I suppose that's for turning the outside fan on and off. So I put the blue wire on the "C" as well, and wallah, everything works now.

Thanks for convincing me it the blue wire was not where it needed to be. Probably wouldn't have made the adventure up there otherwise.
 
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Old 05-04-15, 12:50 AM
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You're welcome Good job.
 
 

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