Heat pump thermostat replacement

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Old 03-28-16, 08:28 PM
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Heat pump thermostat replacement

Hi! about a month ago my furnace failed to start and but came on after I non-systematically monkeyed with t-stat and circuit breakers. Don't know what action fixed it because I didn't allow enough time for it to cycle up before trying something else. Today it did it again but I wasn't able to get it to start up.

I have a demand load thing the power co uses to control a/c in the summer and this fall when it stopped happening, I figured out that it was causing my frequent power issues that the microwave would need to be reprogrammed (but other clocks are OK). So called power co. to check that in hopes that could be causing the problem. When their one to three hour estimate got to be about five hours, I thought I'd take another run at the thermostat to see if maybe another stab at reprogramming it would help. Well, found that the batteries had died but the t-stat still didn't work (blank screen) after I put new ones in. It is a Honeywell RTH7400. Think there are 8 wires but one is not connected (bottom right) -- no insulation so I'm assuming it is a ground wire. Name:  2016-03-28 19.59.59.jpg
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Haven't picked up a new t-stat yet but when I do, do I need to turn off the breaker to that part of the house as well as the ones for the furnace and a/c? To make sure I have the power off which wires do I use to check for power? Is using a two contact outlet tester OK or do I need to use the multi-meter?

Is it at all possible that the problem could be with the demand load device?
Is the disconnected wire an issue?

Electric co guy just left said power was ok, didn't check the demand load device, and said that since the t-stat screen is blank it isn't getting power. Shouldn't the new batteries have allowed the screen to light?

This T-stat is about 8 or 9 years old. I was going to put it in and I have notes (check marks, blank symbols and an arrow pointing from the W2 wire to the Y terminal) but I must have gotten confused as there are three wires with a W but thinking about it now, I suspect that the original installer didn't have a Y label and just put an extra W on the yellow wire. I was doing it the the same day I had an electrician here, and must have asked him to go ahead and do it. Any reason not to follow his wiring? I think it has worked as it should, e.g. furnace warms up house after night cooler temp w/o going on Auxiliary. I'm not sure about the cycle time as when it was really cold (0 degrees F) and running on auxiliary, it seemed to run very often for not long -- maybe 2 minutes on and 1 minute off. But I don't know what it should do.

Although I'm using the fireplace (yeah I know, sucking the heat out of the rest of the house) it would be nice to not have to keep the fire going all night. Is there a pair or more of wires I can cross to run the furnace for a while (assuming the problem is the thermostat)?

Being cheap as well as unemployed, I am hesitant to call in an HVAC person to find out that it is the thermostat that I'd hope to be able to DIY. Or to find out that I needed to have called an electrician because I need a new electric panel. (Bedroom area lights dim when I plug in an iron and I don't think there is that much on that circuit).

BTW we had an hour plus 9000 home power outage last night and I think the furnace came on at least once after that but I'm not sure. With the prior one-time failure I don't know that it is relevant.

Any advice on determining the problem is much appreciated.
 
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Old 03-28-16, 08:34 PM
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Ok.. we need to keep the terminology straight. You have a heat pump..... not a furnace. Is that correct or do you have a heat pump and a furnace. A furnace is usually gas powered.

Your stat wiring looks like heat pump only with backup electric heat.

To check for power you measure from the red(R) wire to the blue(C) wire for 24vac with the meter set on the AC volts scale. The stat needs good batteries to operate correctly.

The power company demand control should control the compressor only. I don't know if they limit power to you in the winter since there is a heat pump there...... most likely not.

Clarify the system type and I'll look at the stat manual.
 
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Old 03-28-16, 08:37 PM
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You can temporarily move the green wire from the G terminal to the RC terminal to see if the indoor fan will run. ( green will be at Rc along with the jumper wire currently there).
 
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Old 03-28-16, 10:03 PM
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Oops sorry about calling my heat pump a furnace I didn't realize that wasn't like calling my photocopier a Xerox machine or something.

Yes I have a heat pump with back up electric heat. As far as the demand control, they aren't supposed to do anything in the winter and as I remember they didn't need to come in to install it, so it makes sense it only works on the compressor (and use electricity for the little green or red light). I don't know how it works so for all I know the compressor could signal back to the blower/fan control. If the device is malfunctioning it might muck things up, as in it shouldn't mess up my microwave clock.

I'll check the red and blue with the multi-meter and report back. I'll check the brand new batteries while I'm at it. They are a Harbor Freight freebie so who knows.

Thanks Houston, I'll check on the indoor fan and let you all know about that too.
 
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Old 03-29-16, 01:32 AM
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Thumbs down No current between red and blue; Indoor blower does not run.

The only good news is that the HF batteries are good. Too bad can't say the same for my brain. I bet I looked for the multimeter for 30 plus minutes. There are only 3 places it should have been. Checked those 3 or 4 times plus a bunch of places it doesn't belong. Sitting down to post that hopefully overnight I will recall where I last used it so I can look there . . . and there it was sitting on the couch by the PC. I have absolutely no recall of getting it out of the kitchen battery drawer and carrying it over to the couch.

SO can I figure out if there is power to the heat pump w/o electrocuting myself? The breakers two double breakers labeled "furnace" and a double breaker labeled "a/c". One of the "furnace" breaker pairs has a connector that rotates and seems a bit wibblely but I think that's the way it is designed. The electric heater has two breakers on the air handler that look and feel OK. BTW this is labeled a Trane TVF030A140A but the filter size is consistent with one of the smaller units in the series. I didn't get the heater model number. I can only stand reading the name plate that is upside down at knee level for a few numbers. Will get it if it is important. Compressor outside is a Bryant 661CJ030E

I saw someplace that there is an outside disconnect switch that I'll need to check out tomorrow. If there is anything I can do to make sure I'm calling the right service people I'd appreciate the input.

Thanks for your help!
 
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Old 03-29-16, 10:33 AM
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It's going to be the two pole breakers labeled furnace. They will be 40A or larger. The other one marked A/C should be a two pole 30A.

I'm trying to find the service info on the air handler. The problem will be in there.... not the outside unit.
 
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Old 03-29-16, 01:01 PM
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Unhappy Thanks for your help.

Well using my Angies list contacted a company that does both HVAC and electrical. Unfortunately their service manager thought it was the HVAC and of course as I was suspecting, I'd wind up paying for the wrong service guy. Should have been the electrician; the panel is bad with the breaker melted on to the bar...... There are some times I would rather be wrong!!!
 
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