Troubleshooting White-Rodgers HVAC thermostat

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Old 06-21-15, 01:30 PM
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Troubleshooting White-Rodgers HVAC thermostat

My Mom says her A/C went out last night. She awoke to a hot house and the LCD display on the thermostat was blank. So she bought new batteries and changed them. Then the A/C switched on but the LCD is still blank.

The unit is a White-Rodgers 1F86-73, and obviously is pretty old.




I've checked the batteries with a battery tester, and they're all good (EDIT: also tested with different unused batteries), but I scrubbed the terminals with steel wool JIC. I've tripped and reset the HVAC circuit breaker. I checked the blade fuse outside in the HVAC unit and it looks okay.

I can't find a manual for this particular thermostat but I used what seems to be the standard reset procedure for the older White-Rodgers thermostats, switch cool-off-heat to 'heat' and simultaneously press both temperature control buttons. I have done this several times and one time the LCD illuminated with a bunch of hieroglyphics, which reminded me of the chicken scratching on the alien's self-destruct device in the film, "Predator":



But it faded to black in probably less than a second and has shown no signs of life since.


I have a multimeter (not that I know how to use it) so I've included a pic of the wiring in case anything might be learned checking voltage or resistance or whatnot across any two points:




For the timebeing, the A/C is on, which I count a good thing, because it's 93F here right now. I have punched the 'Up' temperature control button enough times that if it were having any effect, the A/C shouldn't kick in until the house is about 120. This is an old HVAC unit, and I doubt its had a complete service in 20 years, and on hot days it always runs full-time anyway, never seems to catch up. I don't have a thermometer but right now, the house feels cooler than the 80F my Mom (who wears sweaters in July) usually has the thermostat set to in summer.


Do I need to start shopping for a new thermostat?


One brief update. I should have left well enough alone. The A/C was running non-stop, which I took to be some sort of failsafe mode. Now it's stopped.
 
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Old 06-21-15, 01:38 PM
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R = 24 volt power
Y = cooling
W = heat
G = fan

You can remove the R, G and Y wires and twist them together with a wire nut to prove that cooling still works.

You can't repeatedly bump the R to Y wires since the compressor can't handle short cycling very well.
 

Last edited by Houston204; 06-21-15 at 09:05 PM.
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Old 06-21-15, 02:27 PM
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I get 27vAC from Red to all the other three. And it was cooling just fine, until ~2 1/2 hours ago, when I got to monkeying with it trying to "reset" the thermostat.

So let me see if I'm following you. You're saying that if I cross the red, yellow and green wires, and the compressor comes on, that incriminates the thermostat?

When I unscrewed the wires from their posts and crossed them (R,Y & G), the compressor started (it hadn't been on in better than two hours, despite 90+F temps outside). When I un-crossed them, it stopped, then started again (seconds later) once I had them screwed back down.

At this moment, it's running like a scalded dawg, blowing good cold air, so I ain't touching nothing else (if it can be helped) until I know what (if anything) needs replacing, or it gets to be hog killing weather, whichever comes first. And FWIW, some random segments on the lower half of the LCD display are illuminated now. The upper half is still dark.
 
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Old 06-21-15, 02:51 PM
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Connecting the Red, Green, and Yellow wires together simulates the thermostat calling for cooling. If the A/C works normally (comes on and blows cold air), the system is working correctly, with the possible exception of the thermostat. It sounds like your thermostat should be replaced (no display). They are relatively inexpensive and that should solve your problem.
 
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Old 06-21-15, 03:22 PM
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Looking at Home Depot's web site, I see two available in my local store. Both are Honeywell digital non-programmables. One is $25 and the other -- which says it's for heat pumps -- is $40. This central unit is a heat pump. Does that mean I necessarily need to spend the extra $15?

And is this a straightforward swap job, disconnect the old and hook up the wires as per color code on the new?
 
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Old 06-21-15, 04:23 PM
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Home Depot was closing at the top of the hour, so to be on the safe side I ran over and asked their HVAC guy to recommend a new thermostat. They had a bunch that weren't listed online and I got one that the packaging states will work with a heat pump, programmable, $25.

I'm going to wait until after sundown to swap them out but I'll post back on the results.
 
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Old 06-21-15, 07:03 PM
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Is this unit a heat pump?

I had to buy a replacement thermostat this evening for my Mom's central HVAC system. The new thermostat has a switch with "Gas or Oil" in one position, and "Electric of Heat Pump" in the other. The problem is the heater is gas but it was always my understanding that it also was a heat pump. It's almost a 20-year old unit, and the heater blows lukewarm, not hot air (which sounds heat pump-like), but now I can't find anything to confirm whether it is. The documentation (which my Mom kept) doesn't say "heat pump" anywhere I can find.

The model number is bpgm0420901aa, which Google says is a York unit. The data plate says it is a "forced air furnace with cooling." Which doesn't help a lot because Wikipedia says there is such a thing as a forced air heat pumps.

Any ideas how I should set that switch, or what I can tell you to figure that out?
 
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Old 06-21-15, 08:02 PM
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So I'm reading the installation destructions on the new unit before I disable the old thermostat. There's one configuration item I needed help with, which I posted in another thread.

Another thing is the old unit has too many wires. Besides the customary R, G, Y, W and B, it has two red wires on the same terminal (at the 1:00 position), not the terminal with the single red wire which has 27 volts applied (at the 9:30 position), which do not appear to be 'hot.'

The installation instruction for the new unit makes mention of an Rc wire in addition to an R. The Rc terminal normally is jumpered to the R terminal (sort of a busbar dealie), unless there also is an Rc wire attached.

So what am I two do with the two red wires (which appear to have no voltage applied) at the 1:00 position? Do I remove the jumper and install them both in the Rc terminal? Or just ignore them and hope they'll go away?
 
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Old 06-21-15, 09:07 PM
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I see nothing to indicate that this is a heat pump.

I see that you have a blue wire that is not in use.

A heat pump will use an O or a B terminal and it does not appear that you are using either.
 
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Old 06-21-15, 09:11 PM
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Excellent, Houston204, and thank you for that.

Any idea what I am to make of the two red wires under the post at 1 o'clock?
 
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Old 06-21-15, 09:15 PM
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Fred.... my apologies. I went to move your heat pump post to this thread and it got lost.
We try to keep the same topic in a single thread so that there isn't a duplication of questions.
We, the mods, see all the threads.

To comment on the lost post.... you have a gas furnace and you think you may have a heat pump system. It's possible but not likely. If you had a heat pump system the outside unit would be running in the winter. Does that occur ?

Your old stat has no wiring to indicate a heat pump.

The two red wires is just a jumper from Rc to Rh.

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Old 06-21-15, 09:17 PM
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I see the furnace red wire connected to RH and a jumper wire from RH to RC.

You have 1 red wire. You do not have a 2 transformer system.
 
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Old 06-22-15, 11:49 AM
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I've got the new thermostat installed and apparently working normally. Once I had all the wires in the original thermostat disconnected, I found markings that clarified the roles of the different wires. But being as I'd never attempted anything like this before, and it was my Mom's house, I didn't want to risk going off half-cocked. Silly me, before Houston204's previous post, I hadn't caught on that Rh and Rc meant Red(heat) and Red(cool), which is why I couldn't figure out the deal with the jumpers.

Thanks to you all for your help, and your patience. And apologies for causing the mods extra work with the 'duplicate' post.
 
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Old 06-22-15, 09:31 PM
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Not a problem. Actually the post I thought I lost was indeed in the thread.

Good to hear all is well.
 
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