Replacing old thermostat -- two problems
June 12, 2016 4:25 PM   Subscribe

I'm trying to replace an old analog, mercury tube Carrier thermostat with a programmable one. I'm having two problems.

Problem one is that the terminals aren't labeled -- at least not on the front of the faceplate. Problem two is that I can't seem to remove the faceplate. I'm assuming I need to remove the three screws in the picture to do that. I loosened one, but the other two won't budge and are close to being stripped. There is a fourth screw, recessed behind the face plate, barely visible in this picture at about four o'clock on the dial.

How do I get the faceplate off, and which wires (red/yellow/white) are which?
posted by mudpuppie to Home & Garden (14 answers total) 1 user marked this as a favorite
 
On the wires, the "safe" answer is that nobody knows and the only way for you to know for sure is to go check the wires on the control system for your HVAC system in your basement. You can be pretty sure that the color of the wire in your basement corresponds with the color at the thermostat. I'd go and see what the red, white, and yellow wires are hooked up to in your basement and use that to figure out which wires they were.

If things are hooked up in a standard way, the red wire should be the R (24V source) wire, the white should be the W (completing the circuit for heat) wire, and the yellow should be the Y (completing the circuit for cooling) wire. BUT, there is absolutely no guarantee that things are hooked up correctly, so you should really check.
posted by Betelgeuse at 4:58 PM on June 12, 2016 [2 favorites]


I had to do as Betelgeuse says back when I did this a few years ago. In my case, the wire colors were utterly non-standard, but some poking around in my basement shed some light on what was what.
posted by soren_lorensen at 5:07 PM on June 12, 2016


As for the faceplate (mine was a different abd much older model, so I have no definite advice) see if you can locate any small screws on the bottom or sides of the unit.
posted by soren_lorensen at 5:09 PM on June 12, 2016


I don't live in a basement part of the country. The HVAC unit itself is on the roof. There are no screws on the top, bottom, or sides of the thermostat.
posted by mudpuppie at 5:14 PM on June 12, 2016


A drill bit is very good for removing the heads off of screws. Then you can grab what is left of the shank with a pair of pliers and twist it out - once the offending item is removed.
posted by rudd135 at 5:29 PM on June 12, 2016 [1 favorite]


Only the screw on the upper right looks like the slot might be slightly messed up but the other ones look find. For slotted screws you really need a quality screw driver with the right sized blade. Hardware store sets are notoriously bad and cheap Chinese sets are even worse. It's worth it to pay $10 or so for the right Klein screwdriver. Make sure the blade is seated in the slot and the screwdriver is exactly perpendicular to the screw. Even the slight tilt will make it twist out of the slot and damage it. Hold some pressure on the screw and twist, you just want to move it a little to break it loose. It should be slightly easier then to unscrew it the rest of the way.

Are the screws just screwed into drywall or are they screwed into wood? If they're in drywall it should be easy, if in wood it will be a little more difficult.

Follow Betelgeuse's directions for the wiring.
posted by Grumpy old geek at 5:58 PM on June 12, 2016


Searching for the HH number in the photo gives these user manuals:

http://www.whaleyfoodservice.com/EquipmentManualsIntegration/Manuals/CRC0410.pdf

http://dms.hvacpartners.com/docs/1009/Public/02/50YM-5SI.pdf

You may need to go to the furnace and see which color wires are on the terminals on the furnace to determine how to hook up the new thermostat.

(I recently installed a new Honeywell programmable thermostat on my heat-only furnace, the simplest possible installation-- the instructions were incomprehensible, so if you need to call an HVAC pro, you shouldn't feel too bad.)
posted by H21 at 6:10 PM on June 12, 2016


If you can't access the HVAC system, look very closely at where the wires are hooked up on the thermostat. On every one of the (admittedly small number of) thermostats that I've seen, there have been little notations next to the terminal screws of which wire should be which.
posted by Betelgeuse at 6:10 PM on June 12, 2016


Hardware store sets are notoriously bad and cheap Chinese sets are even worse. It's worth it to pay $10 or so for the right Klein screwdriver.

Craftsman makes pretty good tools, or so I hear. With all of my weight behind the appropriately sized Craftsman screwdriver, I am still not able to loosen it.

Searching for the HH number in the photo gives these user manuals:

Yes, I googled and found those too. Those manuals are for the installation of the actual HVAC unit and do not address the thermostat.

On every one of the (admittedly small number of) thermostats that I've seen, there have been little notations next to the terminal screws of which wire should be which.

Yes, that has been my experience as well on the five or so thermostats I've replaced. This one is an anomaly.

A drill bit is very good for removing the heads off of screws.

Yep! Problem here is that the screws are recessed enough that if I drill the heads off, I won't be able to grip the remaining portion of the screw even with needle-nosed pliers.
posted by mudpuppie at 6:47 PM on June 12, 2016


Well, you can always pry the thing off the wall after disconnecting the wires. Who cares if you shatter it?
posted by jon1270 at 2:51 AM on June 13, 2016


You'd be able to grip the screws with a pair of vise grips or lineman's pliers after the plastic is out of the way. Craftsman tools are now made in China and are not what they used to be. A good fitting screwdriver is indeed very important, but that boat may have already sailed since the heads are half stripped out already. Popping the plastic off as above is also a good suggestion.
From what I have heard, it isn't the end of the world if you have to do a little trial and error wire sorting. Hook up two wires, say for cooling, see if that works correctly. Then start over and hook up for heat, see if that works. If so, you've found the correct wires. If not, a little logic with the connection info above will get you there.
posted by rudd135 at 4:26 AM on June 13, 2016


You could cut the wires from the terminals, then, using an ohmmeter, determine which terminals are connected when the thermostat should be calling for heat, and which are connected when calling for cool. I would start with the assumption that red is hot, so is the common terminal.
Using this information, then do as rudd135 says, and connect wires together to see if you get the desired result.
After this is determined, THEN destroy the old thermostat.
posted by H21 at 6:29 AM on June 13, 2016


"THEN destroy the old thermostat."

Please don't put this in the garbage. Most towns have a toxic waste drop off, and that bulb of mercury is the definition of toxic waste.

Maybe the best way to remove the old unit is to cut the wires off at the terminals, cut the mercury bulb lose, and use a pliers or vice grips to pry the plastic base off. Once it's off, use the vice grips to lock onto and twist out the screws.
posted by Marky at 9:17 AM on June 13, 2016 [1 favorite]


Just a gut feeling, but those 3 screw locations don't look right to me for locations to fasten to the substrate. They look like they hold the thermostat assembly together maybe? There appear to be openings of some sort in the approximate position you would see the screw to secure a receptacle or light switch to its back box.

Is there any way there are screws hidden in those 2 recesses?
posted by misterbrandt at 9:26 PM on June 13, 2016 [1 favorite]


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