Replacing thermostat on HVAC

[High Plains Drifter 11031]

 

Due to unfortunate and one in a million circumstances (don't ask), the digital thermostat in the hallway that controls the central heating and AC for the entire house got completely smashed. Now its just a mother board hanging by the wires. The rest of it (readout, buttons, etc) is a million pieces of plastic.

When this happened, the heat was on pretty much full blast, so I still have heat, but it never cuts off. (I can turn on and off the heating unit and fan using the breaker switches.)

Question: Can I just replace the thermostat with a new one, splicing the appropriate wires? Is there a compatibility issue I need to be aware of? The house is 40 yrs old, the HVAC unit is newer maybe 20 to 30 yrs old, but I don't really know. After searching, it seems the previous owners (who built the house themselves) did not leave any literature about the system, so I don't anything about it, other than its a GE.

[Judge Roybeanbag 11819]

I’m not an expert but thermostats are generally universal.  You just need to check the number of wires and their colors to make sure you get one to handle that configuration.

[Enchubben 5510]

if you can because it's not too destroyed, take a pic of where the wires are currently plugged in on the current broken digital thermostat so when you go to install the new one you know what goes where. 

[Gil Bang 25920]

make sure you write down what colors of wire go to what letters.  You should be able to do it no problem, and I'm pretty sure that you're an idiot.

 

 

I kid. 

[blacklab 11926]

Yes, go buy a thermostat and it should tell you what to do and be backwards compatible as long as your system was installed in the last 30 years or so.

There should be directions in the new one that correspond to the number/color of wires coming out of the wall, something like

If you have blue, red, yellow and black -> hook blue to BL, red to RD, yellow to Y and black to BK

If you have blue, red, yellow  -> hook blue to BL, red to RD, yellow to Y and place jumper from BL to BK

 

[Enchubben 5510]

If you want to choose the most popular go the nest route.  You can actually see if your system is compatible online for free before buying as well.

[Liquor and Poker 3659]

Yeah, this is slightly harder than replacing a broken lightbulb if you have a standard HVAC.  If you're unfortunate enough to have a heat pump, it rises to the level of maybe replacing a light switch.

[Dbeasy 15042]

If you want to choose the most popular go the nest route.  You can actually see if your system is compatible online for free before buying as well.

Is it easy to do manual control occasionally on the nest?

[ISUCyclones 216]

44 minutes ago, Dbeasy said:

Is it easy to do manual control occasionally on the nest?

Turn the unit (unit is a big dial) to adjust temperature. Literally couldn't be easier. 

[Upgrayedd 5810]

6 hours ago, High Plains Drifter said:

 

Due to unfortunate and one in a million circumstances (don't ask)

What happened?

[Enchubben 5510]

1 hour ago, Dbeasy said:

Is it easy to do manual control occasionally on the nest?

You buy the nest for ease of use and install and potentially the looks/cost.  Everything on it is incredibly easy to use and the compatibility with other smart network stuff is a breeze.

To adjust manually it’s literally turning it. When you first install, as you adjust it, it will learn what you like and auto adjust based on day of week and time. 

Edited October 29, 2020 by Enchubben

[High Plains Drifter 11031]

51 minutes ago, Upgrayedd said:

What happened?

 

Carrying a big ass extension ladder in to measure some windows that are up high (living room has a really high vaulted ceiling, so windows on one wall are at second story level). As I was shouldering the ladder around a corner, daughter opens door to her room, allowing dog to run out between my legs. I tripped on the damn dog, and the end of the ladder crunched directly into the thermostat, which was open because the wife had recently adjusted it, which requires (required) you opening a little door to see the readout, so the thermostat door was open which meant the crunch of the ladder was taken completely by the face of the thermostat.

Basically, some Final Destination series of events.

[High Plains Drifter 11031]

5 hours ago, blacklab said:

Yes, go buy a thermostat and it should tell you what to do and be backwards compatible as long as your system was installed in the last 30 years or so.

There should be directions in the new one that correspond to the number/color of wires coming out of the wall, something like

If you have blue, red, yellow and black -> hook blue to BL, red to RD, yellow to Y and black to BK

If you have blue, red, yellow  -> hook blue to BL, red to RD, yellow to Y and place jumper from BL to BK

 

 

sounds complicated, but I think I can pull this off.

 

 

[HiggyBaby 2529]

Yeah take a pic. We would all like a chance to burn your house down. Also, shut off the circuit breaker to your hvac unit. If you cross the wrong wire you’ll blow an internal fuse, making your evening slightly more sucky.

[Jkwellborn 6691]

Nothing to changing a thermostat. Make sure you put the new one on the wall level if it is square.

[Revolution512 1487]

Most have a level inside now

[High Plains Drifter 11031]

 

I know you guys are waiting with baited breathe right now, so I'll give you an update:

The weather has turned very pleasant, and currently neither the AC nor heater is needed. Open a few windows in the day, close em at night works just fine. So, I've kind of just put it off. Might get to it this weekend.

I'll keep you guys posted.

[shakahorn 1645]

On 10/28/2020 at 12:17 PM, blacklab said:

Yes, go buy a thermostat and it should tell you what to do and be backwards compatible as long as your system was installed in the last 30 years or so.

There should be directions in the new one that correspond to the number/color of wires coming out of the wall, something like

If you have blue, red, yellow and black -> hook blue to BL, red to RD, yellow to Y and black to BK

If you have blue, red, yellow  -> hook blue to BL, red to RD, yellow to Y and place jumper from BL to BK

 

what if he's color blind?

[Jkwellborn 6691]

what if he's color blind?

Then tie all those gray bitches together.

[TKthunder2 4985]

Installed two nest thermostats today, had a bitch of a time figuring out how to tie in the heat pump but I got it all working.  Without the heat pump it would have been a breeze because nest walked me through every other wire.

[Baboontyme 2809]

Not really sure what to do. I installed a Nest maybe 2 years ago. I noticed last week that it was dead on the wall. I took it off and charged it via micro usb. Put it back on the wall. It died again. I tried chatting with nest support. They told me to fully charge it and try to connect to wifi. I did. It would not connect to my wifi. I turned wifi off. It still died. So I'm down to either one of two things - bad unit, or that suddenly the power to the unit stopped. I was obviously hoping for the former. I ordered a new Nest and installed it. There's no power. Do I call an HVAC person or an electrician? Any tests I can do myself? I am a little paranoid of cutting the wires too short if I try to start re-stripping them. 

[Jkwellborn 6691]

Call an HVAC company you trust. It should be getting power from the board on the indoor unit.

[High Plains Drifter 11031]

 

I just realized I never updated. Sorry to leave y'all hanging.

I got the new one installed, no problem. AC and heater work great. Seems like the heat actually blows hotter(?). Not sure how that works.

In order to avoid having to drill  new holes, and not having old holes exposed, I had to mount the new one rotated 90 degrees, so its sideways when  you look at it (I didn't think to check the arrangement of the mounting bracket when i bought the new one), which is actually good because the wife and kid can't figure out how to read it, and they are too afraid of looking dumb to ask, so they don't mess with it. Thermostat works, because it doesn't have the little level thing with mercury or whatever the liquid is in it that tilts back and forth.

Went with the cheapest they had at the hardware store. #NoNest #cheapbastard #luddite

 

[dirtonia 269]

The transformer of your unit should provide 24v to your t stat. Should be a red and black wire, test red to ground.

[Sam Lin 3551]

Bumping this for help. I have two Trane XL 14i heat pumps that I just tried to upgrade to Nest thermostats so I could monitor over the internet. One seems to work correctly, the other is behaving oddly.

The existing thermostat was Trane TCONT800. Looking around on internet and looking at info from Ecobee's site, I should be able to treat the existing B wire as a C wire for Nest/Ecobee, and I should connect existing O to the O/B terminal. Snippet from Ecobee site:

 

Existing wiring with the Trane TCONT:

 

How I wired into each Nest:

 

Here are the software settings on the Nest that works correctly, O wire heat pump:

 

The other unit, if configured the same way, seems to cool when the thermostat is trying to heat. So I tried switching the software setting, heat pump to B wire, and then the unit doesn't cool when the stat is instructing to heat, it just blows fan, but still no heat. It doesn't make much sense to me that two identical builder-installed units would be wired differently in my house (one unit using O wire and one using B)? What should I look at next? (I have not gone to the air handler in the attic to check the wiring yet - I was hoping to avoid needing to go up there.)

I did not disconnect power when I swapped thermostats, but I was very careful to protect each wire and not short it. However, if I shorted a wire and blew the internal fuse in the air handler, would the symptom be the behavior I'm seeing? I can also try swapping the two Nest thermostats themselves to see if one is actually sending a different command.

I also read while Googling that even on heat, the heat pump will periodically run a cool cycle to defrost. Is it possible that I was in a defrost when I tested? If so, how long is a typical defrost cycle? (How long do I need to let it run before I'm sure it is/is not correctly heating?)

[TKthunder2 4985]

I have a trane with a heat pump but I have an orange wire in OB and the white going to the W.

[MonkeyDoughnut 10229]

yeah, if wiring same not having same results I think you are going to have to go into attic and look at how each is wired there.

[Sam Lin 3551]

I swapped the two Nests and got same results, so it isn't the thermostat. I swapped the finicky thermostat back to the original "dumb" Trane thermostat and everything (heat/cool) worked correctly. Will throw the Nest on a shelf and try again in the future when I have more time. Very confused.

 

[UT_OB1 2217]

@Sam Lin were both units wired the same as in the first picture, or just the one that ended up working correctly?

[Sam Lin 3551]

Both were wired identically (both Nest thermostats) - what I don't know is if the wiring in the wall to the air handlers is the same, though builder spec house, identical heat pumps, it <should> be. But one Nest worked correctly by default, and other cools instead of heat unless I switch that OB wire setting in the software, and then it just blows fan, still doesn't heat. But works fine swapped back to the dumb Trane TCONT800 thermostat that was originally there.