I bought a new Nest thermostat which is supposed to be compatible but needs a professional installation (advice that should have been heeded by non-professional electrician me). This is to replace an old finicky thermostat that must be original with this 20 year old house. After wiring up the Nest and starting to go through the menus, I realized it was better to let a pro do it. So I put everything back the way it was with the old thermostat.

... Or did I... With the old thermostat back on, the heat keeps going up and up without being able to be switched off or lowered by the thermostat. I can get the HVAC to turn off by removing the front face of the old thermostat (like in the pictures below) but that's all the control I have.

Here's the wiring before I attempted any changes:


And here it is after I tried to get it back on:


I've scheduled a pro to come out and install the new thermostat this coming Monday... But that's a while to wait now not being able to control the heat. Plus I'd rather have the old setup working before he gets the new one put in. So... HELP!

Additional information - maybe not relevant:

I did have the breaker to the house completely turned off while switching out thermostats. The first time I put the old one back on, the display went nuts flipping through numbers. So I took it right off, flipped the power to the house back off... Double checked the wiring and how well screwed in it all was... Then put it back on and powered back up. After that, the display is completely normal, but the HVAC ignores the settings still. The thermostat even makes the same "click" that it would if I changed the temperature above or below the current temperature, but nothing changes still.

  • The problem is, that without pictures of either how it was originally wired, or of the other end of the cable, there is nothing we can help with. Jan 24 '15 at 14:59
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    Thanks, Brad! The top picture does show how it was originally wired. Anyways, it appears to have been fixed... With the power to the HVAC cut off, I waited until the temperature dropped below the desired temperature, put the thermostat back on and restored power and all was well after that. Must've just had one very confused thermostat. Jan 24 '15 at 15:35

Nest states their thermostats do not require a Common 24 volt leg of control power to be provided to the Nest, However there are some situations where the Nest will not stay charged after a bit thus requiring a Common leg of the 24 volt power sent to it, they allege the Common leg back feedsenter image description here to the thermostat by unused circuits and it does in many if not most cases however there are many complaining after a week or so of the Nest no longer operating or operating normally, this is due to no Common leg in most cases.


A suggestion - return the NEST and get an Ecobee. It's a much more sophisticated device with remote temp sensors and some nice apps.

WRT the 24VAC needed to power, Ecobee comes with an adapter for your basement equipment to put the 24VAC on your existing wiring to power the Ecobee. You don't need it if you have a "blue" 24VAC, but it's very cool theway they did it.

Note your setup indicates (via the RH, RC, and Orange) that your heat/cool switch can change the furnace mode via the thermostat. I know Ecobee can't do the auto-switchover function and I doubt Next can either - you're going to have to do the the RH RC connection in your control panel to swtich over to call for heat/call for cool.

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