heating and cooling turns on at the same time

[wpelgrum]

I replaced a Honeywell thermostat with a NEST learning thermostat but now the AC is turned on together with the heat

The issue is somewhat similar to https://www.googlenestcommunity.com/t5/Nest-Thermostats/Issue-with-Nest-Setup-AC-and-Heat-are-turnin... but I have different wiring

C = Blue = Common

RH = Red = 24V

W1 = White = heating

G = Green = Fan

Y1 = Yellow = AC

I probed the function with a multimeter

• Between C and RH I have 26V AC
• If I turn the fan on in the Nest, the Green wire shows 26V relative to Blue (common) and the fan starts working
• If I turn the heat on, I see 26V between White and Blue --> Expected
  • The fan (green) is not activated by NEST in heating mode, but I assume my furnace turns the fan on automatically
  • I also see 26V on the Yellow (AC), which turns on my AC system

If I pull the yellow wire from the NEST mounting plate the AC stays off and heating works fine (the NEST is now in a heating-only mode, AC options disappeared). So I don't have any shorts in the wires

The common wire appears OK, I can toggle heat and fan just fine, voltage between blue and red stays above 25V at all time

I have a standard AC, no heat pump

Any ideas?

[Panda5466]

I am having the SAME EXACT ISSUE the heat and the air both turn on and it’s hot as heck in my house I don’t know what’s going on I’m not technician but seemed pretty easy to install I also replaced my Honeywell thermostat with this one 

[Nest_attempter]

I got this figured out… just had to do a full reset (had to set all programs up again, connect to Wi-Fi, etc) and that worked. The connections were ok but I think I had them incorrect at one point and the Nest tries (and fails) to reconfigure itself to make it work.

[EricG1]

I have the same exact issue. I also replaced honeywell with this one. Reset doesnt work for me. 

[Doug12242000]

I also have this issue. Do you get resolution?

I think I'm going back to my old one at least it works reliably 

[richleon]

I have the same issue.  Was getting this weird smell whenever the AC will kick in.  Somewhat similar to when you start your iron.  Over time, the air handler started making this loud noise so I decided to open it and take a look.  To my surprise, the heating element had broken and pieces fell into the squirrel cage.  The remaining element was brittle.  Guess running heat and AC at the same time created condensation until the element broke.  Condensation was the steam I was smelling with the heat.  Figured the only thing causing that is the thermostat running them both at the same time.  Now need a new heating element.  Cleaned up the cage and setup the thermostat to AC only, rather than automatic AC/Heat.  Not sure that will solve the issue.  Will try a reset when I install the new element and see how it goes.

[EdwardT]

Hi richleon,

Thanks for posting. I'm sorry to hear that you had this same issue. Please keep is posted once you've installed the new heating element.

Thanks,

Edward

[EdwardT]

Hi richleon,

It's me again. I wanted to check back in to see if you have other questions and concerns. Feel free to let me know if you do. 

Thanks,

Edward

[EdwardT]

Hi richleon,

I'm just checking in to make sure that you've seen our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

Thanks,

Edward