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Discussion Starter · #1 ·
We just moved into a new house, and was upgrading the thermostat from a "semi smart" RadioThermostat(brand name) which was working fine for heat and cooling. This thermostat utilized 4 AA batteries.
Wiring on the original thermostat was as follows
(C) - Brown
(Rh) - Red
(Y) - Yellow
(G) - Green
(W) - White
(O) - Blue

I powered off all breakers to HVAC and followed install wizard on Nest - and being smarter than your average bear i followed the letter codes and put them where i thought they should go... (matching C-C, Y-Y, etc) - When complete the Nest failed to power up at all....

So i started digging into the wiring
For reference the Air Handler is
United Refrigeration TZHLL-4821JA (R410a unit)
Compressor is
United Refrigeration TZPL-442-2B

Both units were installed/manufactured in 2017

When i started investigating the wiring - i found that the Brown Wire to thermostat (was wired to C) was NOT connected at Air Handler at all - so just open wire to nowhere (so this explains Nest not powering on)

So the NEST suggestions (which i now followed because apparently i am not smarter than the HVAC system) suggested the following
(C) - Blue
(Rh) - Red
(Y) - Yellow
(G) - Green
(W) - White
(O) - Brown (picture shows kind of orange, but orange was never connected to original thermostat and is not connected anywhere else so i used the BRN here - not at the time realizing it was open circuit / not going anywhere)

Once i swapped the Blue wire to (C) - the Nest powers up, blower works, cooling appears to work (was not very warm that day so hard to tell) - but heat blows COLD (blower comes on, discharge are is cooler than ambient so it is actually cooling, not just running the blower)

Attached are complete diagrams how it WAS wired with old thermostat along with color codes from condensor and air handler, along with the CURRENT wiring (not working for heat) with the Nest thermostat

I think i know what to do (wire up the brown common so that it actually does something at thermostat)

The overall quality of wiring job is suspect - wire nuts not engaged - brown wire which i think was supposed to go to thermostat was instead wired to unused portion of wire used for float switch (wire not connected at other end) - etc - enough things that i dont 100% trust what is there already

But i had a couple other questions
1. the "D" wire (defrost) at condensor is currently wired to WHITE - is this correct
2. I have read conflicting opinions on the float switch (air handler is in attic) - some say break power (R), others say break (Y) to kill the cooling but leave heat / blower functional
3. In diagram of air handler it DOES suggest wiring W1 and W2 together for faster heat up (like i currently is) - but are there advantages to wiring W1 and W2 separately to thermostat? If so which one should connect to D at the compressor (if that is correct at all - but is the way it is currently wired)
4. I live in Georgia if that makes a difference to #3

So my question is - what should the proper wiring be to the Nest - AND do i need to do a reset on it and start the setup all over, or can i just rewire properly and reconnect it and it will sense the change as needed?

Thanks in advance for any advice / help



· Registered
62 Posts
Connect blue wire to O/B not 24V common. Keep float switch to break R. If air handler has a control board with a C terminal then attached the brown wire to C on nest and control board.

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· Registered
17,809 Posts
You have a heatpump.

Nest is not a good stat and I would return it.

Plus heatpumps with electric backup should be run at a constant temperature anyway so wifi isn't really helpful unless you go on vacation and want to monitor/adjust remotely.

Wiring info:

B goes to O/B on the stat which controls the heatpump reversing valve, had you switched it to cooling, the low voltage fuse would have blown at best - at worst low voltage components may have been damaged.

Your heatpump needs the reversing valve energized in heating mode - you have to go into the stat settings and change that.

Connect an unused wire from C on the stat to the common on the air handler for any stat that uses the common.

The defrost output from the heatpump is supposed to connect to W1 or both W1/W2 to turn on the heat strips during defrost cycles so you don't feel a cold draft.

It is better to have the thermostat stage the backup so second stage is only brought on when absolutely necessary and it can down-stage to low.

Another advantage of t-stat staging - without W1/2 jumper, only the first stage is used during defrost cycles which saves some energy.

Most consumer t-stats don't offer aux heat staging, you need a higher end honeywell, white rodgers or ecobee to do that.

Doubt the nest supports it, it's an over-priced low quality consumer grade thermostat. Most people helping on this board hate them.
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