why does nest thermostat need a common (c) wire for heating but not for cooling?

[idanko]

I have separate AC unit and furnace each with its own transformer. My old Honeywel thermostat had two sets of wires: red + white coming from the furnace and red + yellow + green from the AC system. I decided to replace the old thermostat with a Nest Thermostat that I got at deep discount from my electric company, but unfortunately it cannot handle two transformers. So in the Summer, I just hooked up the AC wires (red+yellow+green) and was able to run the cooling system without any issue. Now the heating season is approaching, so I am trying to switch the wires in the thermostat to the ones (red + white) from the furnace instead of the AC wires. When I try to setup the Nest Thermostat with just red + white wires, the app says it needs the common (c) wire (or the nest power connector). The thermostat has its own batteries. I do not understand why it suddenly needs a common wire to get power to run the heating system, while it did not need a c-wire for the cooling?

[Patrick_Caezza]

@idanko 

I'm surprised that your system is working using the Google Thermostat (2020).

Only the Nest Learning Thermostat 3rd and 4th Gen can handle a dual transformer system.

There is a very good chance that your furnace is only looking for a switch closer to turn it on and off which means there is no way for it to get power.

What type of furnace do you have?

[idanko]

No. The thermostat CANNOT handle the two transformers because it has only a single red wire connection. So I only connect ONE system at a time. In the Summer, I had only the AC wires (red+yellow+green) connected to the thermostat and it was running the AC fine without a common wire.

Now, I disconnected the AC wires and I am trying to connect the wires from the furnace (or rather the boiler) to the thermostat. There are only two wires: red+white coming from the heating system. However, the app now says I need a common wire (or the nest power connector). Why does it need a c wire for the heating system but not for the AC? The thermostat has batteries why does it need the extra power for. I guess for continuous wi-fi connection. But why didn't it need the c wire when I had only the AC connected to it? 

[Patrick_Caezza]

@idanko 

I understand now.

A two-wire furnace requires a C wire to be installed as heat only normally doesn't supply power as the AC does. It's just the way manufacturers make heat-only systems.

Follow this like and it tells you what type of systems are compatible with which Nest thermostat.

Nest thermostat compatibility 

[idanko]

That is what I do not understand. Where does the extra power comes from with the AC wires. I thought the three wires (red+yellow+green) from the AC are all control wires. Is one of them actually a common wire (ground)? Can I use that wire also with the heating system?

[AxelD]

Hi @idanko,

Thanks for posting in the Community! I understand your confusion about the C-WIRE requirement for your heating system but not your AC system. As @Patrick_Caezza explained, heating systems often don't supply power like AC systems do.
 

After reviewing your AC system's wiring, it appears there's no common wire (C-WIRE). Your current wiring seems to be:
 

Red: Power
Yellow: Cooling
Green: Fan

To ensure consistent power for your AC system, I recommend installing a C-WIRE or a Nest Power Connector.

Please let me know if you have any further questions.

Regards,
Axel.

[idanko]

My nest thermostat was working fine with my cooling system without a C wire. No problem there.

But when I try to setup the thermostat with the heating system (two wires only, no C wire). It does not want to proceed without C wire. That is what confuses me.