So I am trying to change my old mercury thermostat from the 60's to a new digital one. Currently I'm thinking about the Nest for my heat only furnace system. As I mentioned, it is an old system without labels on the thermostat or the furnace. I only have a picture of my thermostat wiring at the moment (attached). I'll try my best to describe the rest of my heating system to you.

There is a Red and White wire in use. Also a Black unused wire tucked away on the thermostat and the furnace end. At the furnace end, the Red is connected to a device that has the gas pilot on it. The White is connected to a "box" that leads to an AC outlet on one end and a fan control on the other end.

(I'll try to get a picture of the furnace unit tomorrow)

After doing some research people are saying that the Red is usually the 24V while the White is the heater. But it seems like in my case it is the reverse?

Can I just test compatibility of the Nest by swapping the R and W wires between the R and W connections on the Nest unit? Will I risk destroying the Nest unit if I do not hook it up correctly?

Also I'm getting mixed responses regarding the requirement of a Common wire for power to the Nest. Will/should I need one? If so, can I use the unused wire and make it a Common? If so, where do I connect that to on the furnace end?

If the Nest does not work because of powering issues, what would you guys recommend? I do not necessarily need all the fancy bells and whistles of the Nest but I figured it would be nice. I'm okay with a Honeywell one as long as it is digital.

 

A Honeywell RTH6350 would not require a common wire.

You might try posting some pics of the furnace end of that thermostat wire along with the furnace make and model number.
Someone may be able to tell if it is 24 volts ac, millivolt or something else that your old stat was working with.

 

if the new subbase has a terminal for the C wire yes that would be a solid 24vs on the subbase to power the show

 

I actually thought it through and since I'm in Los Angeles, I'll probably only use heating 2 months out of the year. I just bought a cheap $25 Honeywell. I got it to install fine by sticking red to R and white to W.

I did discover that I have a blue and a green used wire though. I guess even in the 60s they "future proofed" the house in case I wanted air condition.

Either way I attached the picture of the furnace end and also a diagram. I still don't 100% follow how my wiring is flowing compared to the diagram.

 

You have an old 24vac switching system.

I marked common on your picture if it's needed.

 

So I can take an unused wire and hook it up there if I need a common line?

And when you say old - would it be much more energy efficient to switch to a new unit or wait until mine fails?

 

Hi. I have a similar setup (gas furnace - hot water heat only) and purchased the Honeywell WiFi RTH6580WF thermostat. I have a white wire currently connected to W and a red wire currently connected to R and there is also a jumper between R (where the red wire is) and Rc. I am currently using a non-programmable Honeywell thermostat that is powered by 2 - AA batteries. Since there is no wire to plug into C, do I have to run a new wire or can I do something with the 2 wires I have? Maybe split the power from 1 of these wires??

I am assuming that I need a new wire to plug into C that will provide constant power to the thermostat, where my old thermostat used batteries for the power.. if that is what C is for and I have to run a new wire, where can I run the new wire from to get the power?

For some more info on my system.. Off of the same furnace, an addition was setup with radiant heat in the floor. That is why 1 of the pictures shows the split, sending out 2 sets of White and Red wires (1 goes to the thermostat for the radiant heat and the other to the thermostat that I am trying to replace which controls the rest of the house (Colonial - 1 zone). There is an old tag hanging on the wires that I included a picture of.

Any suggestions would be greatly appreciated! I need to get a programmable thermostat hooked up ASAP to help with my high gas bills! Thank you!

 

You didn't just buy a "programmable" thermostat as one of those could run fine on batteries. You purchased a WiFi enabled that requires power to transmit to your computer network.

In the picture with the yellow Taco card.... your C terminal would be the wire that connects to the #2 terminal on the zone valves.

 

Thank you for the quick reply! I bought the WiFi one because my gas company is offering up to a $100 rebate on them (I paid $89).

I no longer have a Taco Thermostat, so does that diagram even apply to my current situation? Are the zone valves you are referring to what I took a picture of with the red and green wires (1 for the radiant heat and the other for the rest of the house)?

If you can tell based on the wiring pictures I provided, can I use the 2 wires somehow with the new thermostat or do I have to run an additional wire to plug into C? If I need a new wire, can you describe where I can run it from (perhaps splice a new wire from that green wire)?

Thank you!

 

You ended up paying $89 for the Wifi thermostat. A non WiFi programmable thermostat would probably have been less than $89. That thermostat requires a third wire for the C terminal.

In your picture you have two zone valves..... probably one for the radiant and one for the rest of the house.

Your two zone valves are just like what's in the Taco picture. The middle terminal is what you need to connect to. It doesn't have to be right at the valve. You can trace that wire and connect where it's convenient. I can't see enough detail to tell you what color it is.

I'm assuming you'll be using the new thermostat on the house zone.

 

So I finally got my plug-in transformer to supply constant power to the WiFi thermostat. I can a thermostat wire from that 24v transformer and connected the white wire to the C and the red wire to the Rc (in addition to the 2 original wires that I have from the old thermostat). That thermostat works great and the heat is working fine in the main part of the house, but now the radiant floor doesn't work! The thermostat shows that it is calling for heat, however the floor doesn't heat up. There are 2 valves as pictured below - 1 for each thermostat. Why would replacing 1 thermostat (that is working perfectly) affect the other? The wires are split from the furnace to each of the 2 zone valves, but from each zone valve there is a dedicated thermostat wire. For more info - the pipe from the furnace to the radiant floor is burning hot (so hot water is running to the floor), but the return pipe back to the furnace is cold.. so it appears as though the thermostat is calling for heat and the valve is letting it through, but it is getting stuck somewhere at the end of the pipe that is supplying the hot water.. is there another valve below the floor as well that maybe got clogged coincidentally? The floor stopped heating right after I replaced the other zone's thermostat with the WiFi one. PLEASE HELP!

 

Have you confirmed that the valve is really open. There is a manual release arm on it that should be loose when the valve is open and hard to move when the valve is closed.

 

So I guess it was a coincidence that my latest problem started right after I changed out my other zones thermostat.. it turned out to be a bad zone valve. The thermostat was calling for heart and the valve was open which was letting the hot water get into the pipe, but the valve was not telling my circulating pump to turn on to push the hot water through the floor. Changed out the valve had and works fine again! Good luck