Adding two 3-wire nest thermostats to a legacy Erie (Taco) SR601

mbroadst

Hi! I'm trying to replace two Honeywell 2-wire thermostats with Nest Thermostats (not the Learning models) and having trouble getting the second thermostat to recognize the C wire.

I ran new wire to both thermostats with at least 3 wires (they are different cables): Zone 2 has R-W-G, Zone 4 has R-W-B. On the relay itself I wired the R and W wires for both thermostats to the same R/W ports they used to be connected to, and then I joined the C wire for both into a shared extension wire to the C port in the top left of the unit. Please see the attached image for more detail:

• I labeled the "C" in the top left, which the extension is being run from.
• The other end of that extension shows that "G" (from Zone 2) + "B" (from Zone 4) + "C" have been joined together

This works just fine for the first thermostat in Zone 2, but the thermostat in Zone 4 says its not getting power from the C wire. One notable difference between zones is that Zone 4's wire is probably close to 80-100ft away from the relay - is it possible that's too far to maintain the 24v?

Thanks for your help.

pecmsg

🤭

Jamie Hall

Try swapping the R and W wires at the misbehaving thermostat.

EdTheHeaterMan

When you connect a jumper to the Right T on zone one and Left T on zone 2, and the Zone 1 lamp lights, the the R terminal is on the Right T Terminal of all the zonesWhen you connect a jumper to the Right T on zone one and Left T on zone 2, and the Zone 1 lamp lights, the the R terminal is on the Right T Terminal of all the zonesWhen you connect a jumper to the Right T on zone one and Left T on zone 2, and the Zone 1 lamp lights, the the R terminal is on the Right T Terminal of all the zonesIn order to make this work, you may need to use an additional transformer. Is this control an ERIE or a TACO?

I see a transformer that is rated at 24 VAC / 25 VA. the VAC rating is volts of AC current. that means that it will operate with a 24 volt thermostat. The 25 VA stands for Volt/Amps. which is kind of a Watts rating. if your total amprage draw from the transformer is less than 1 amp traveling thru the relay coils and any other load, that transformer should work just fine. If however you add so many relay coils and other loads (like the power required by the thermostat's wifi operating system) that the transformer is required to develop more than 1.08 amps at 24 volts, the load would be too great and the thermostat would over heat and all the factory installed smoke will be released.

Not knowing the amp draw of each of those relays, I can not tell you with any certainty what the total amp draw when all the relays call for heat at once and if the additional load of the thermostat(s) might be. Basically I'm saying the transformer may not have enough juice.

But if you did have enough juice for the two additional thermostats, you must connect the thermostats properly. You must connect the R of the thermostsat to the R of the zone relay. Since the zone relay termilans for the thermostat are labeled, T1 T1 and T2 T2 and so on, you do not know which T is the R from the transformer and which T is the W terminal. You must do a test on the control to determine which T = R and which T = W. The test is simple.

When you connect a jumper to the Right T on zone 1 and Left T on zone 2, and the Zone 2 lamp lights, then the R terminal is on the Left T Terminal of all the zones

When you connect a jumper to the Right T on zone one and Left T on zone 2, and the Zone 1 lamp lights, the the R terminal is on the Right T Terminal of all the zones

Once you know this you can then connect the R from the thermostat to the proper T on the zone control. Then the W from the thermostat can be connected to the other T on the control. Now you need to determine which 24 VAC terminal on the board is equal to C or Com. on the zone control board

Once you determine the proper R terminal, take a meter to any zone R terminal on the control and connect a meter lead to that terminal, then connect the other leas to a terminal on the transformer load side. If you get no voltage reading, (shown by dotted lead line) then you are on the R terminal of the transformer. If you get 24 VAC on your meter then you are on the C or Com side of the transformer. That is the terminal you connect the thermostat C terminal to on the zone control

mbroadst

@Jamie Hall 's idea worked! Would you mind explaining why, just for my own edification?

@EdTheHeaterMan Thanks for your very helpful comment! I've read a number of other posts related to this relay unit, and it seemed like when folks tried adding 3+ thermostats they start getting "clicking" at the relay likely due to overloading the transformer. So I should do my homework on what the draw of each of the relay coils and the nest thermostats to ensure its under 1.08 amps @ 24 volts? I'll get started figuring out how to do that.

EdTheHeaterMan

Check my diagram edit above @mbroadst

EdTheHeaterMan

@Jamie Hall 's idea worked! Would you mind explaining why, just for my own edification?

If you can see the wiring behind the printed circuit board, you will see that you connected the R wire to W on the thermostat and the W wire to R on the thermostat. The thermostat MUST have R connected to R on the thermostat and C or Com connected to C on the thermostat to have a complete circuit. You are using 24 volts to power the computer on the thermostat. when you connected R on the thermostat to W on the control, there was no completed circuit. It is just like connecting both terminals of a flashlight bulb to the same side of the battery….

This slide from one of my classes illustrates what I am saying. There is no complete path from the light bulb from the source then back to the source.

Jamie Hall

To add to @EdTheHeaterMan ;s comment above — I've often said (drives people nuts) that electricity is colourblind. Your situation — which is very common! — simply had the wire which was red in colour functioning as the white (switched) wire, and the wire which was white in colour functioning as the red (power supply) wire.

Don't feel bad. Happens all the time — since two wire thermostats just don't care what colour the wire is or what it is supposed to be doing.