My previous post was deleted by the Community. I'm incorporating very helpful comments from Stack users here in this updated entry.

I have hydronic baseboard heating that uses a Weil McClain oil boiler. Two zones, one for the first floor and one for the second. I have central A/C with the air handler in the attic and the condenser outside. The A/C also has two zones, one for each floor.

I have two thermostats, one on each floor, with OFF, HEAT, COOL, FAN ON, FAN AUTO options. Each thermostat controls both heating and cooling. My first floor thermostat is currently a Honeywell T8775X (digital round, non-programmable). My second floor thermostat is a Honeywell CT87 (classic round, non-programmable).

Images of thermostat wiring, boiler, and A/C

Please see included images of A/C, boiler, and thermostat wiring in link above.

I plan on installing two Ecobee3, one for each zone. What would be the best way to connect the Ecobee3 - to the A/C or to the furnace? Is it ok to connect both thermostats to one system (i.e., both to the A/C or both to the furnace)?


  • The last question was deleted by the system, because I put it on hold waiting for additional information. It looks like you've provided that information now, so this question is now answerable. For future reference, the site works best with Imgur for images. So if you upload the images to Imgur, or directly to this site, that makes viewing them much easier. – Tester101 Aug 26 '16 at 18:36
  • I'll sign up with Imgur for future use. Stack's 2mb limit is tough to get under now... – achao Aug 26 '16 at 20:21
  • I posted a question and answer, that should help if you need to install a second transformer. – Tester101 Aug 27 '16 at 11:53
  • I also found the documentation for the zone control board. It appears to have a 2.5 ampere thermal breaker built in, to protect the thermostat and damper field wiring. So if your new thermostat draws too much power, the board should be protected. It says that if the breaker trips, all the LEDs on the board will shut off. If tripped, the breaker should automatically reset, once the it cools back down. – Tester101 Aug 27 '16 at 11:56

Looks like there's an extra blue wire in each of the thermostat cables at the zone controller. Simply connect the blue wire from the thermostat cable, to the C terminal of each ZONE# T'STAT terminal block. Then connect the blue wire to the C terminal on the new thermostats. This should give you power to the thermostats.

enter image description here

However, since it looks like it's using the transformer from the air handler to power the zone controller. You'll likely have to also install a jumper from the C terminal on the 24VAC Transformer terminal block, to the C terminal on the System terminal block.

enter image description here

Depending on the transformer that's in the air handler, the additional load of the thermostats might be too much for it. So make sure the secondary side of the transformer is fuse protected. If it is, and the fuse blows. Then you might have to upgrade the transformer, or use a second transformer to power the zone controller and dampers.

WARNING: If the transformer is NOT fuse protected, and you draw too much current. You will end up damaging the transformer.

  • Thanks for the directions and clear photos! A few follow ups: 1. If I try connecting one common wire for now to test out one thermostat, that should be fine? 2. What do I use to create a jumper from the 24VAC Transformer terminal to the System terminal? Would a random piece of wire (same type) work? 3. How do I determine if the transformer is fuse protected? 4. If I have to go with a transformer upgrade or a 2nd transformer, how would that be installed? – achao Aug 26 '16 at 20:21
  • Yes, you can connect one new thermostat as a test. A piece of the same size or larger wire can be used as the jumper. I'd use white if you had it, just for consistency. You'll have to check the schematic, or look at the control board in the air handler, to determine if the transformer has a fuse. I'll add the steps to using a second transformer to the answer, but you'll have to wait until later tonight, or tomorrow. – Tester101 Aug 26 '16 at 22:45
  • No rush at all! I appreciate the help. This may very well have saved me from hiring an electrician to do unnecessary work. Plus I have no plans to go into an attic when it was almost 90F today. – achao Aug 26 '16 at 23:26
  • I connected the zone 1 common to the board and fished it out of the wall at the tstat end. Installed ecobee with no issues! Reading your question and answer to see if that falls within my DIY capabilities – achao Aug 28 '16 at 14:34
  • This is likely off topic, but related specifically to my question. Are volt-amperes additive? That is, if I have a 24v 40va transformer, and I know the thermostat uses at most 3.5 VA, does that mean I have 36.5VA left? – achao Oct 14 '16 at 7:48

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