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Weird problem with smart thermostats and multi-zone furnace controller

Hi and thanks for reading this. I hope it makes sense to someone, because it doesn’t to me.

I’m seeing a problem that I can’t quite wrap my head around. I have a Taco SR503 3 zone controller attached to my furnace and it is set up for 2 zones in my house. Until recently I had two Honeywell 2 wire thermostats with 1 assigned to each zone. They both worked properly. 

Recently I replaced them with a pair of Amazon Smart Thermostats. Both are wired to their respective zones with R,W,and C wires (brand new 18/5 only using 3 wires). The thermometer function works well in each zone, and my Zone 2 (upstairs/master bedroom) thermostat will activate and heat the zone as it is supposed to. However, while my Zone 1 thermostat appears to come on when it is activated (downstairs/dining room), it also delivers heat to my upstairs zone. My downstairs zone gets no heat or very little heat. As a result, the second floor of my house gets to be 7 or 8 degrees warmer than the first floor.

What I’m trying to figure out is why this is happening with these thermostats, and I’m pretty perplexed. In the past 8 months I’ve tried this with a pair of Nest thermostats and had some significant problems, and I replaced the controller on the furnace thinking that maybe the original controller got damaged somehow. At this point I’m not convinced the original controller was a problem, but with the current controller having a Common terminal that is available makes it much easier to work with than the original, which didn’t have one.

The problems with the Nest thermostats were actually worse because I couldn’t get them to function at all, even with new wiring. The Amazon thermostats, as I said, do appear to be working properly. The controller wiring is pretty straightforward, and both C wires are connected to the Common terminal on the controller board. When I activate either thermostat the appropriate zone for whichever thermostat I activate lights up on the controller board, which makes this a little more weird.

Is it possible I’m missing something as far as the wiring is concerned? Or could it be a problem outside of the controller? I’m honestly not sure where else to look, and any insight would be really appreciated.

Thanks very much in advance.


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    So. When you activate the downstairs thermostat, the downstairs light on the board comes on, but NOT the light for the upstairs zone? But, I take it, the upstairs zone valve opens, not the downstairs one?

    Is that a useful snapshot?

    If so, the very first thing I would do is double and triple check all your wiring. Do not depend on the colour of the wire -- the electricity doesn't care. Make use that each individual wire goes exactly where it is supposed to go.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,166
    edited October 2022
    Did a professional do the control wiring for the Nest thermostats or the current amazon thermostats or both, or is this all DIY wiring?

    Next can you take pictures of the wires inside the Taco ZCR box, and the wires on the thermostats so I can see what might be the cause? example of thermostat wiring picture.
    When using only 2 wires from a thermostat the R and the W are interchangeable.
    When a C wire is required, the R and the W are not interchangeable. The proper placing of the R and the W becomes important.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • pedmec
    pedmec Member Posts: 998
    Sounds like the second floor zone damper might be open all the time. Shaft could have come lose from the zone actuator. I mean if all you did was change the thermostats can't see any reason why the zone would be opened when its not calling. Zone dampers are designed to be forced closed when a call is initiated. so when the first floor zone calls for heat the second floor zone should be powered closed. Newer zone actuators have green and red lights on the actuators indication open(green) or closed(red). Not all zone actuators have lights as older models wont have them.

    I would just check the zone dampers to see if they move before condemning the wiring only because you want to eliminate the obvious. It would suck to keep going after the thermostat without realizing a zone damper motor failed. Not saying it couldn't be the thermostats but it only takes a minute to check.

    *Just reread your post and i just want to clarify. you said a furnace when you started the post so my response is with a hot air system with hot water coils. Or are you talking about just a zoned baseboard system with a boiler because my response will leave you confused. Sorry if it doesn't apply as i realized it after offering but just in case it does i'll leave the post.