Two thermostats to One

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Old 12-10-20, 11:00 AM
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Two thermostats to One

Would it be reasonable to replace two existing old low voltage thermostats that are right next to each other win one?
Currently I have two old heat only Honeywell mercury thermostats next to each other. Each controls a single hot water baseboard unit, which run next to each other on the same wall. These are both low voltage thermostats. I'd like to simply plug both R wires & both W wires into the same terminals on a new thermostat. Obviously I would lose individual control over each base board but always use them simultaneously anyway.
 
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Old 12-10-20, 12:21 PM
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Theory is that both thermostats go to the same boiler system with same transformer. If so, yes, connecting both of the W's to R will trigger both baseboard units. Issue is that many times the R and W wires get swapped by the time they get to the thermostat so needs verified. Get a multi-meter and measure the voltage between the Rs on each thermostat. Should be zero. Test voltage between Ws and also should be zero. Then connect the Ws together and verify the voltage between the Rs is still zero.
 
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Old 12-10-20, 01:19 PM
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Got it. Will give it a shot and see. Thank you
 
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Old 12-10-20, 05:50 PM
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Using a standard thermostat like a Honeywell would be no problem. Most wifi and electronic thermostats will be hard pressed to run two zone valves. You need a thermostat that uses a tried and true relay for switching,
 
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Old 12-11-20, 07:43 AM
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I was hoping to use the same Honeywell Wifi thermostat I used elsewhere. Are you saying you don't think this Honeywell wifi thermostat would be capable of running two zone valves?

I suppose I can try it and see. If I don't get heat I can just convert back
 
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Old 12-11-20, 11:35 AM
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If the transformer is the same for both baseboards, you might be able to do some interesting things to keep them separated yet still controlled by the same thermostat.

If the thermostat lets you program as different system types, you could program the fan output to turn on when heat is called. In this case use one red on the R terminal, one white on the W terminal, and the 2nd whiteon the G terminal.

If the thermostat has two stage heat, drop one white on W1 and the second white on W2. Of course this means only one baseboard will turn on unless you either bump up the temp by more than 1-2 degrees or it is taking too long to heat the space.
 
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Old 12-11-20, 12:19 PM
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I know the new thermostat has a W1 & a W2 & believe it can handle two stage heat. So if I want to still control each baseboard separately are you saying I hook one white to W1 & the other white to W2 but attach both reds to the R terminal. Regarding the first part of your post not following exactly - what do you mean by different system types? Obviously there is no fan being that these are baseboard units - would this allow for individual control of baseboards?
 
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Old 12-12-20, 07:57 AM
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Normally a gas HVAC control boards will automatically turn on the blower fan when a call for heat is needed. If the system is a heat pump or electric furnace, sometimes the fan wire needs to be energized when there is a call for heat.

If you wire up as two stage keep in mind you won't exactly be able to control separately. You'll have one baseboard that is always going to turn on when heat is needed and a second that will periodically turn on.
 
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Old 12-12-20, 08:59 PM
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If that thermostat is just going to be running the heat...... connect one zone valve to W and connect the other one to G. When programming set the blower control to electric heat. That would mean that G becomes lives on a call for heat.
 
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Old 12-12-20, 09:45 PM
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Would both Red wires be connected to the same R terminal?
 
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Old 12-12-20, 11:03 PM
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I'm guessing that you have four wires. More than likely they share the same transformer.
What you can do is to put one set of wires on R or Rh and W. The other set on Rc and G.

The only problem is there is no C wire. How do you plan to power a wifi stat ?
 
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Old 12-13-20, 06:04 AM
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You are correct about only having four wires. I'm not sure what I would do for my C wire - I've had success using these 24v transformers you plug into an outlet on two other thermostats I installed. Perhaps I might try that especially because I have yet to locate where the zone valves are for these units. The only issue is the 24v transformers I have used require one of their wire to C and their second wire to RC & than removing the jumper on the new thermostat. In the setup you describe above you have one of the wires already going to the RC terminal. Can I share this terminal with that of the transformer?

With the set up you describe I assume both baseboards would come on simultaneously when I call for heat?

 
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Old 12-13-20, 08:01 AM
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Don't share your add-on 24VAC transformer with the boiler control one. If you have a WiFi stat and need continuous power, use the add-on 24VDC on RC and C - this will power the unit but not cross over to the boiler transformer. Don't install a jumper between Rc and Rh if doing this.
 
 

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