C wire question/ Wiring transformer

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Old 10-23-20, 12:44 PM
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C wire question/ Wiring transformer

I want to install two Honeywell Smart Thermostats. My existing thermostat has only 3 wires going to it, two are used for RH and White the third is free. I am only using this for heat, AC not normally useful in Alaska. I bought a new Transformer but I don't want to hook it up at the thermostat end but in the furnace room. My plumber told me I could hook one side of the new transformer to the free wire going to the C connector on the thermostat end and the other side to the white side of my existing transformer in the furnace room. I just want to verify that that will work. Also, will my new transformer (24VAC/500 milliamp) power two Smart Thermostats? Any help is appreciated.
 
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Old 10-23-20, 12:48 PM
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Welcome to the forums.

What are the thermostats controlling ? Zone valves.
It may be a better idea to increase the transformer size here and use the for C.

Connecting one half of each transformers is occasionally done but doesn't always have the expected results.
If the transformers aren't connected "in-phase" you could technically see 48 volts at the thermostat.
A voltmeter could be used to check for the correct voltages.

24VAC @ 500 milliamp = 12 watts of power. 6 watts per thermostat could be on the low side.
 
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Old 10-23-20, 01:02 PM
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Thanks for your reply. They are controlling zone valves on a heating oil boiler. Not quite clear on your response but I do have electrical training so I can muddle through. How would I check to see if it's in phase or not? Would I check at the thermostat end for 24VAC across the C wire from the new transformer to the White or RH at the thermostat end?
 
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Old 10-23-20, 02:06 PM
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I left you two diagrams. If you add the second transformer.... check from R to C at the stat end and from R to W. Both should be close to 24vAC.


 
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Old 10-23-20, 02:14 PM
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Great, thanks. My plumber told me to hook one leg to the white on old transformer but what you diagrammed makes more sense. Do you think my existing transformer can power both new thermostats or at least one of them? That seems to be the easiest way to provide 24VAC.
 
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Old 10-23-20, 02:57 PM
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The size of the transformer should be stamped on it.
It'll be 24vac and ?? va.

I used arbitrary wire colors but red is typically R/24v and white is W.
Since we're working with AC there isn't a polarity on the transformer. I
 
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Old 10-23-20, 03:12 PM
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Transformer says 25V, 40VA. I measure 26.8 volts across it.
 
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Old 10-23-20, 04:34 PM
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That is large enough for at least one stat and possibly both depending on which ones you use.
 
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Old 10-23-20, 05:27 PM
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Ok, thank you. I am using the Honeywell MN- RTH6580WF. I put a bit of effort including calling Honeywell to find out the current draw for it with no luck. Their Tech support was useless trying to find out any real information about my particular setup for a C wire and Electrical Specs. I really appreciate your help.
 
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Old 10-23-20, 09:55 PM
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Since Honeywell is no longer king of the thermostat hill..... tech support seems to be lacking on their end.

The stat specs state .04w standby current. That would seem to be the least amount of power it requires. I would figure on 4 watts max operating current per thermostat.

Stat tech specs
 
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Old 10-24-20, 04:31 PM
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Something is not right. I hooked it all up like you said, nothing. Tried reversing the wire to the transformer to the other side, nothing. The way I figure it I need to have 24vac between C and W, correct?. Which I did have. Just to say in case it matters, the R from the Transformer is going to the White wires to the Thermostats, The White is going to the Zone Valves. Seems to me it's AC and shouldn't matter.
I then hooked up the new transformer as you said, nothing. So I tried hooking up the transformer direct, the instructions for it say to hook one side to RC and the other to C. That powered it up. My question is this, the Thermostat came with RH jumpered to RC, I removed that because I thought I'd read that you should. Should I leave that jumper in?
 
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Old 10-24-20, 04:55 PM
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Try removing the R jumper. You'll use Rh only.
It's hard for me to diagnose by colors.
One wire from the transformer goes directly to the stats..... no where else. That is R.
One wire from the transformer goes to both zone valves.... that is C.
 
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Old 10-24-20, 05:27 PM
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Ok, I had already removed the jumper from RH to RC, I'll try it again with the jumper in. I'm a little confused because the one wire from the transformer goes directly to the white wires which go to all four thermostats. The wire from the ZV goes to the red wire and up to the thermostat. Sorry to be such a pain. I really appreciate your help.
 
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Old 10-24-20, 08:36 PM
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Your not being a pain. When just two wires are used..... there is no need to connect red and white a certain way. When you introduce a third wire.... the C wire..... the red must carry the 24v from the transformer to the R terminal and the W terminal must go to the zone valve. If there are Rc and Rh terminals.... use Rh as you have heat only. The Rc supports cooling functions and tells the stat to get C from the A/C unit.

It appears your colors are backwards. It may be advisable to change them at this time.
 
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Old 10-26-20, 09:20 AM
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Last winter I went through and did something very similar; I replaced two dumb stats with two ecobees to control two hydronic zones separated by zone valves. I opted to swap out the old transformer (which originally powered just the valves) for a larger transformer to handle both valves and stats. Here is my original post, which may be a useful resource.

I am not sure what type of ZV you have, but typically they have three or four terminals. In my somewhat limited experience, all zone valves have two switches in them. One that is engaged by your stat and opens the valve for water flow. The other is an end switch that opens after switch 1 engages (which then sends signal to boiler). In three terminal valves, the two switches share a common. In four terminal valves, they have their own common terminals. Whichever, you have, you only need to mess around with switch 1.

Hot (red) comes from the transformer and goes to Rh/Rc on your stat. Common off of the transformer goes to "C" on your stat and terminal "2" (note, terminal numbering is arbitrary, check your install sheet for the ZV to confirm). "W" on your stat goes to terminal 1 on your ZV.

Here is the work flow: current from your transformer goes to Rc/Rh. The closed circuit back to the transformer common terminal via "C" powers your stat. When stat calls for heat, it contacts the "Rc/Rh" hot terminal to "W". This sends current to terminal 1 on your ZV, which subsequently has a closed circuit back to the common terminal on the transformer via terminal 2 on the ZV. This energizes switch 1 at the valve, which opens the valve. Once switch 1 is fully closed, it then energizes switch 2, which turns on boiler/circ etc.

It is possible you have zone valves that don't operate the way I described above. Model numbers can help us confirm that.

Good luck.
 
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Old 10-26-20, 12:06 PM
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PJMax... I wired it all up again this time with the jumper between R and RC installed and it fired up and looked good. Setup was ok but as soon as the thermostat called for heat it started flickering on and off. So I gave up with all that and figured out a way to run wires from the new transformer to both thermostats and all ran well. FYI, the new thermostat rated at 500ma runs both thermostats without a problem, so far. Again I appreciate all your help.
 
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Old 10-26-20, 12:19 PM
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Post deleted as problem was solved.
Geo🇺🇸
 
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