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Category: HVAC
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Experience:  15yrs in the HVAC industry with certificates and degrees.
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I'm trying to wire a Sensi wifi thermostat (heat only) to my

Customer Question

I'm trying to wire a Sensi wifi thermostat (heat only) to my Viessmann vitodens 100W wall mounted gas fired boiler. Currently, the old thermostat I'm replacing (round, heat only, Honeywell) connects with a red and a white wire. The new Sensi, for heat only purposes, requires a c wire connection as well. On the thermostat side, there is an unused wire that I believe can be used as a c wire, and I understand how to connect these 3 wires
I assume that the c wire connection must also be connected at the boiler in order for the thermostat to be operational. My question to you is, which terminal at the boiler should I use to make this c wire connection?
Thank you for your attention to this question!
Submitted: 13 days ago.
Category: HVAC
Expert:  HVAC Guru replied 13 days ago.

Hello, one minute while I get some information together.

Expert:  HVAC Guru replied 13 days ago.

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You need to connect that wire to the COMMON side of the low voltage transformer. That boiler does not have a terminal to connect that to. You would have to get a low voltage transformer to supply the 24 volts to your thermostat separate as you can not do it from the boiler itself.

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Customer: replied 13 days ago.
How and where would I connect the low voltage transformer?
Expert:  HVAC Guru replied 13 days ago.

First what thermostat were you previously using? A battery one? or mechanical?

Customer: replied 13 days ago.
Expert:  HVAC Guru replied 13 days ago.

Yes then I would either use a battery one as you will need two wires to run the powered unit.

Customer: replied 13 days ago.
Do you mean a battery powered thermostat? Because I was trying to avoid that because the batteries would be an unreliable power source while we are not at home.
Expert:  HVAC Guru replied 13 days ago.

Yes a battery powered unit, I use one and have never had an issue that would be the easiest route to go honestly

Customer: replied 13 days ago.
Is your thermostat wifi? And if so, how long do batteries last? We were told only a couple of weeks with wifi.
Expert:  HVAC Guru replied 13 days ago.

Yes they do have to be recharged more, or you will have to get a low voltage transformer and wire that in. You just need to run 120 volts to the transformer then 2 wires to the thermostat from the transformer then to do it.

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