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Heating & Air Conditioning The Old House Electrical Wiring

older home ... no A/C just a two wire stat can I put a 3 wire stat in

3 years ago

I just have a two zone baseboard heat no a/c ... I want to put in a newer type of thermostat but it looks like it needs a third wire. I was gifted two units both don't accept batteries. what can I do to make them work. Any help here would be great thanks.

Al

Featured Answer

mike_home

3 years ago

The first thing I recommend people do is to turn off the learning mode of a thermostat regardless of the heating system. The occupants of the house are smarter than the thermostat. I am a firm believer of setting back the temperature at night or when no one is home. We can review this once the thermostat is installed.

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Comments (6)

mike_home
3 years ago
What is the brand and model number of the thermostats? Do you have a boiler with hot water baseboard heat?
BT
3 years ago
Could this be an option

https://www.amazon.com/Venstar-ACC0436-2-Wire-24VAC-Thermostats/dp/B00755BZZC

You have to power it, from an outlet
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big_al_41
Original Author
3 years ago
Model # is a Honeywell RTH9585WF and it is a boiler with baseboard heat
mike_home
3 years ago
last modified: 3 years ago
This thermostat needs a C wire connection. If you can't run a third wired back to the boiler, then the other option is to provide the power from a 24V transformer.

The Venstar-ACC0436-2 would work but it comes with a relay which I don't think you need. The Fyve 24 V transformer C wire adaptor should work. You need to figure out which of the two wires is the W wire. It is usually the white one. The Rc and C would be connected to the transformer. The other wire that was going to the thermostat (usually black) is connected to Rh with the jumper to Rc removed. You would need to confirm this with Honeywell customer support. They should be able to walk you through the installation.
kats737
3 years ago
Check out this info and video here.

https://www.honeywellhome.com/us/en/support/everything-you-need-to-know-about-a-c-wire/

We replaced our traditional digital thermostat with a moderately smart learning thermostat (not wifi) for our hot water radiators boiler. One thing we didn't know is it defaulted to a setting where it 'learns' how long it takes for your room to come to temperature and then when it runs the programs, it takes that into consideration. This resulted in our boiler being on A LOT because it takes some time to come up from the nighttime tempearature. So we had to change that setting at some point - be aware.

I think smart/learning thermostats are great but you lose a lot of the functionality with a boiler system just because you generally shouldn't have as big of a swing in temp. I can see the advantage of monitoring your temp via wi-fi, especially if you leave regularly/have pets/etc... But if you leave during the day, it's probably more efficient to keep your overnight temp something moderate and just leave it there, than it is to drop low overnight, and have it run for an hour while you get ready in the morning.

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