DIY Doctor

Old Honeywell Two Wire ThermoStat to New Thermostat

Postby anav_ds » Tue Sep 18, 2018 12:06 pm

Scenario. Old two wire thermostat Honeywell (R and W wires), the R was hooked up to 24v transformer**, W connected to one of the yellow wires going to the motor on the zone valve, and the other yellow wire attached to the zone valve motor goes to the other side of the transformer**. For those that don't know, the circuit is open (no juice) until the room temp is lower than the thermostat setting. If so, the thermostat closes the circuit (connects the R and W wire allowing juice to flow. This in turn cause the valve motor to open (and when in the complete open position further closes another the circuit to the pump calling for it to turn on) - We have a Multizone system.
Bought a lyric T5 honeywell. Stated I needed a Common Wire for 24 volt power for the unit itself. Luckily I thought, I found a folded back third wire I could use as common. I attached this wire in the propane boiler room to a transformer thinking aha, 24v power. Apparently I was wrong just like the R and W loop (2 wires), I would need a fourth wire to complete a loop for power through the common wire???

Hence my dilemma.
Possible solution is perhaps tap off at the thermostat the 24v R wire and jump this to the C wire attached to the other side of the thermostat thus creating a live circuit. This would have no effect on the zone valve because the thermostat doesnt connect R and W until there is a call for heat (temp lower than thermostat setting).

Is this possible, safe credible, or am I hosed with 3 wires???
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Postby Mr White » Tue Sep 18, 2018 6:45 pm

Without an actual drawing it is hard to be definitive.

But from your description I can not see why a 4th wire is required, since you need 24v to power your new stat, and the 3rd wire you have is the switch wire.

Using your old stat as an example:
Although you only have 2 wires, one is supply (Let us say +ve), the other is switched, and the one you are missing is the supply -ve.

So if everything is 24v as you say it is, then there is no need for 4 wires.
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Postby ericmark » Wed Sep 19, 2018 1:08 pm

There are two Honeywell Lyric T6 thermostats T6 and T6R you state T6 which is a two unit device, unit one will be mounted where you have the motorised valve and has 12 connections, some will be unused so you must have 6 wires connected line, neutral, pair to motorised valve (24 volt in your case) and pair to actual thermostat, you say you have two wires to thermostat so what is the problem.

The T6R needs 4 wires to base unit and is wireless to actual thermostat so as name suggests no wires.

The contacts in the base unit are volt free, so the 230 volt supply can be completely independent to 24 volt, in theory you should have a sticker to say two independent supplies are used, but never seen any one bother with them.

So what is the problem I have clearly missed some thing.
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