2 and 3 wire thermostats

Derek Brazier

Could some one explain why some installations require 3 wire thermostats.
Thanks

Mark Eatherton1

Some zone valves

require power to close. They require power to open AND to close. This requires three wires. An OPEN wire, CLOSE wire and a COMMON wire.

Most thermostats currently on the market today are a spring closed type of valve, and therefore only require power to open. Once the power is taken away fron the valve, a spring causes the valve to close.

In both of these cases, we're just talking about the wires between the thermostat and the valve. If were talking about zone valves with end switches, then we're talking about two additional wires...

Hope that helps clarify your question.

ME

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Big Ed_2

If we go back to the old days.......

............three wire thermostats uesd in the MH series 10 controls used the 3rd wire to power the heat anticipator. W and B was TT and R was the power for the anticipator. The anticipator heater was a ceramic disk which heated up within the thermostat . You had to change the disk to adjust the heat cycle. Just incase you run into one of these old systems or ever wondered what the "R" terminal was for...........This is another three wire thermostat

Glenn Harrison

Two stage heat

I worked on an old gravity hydronic system last year that had a forty plus year old gas conversion burner with a two stage gas valve on it controled by a two stage (3 wire) t-stat

Glenn Harrison Residential Service Tech

Althoff Industries Inc. Mechanical and Electrical Contractors

Crystal Lake, Illinois

Dale

Besides what Mark E. said

there are zone valves ( the auto mag is an example) that are powered closed so if the valve fails you still have heat to that zone if another zone calls. The valve is powered closed by the separate zone valve power source ( DC). The zone valve just has 2 wires and no end switch. So with no end switch how do you start the boiler or pump or in other words how do you close the TT terminals on the burner? You can do this with a 3 wire stat where if the call for heat opens R and W the W the out wire to the valve ( white) loses power and the valve opens and the stat switch wires R and B which are connected to TT close which starts the burner. The 24 volt potential is at TT. Sure this could be done with an end switch closing TT but it adds complexity to the valve.