Wednesday, November 24, 2010

How does a 2 wire 24 volt ac electronic thermostat work to control a gas fired furnace ?

Have moved into a house where previous owner replaced simple mechanical type thermostat (with mercury bulb) with an electronic unit. Mechanical type obviously just closes a contact to complete the gas valve circuit and so get 24V across gas valve coil.

Obviously 2 wire electronic unit cannot do this because the unit needs to draw power for the electronics.

The new thermostat is giving unreliable operation, am thinking of replacing it with the old unit which I found and appears to be functional (checked with multimeter).How does a 2 wire 24 volt ac electronic thermostat work to control a gas fired furnace ?you are correct, the electronic tstat needs to be powered, some cheaper thermostats will operate on 2 AA batteries(junk in my opinion) or the other will actually be powered from the 24 volts on the unit transformer, verify that its wired correctly, is this just a furnace or is there also a cooling unit with this system?



heat only:

you should have 3 wires, one wire connected between Rh on unit and Rh on stat, one wire between C on unit and C on stat, and one wire between W on unit and W on stat.



the wires to Rh and C will power the stat, W is the open contact on the stat that will close when a call for heat has occured.How does a 2 wire 24 volt ac electronic thermostat work to control a gas fired furnace ?An electronic tstat works the same way a mercury bulb tstat works. A tstat is just a switch. The only difference is that a mercury bulb tstat uses mercury to complete an electric circuit. Voltage passes through safety limit switches to one side of the gas valve through the tstat mercury and back down to energize the gas valve.



On an electronic tstat the current still travels as stated above however, the %26quot;switch%26quot; is completed using a printed circuit board.



For the most part electronic tstats are considered morefficient, but you should chanfe the tstat if you're not happy with its operation.
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