# Install New Wifi tstat with 3 wires



## Tailbent (6 mo ago)

I have a heat only boiler for under slab heating. The heating zones are now on old two wire mercury contact tstats. Luckily the original installer used 3 wire cabling but left the third wire unused.
If the needed C wire for a smart tstat is actually a 24 volt comon wire, could the unused third wire be connected to the C terminal in a new wifi tstat and connected to the 24 volt common at the boiler to supply 24v constant power to the wifi tstat?


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