Join Date: Feb 2010
Location: Leesburg Virginia
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If I had to guess it is probably the fan turning on and off that is causing the noise versus the t-stat switch itself. I think you probably have 1 of 2 things going on.
1. the fan and the intercom are on the same circuit? If so then I would try to run a dedicated circuit to the fan (Same thought if you are using the same neutral for the 2 circuits)
2. the fan is on the same "phase" as the intercom system, if you look at your panel you should have 2 "hot" (each one is a phase) wires in it, they alternate breakers as they go through your panel, Circuits 1&2 are on phase 1 or A, 3&4 B, 5&6 A, 7&8 B and so on, sometimes if they are on the same phase you can get noise. (Just move the circuit for the fan to the alternate phase to see if that helps any.
Just thought of another possible issue, your intercom system, does it have its own wires from each location (either Cat-5 or other data cables) does it work through RF or does it send the signal through your electrical lines?