I have two controllers in parallel to a bathroom fan: Honeywell HVC001 and DewStop FS-100. The DewStop functions properly but the Honeywell apparently resets its clock/timer whenever the DewStop powers the fan (otherwise the Honeywell operates as specified; the Honeywell control display will blank out while the DewStop is running the fan.) How should I connect these in parallel (or what do I add?) Honeywell's tech support says that they've no ideas or suggestions. While I could rewire these controllers so that only one controls the fan, both are required for this application (one be city code; the other by user specification.)
This is a somewhat difficult problem to solve. The issue is that the Honeywell controller uses current leakage through the load to power its internal circuitry. It relies on having 120V across it when it's not on to keep the clock and display running. When the other controller turns on, it backfeeds the load side of the Honeywell controller and thus eliminates the voltage across it, so it has no way to power itself. I assume the DewStop controller must have a neutral connection, an internal battery, or not keep time. The most obvious solution to me (the only feasible solution I can think of, in fact) is to use the Honeywell controller to operate a contactor. Wire the contactor's normally open contacts in parallel with the DewStop controller. The Honeywell controller will then operate by leaking current through the contactor's coil regardless of whether the DewStop is activated or not.