This system is from the 60's era - the furnace has a two T connectors for the R and W (thermostat) and two F connectors - one labeled "Thermostat Load 24VAC 0.2AMP, the other for the Burner Motor Ignition. I want to use a Honeywell Sensi wifi thermostat and have no problem running a new wire to the thermostat for "C", but where would I connect it at the furnace considering the limited connectors (T T F F)? Thanks!
This is an interesting case since your oil burner control integrates the 24VAC control transformer. Normally, you'd look for either a C terminal on the control, or the control transformer itself. However, your control lacks a C terminal. What it does have, though, according to the internal schematics from Honeywell's manual, is one of the F terminals for the flame safety photodetector (CdS cell) that goes back to the opposite side of the transformer from the upper T terminal that goes to the red wire.
You'll need a multimeter on continuity (or its lowest ohms range if you don't have a continuity buzzer on your meter) mode for this procedure.
- Turn the service switch or disconnect OFF before starting to avoid damage to parts (it may be a switch on the unit, or you may need to use the circuit breaker to disable it).
- Put one meter lead on the upper T terminal (the one with the red wire on it) and the other on each F terminal in turn. The one that beeps (or reads low ohms) is equivalent to the C terminal, and can be used for a C wire connection. The other is how the CdS cell connects to the internal circuitry, and will not cause your meter to beep (or reads a high resistance on the meter).
- Connect the C wire to the correct terminal (it should be the bottom terminal according to the manual).
- Hook up your new thermostat.
- Turn the service switch or disconnect back ON.
- Enjoy the new thermostat!