Older furnace + new wifi thermostat

[Theodore]

Hi,
Our community center has an older oil burner and furnace (forced hot air only, no air conditioner). The burner has two "T" terminals on it that run to a mercury switch thermostat. We want to upgrade to a wifi-enabled programmable thermostat. We already bought the 24vAC transformer for the "common" wire we have to add, and have run a new thermostat wire (5C-18AWG), and will properly dispose of the mercury thermostate. But I've also read that we have to add a relay to protect the existing cad cell relay (on the burner assembly). This is beyond my amatuer capabilities and we're waiting on our HVAC professional to complete the install.

Just curious if anyone can recommend an online tutorial or video or document that explains all of this? i.e. the exact type of relay, wiring diagram etc? I want to understand what will need to be done, and why.
Thanks for any guidance.
Theodore.

 

[Jadnashua]

Look up isolation relay. RIght now, your existing thermostat is just acting like a simple switch. It sounds like your new thermostat requires 24vac to run it. So, it will still act like a switch, but now would output 24vac, which may (or may not) be compatible with the voltage requirements in the old system. The simple way to handle that is to take the two wires that now go to the old thermostat and put them on the normally open and common terminal of an insolation relay. Then, hook one side of the output of the new 24vac transformer, and the switched lead from the new thermostat, and when that 'closes' it would be connected to the hot side of the relay coil, it would then then energize that relay, connecting the two old thermostat wires together, as if it were the old thermostat.

IOW, connect the existing thermostat wires to the relay contacts so that they replace the original thermostat as a switch. Then, use the new thermostat to control the coil of the relay, when there's a call for heat, the thermostat applies power so that the new relay will close, emulating the old thermostat's switching action.