Ecobee premium wiring from old honeywell

If you need a C wire, them you might want to get this

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Carlin-Combustion-98435S-Product-Overview.pdf

or https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Beckett-51950U-User-Guide.pdf

Shame to replace that "outdated" thermostat. It's more reliable — and precise — than anything you can buy today. A lot simpler, too.

I like the Carlin unit that @EdTheHeaterMan suggested, but there's nothing wrong with the other one.

Frankly if it were mine I'd replace the whole burner control with a Carlin 70200 control — a lot more functionality. But that's me. It I were going to do it at all — which I wouldn't, since I wouldn't replace that old thermostat unless I were paid — well — to.

Would this wiring diagram help (it worked for me)?

YES. But why don't you disconnect all the wires at both ends and use the Red wire on the R at the thermostat, use the White wire on the W on the thermostat and use the Blue (or Black hard to tell) for the C on the thermostat. Electricity is colorblind, so make it easy for the next time so the humans don't get confused.

Do you know where to connect the C from thermostat?

You do not have the R8184G primary control, so that will not work.

You have only T T available on the GeniSys control. The F F that is inside the box is not wired up internally the same way as the analogue R8184G. You can not use the transfromer in the GeniSys control to power that thermostat.

The first diagram you sent was correct. and you need a 20 VA or better transformer to power the isolation relay.

You may find the Kool Kit or the A C ready kit less transformer is less expensive, the nyou can purchase a lower cost 24 Volt 20VA transformer to operate it

Better yet. As I suggested above, "new" is not always "better". Keep that old mercury beauty and relax. Oh yes. It is permissible to turn those old thermostats up or down as wanted by hand…

I like the Honeywell 70200. It is not all that hard to wire to a different burner — but it isn't plug and play either. You do have to have some idea what you are doing, and why you want to program it, since it gives you a variety of options.

I'd still keep the old thermostat, though, unless you have a good reason other than programmability or a modern look to change.

You will need to get this control. https://www.supplyhouse.com/Packard-FC90113-Fan-Control-Center-120-VAC-Primary-24-VAC-Secondary-SPDT-Relay It is the lowest cost control of its kind. You will also need to get a https://www.supplyhouse.com/DiversiTech-PI362-4-L-x-4-W-x-1-1-2-D-Galvanized-Steel-Utility-Box-w-3-4-and-1-2-Knockouts?_br_psugg_q=1900+box to put it on.

Here is the diagram:

You can also do it with these two items that you can add to an existing electric box with 1/2" electric knock outs available.https://www.supplyhouse.com/Functional-Devices-TR20VA001-Transformer-20VA-120-to-24-Vac-Foot-and-Single-Threaded-Hub-Mount and https://www.supplyhouse.com/Functional-Devices-RIBU1C-Enclosed-Pilot-Relay-10-Amp-SPDT-w-10-30-Vac-DC-120-Vac-Coil?_br_psugg_q=rib+relay.

Here is that diagram

Both options are under $45.00 at SupplyHouse.com