I have a gravity flow furnace currently connected by 2 wires to a battery powered t-stat. Due to the home / location of the t-stat, I want to upgrade to the Honeywell YTH5320R1017, which requires a "c" wire.

Question: can I wire a 24v transformer so the common wire goes to C on the t-stat, the other wire from the transformer goes to the R slot and then the 2 wires from the furnace would wire to W and R (so there would be 2 wires into R)? Would I need a 24v relay switch?

The YTH5320R1017 is a bit expensive and I don't want to fry it by connecting the 24vac incorrectly. Thanks!

Anyone have any input on this? Seems like it should be intuitive but I don't want to fry the t-stat!