I have an old Gaffers and Sattler mercury 2 wire ( red and white wires) thermostat.Can I convert it to a programmable multi-wire system by hooking the red wire to R or Rh and the white wire to W1?

Okay, I told you I couldn't help, but hubby might be able to.

He would like to know what brand and model of programmable thermostat you're trying to hook up.

Okay, hubby re-read your question, and said "yes, this sounds right. But, if it isn't, trial and error. It's not going to hurt the system. It either runs when you do it and you know it worked, or it doesn't run and you know it didn't. You won't hurt the system by trying this. But, having said that, what you posted does sound right".

Disclaimer: This is hubby's advice, and all you experts that may disagree (heck you probably won't because hubby is actually really knowledgeable about this stuff, but if you do disagree) this was hubby, so don't negative rep the messenger! ;)

A 2 wire is heat or air only. For a multi wire system, you will need to install new thermostat wire and make sure the voltage on the transformer is correct for the thermostat. Some older systems 2 wire used 12 or 16 volt supply. Almost all new units use 24 volts.