The old thermostat is a Lennox t-90 mercury thermostat. It has 4 wires with an ac unit and a heating unit. There are no heat pumps on the system.
The old t-stat Lennox T-90 has 5 terminals labeled G,F,C,H,and V. It has a jumper wire between the G and F.
New thermostat is a Honeywell RTH230b. It has Rc, Rh, (with jumper), W, Y, and G terminals. The instructions do not list the terminals on my old t-stat.
Questions are With the old terminal designations, where do they match on the new terminal designation?

I have done this before, but not replacing such an old t-stat (circa 1968). Not sure which wires go where.

This link (https://www.askmehelpdesk.com/advice/t-39609.html) may be what you need.

Good Luck,
WallyH

I know this post is old, but I just replaced this same unit successfully, so I figured I'd get some karma out of it and make it easy for anyone else looking.

Just to clarify, it's an ITT General Controls" thermostat, model T-90H110A137, 24 volts max, 0.8 AMP. I have a forced air heat and cooling system, which is also Lennox.

The main issue is the letters/labeling for each terminal does match "modern" standards. In addition, my wiring did match the colors either, so it was a combination of research and guesswork to get it to work. (Two disclosures: I'm not a professional as this is the first thermostat I've replaced and I changed this is March, so I didn't run the A/C for long.)

You can probably ignore the wire colors (in parentheses) but I included them just in case.
Old Lennox - (Wire color) - New Thermostat - [Function/what it does]
G - (Orange) - C - [Ground]
F - (Blue) - G - [Fan]
C - (White) - Y - [Cooling/Compressor]
H - (Green) - W - [Heat]
V - (Red) - RcRh - [Power/Voltage]

Looking at the old labeling, it makes a lot more sense since the letters actual mean something.