Rather than play with a new thermostat, consider a new boiler. That old system is using 20% to 40% more fuel than a new system depending weather it's using oil or gas.

Gas systems can be as efficient as 93% and a good oil system about 86%. With energy prices where they are, payback can be in about 5 years.

With a high efficiency boiler consider an indirect fired water heater. Your stand-by losses will be reduced as the boiler will only get hot as demand requires.

The boiler routinely clocks out at 77% efficiency, and has since Mom and Dad moved into the place in 1999.

Installing a new boiler and high efficiency water heating system and lining the 100-year-old, nearly 50-foot high chimney isn't an option.

Even punching up to an 87% efficient oil boiler with direct venting isn't an option.

77% is combustion efficiency. Stand-by losses should be your concern. Your boiler is maintaining water temperature on an aqua-stat. Air from the basement is passing through the boiler and up the chimney carrying heat along with it. The aqua-stat does not care if the heat goes into your home or up the chimney all it does is maintain the water temperature. AFUE takes into consideration stand-by losses in their calculation. Like I said earlier, new boilers hold very little water to minimize stand-by losses.

You do not have to line the chimney if you powervent the boiler out the side of the house and abandon the chimney.

More than happy to accept donations.

More on topic: Were the other two zones added later, or possibly wired differently by mistake? My old boiler was wired with the thermostats controlling only the zone valves. An inspector mentioned that it could be wired to go cold and stop the pump while the thermostat isn't calling for heat. Maybe you could wire all three to match?

Don't want to wire all three to match. It would require pulling new wiring two two zones and would require finding thermostats that would do it.

An easier solution would be to remove the burner control wire and cap it off. It would do essentially the same thing, I think.

The main zone and the front zone were added at the same time when the house was converted to forced hot water in the 1950s.

The back zone, for the kitchen only, was added in 2003 when Mom and Dad redid the kitchen.

I don't know exactly why my Grandfather (Dad and Grandpa installed the boiler in 1954ish and as far as I know did the wiring) wired the front zone differently from the main zone, but I suspect it might have something do do with the fact that the front zone controls rooms that he and Grandma really never used unless there was company.

What I'm finding to be odd is that no one I've talked to in the HVAC community has ever seen an EL terminal on an R845A controller.

I'm going to try calling Honeywell and see what they have to say.

Is there any chance the terminal that you believe is "EL", might be a worn "B1"? (burner control)

Hum...

I suppose it could be, but it seemed to be marked very clearly EL.

I also don't see why it would be burner control when that's normally one of the two T terminals.

I was looking at the R845A diagram and it seemed to have B1 & B2 terminals. .. maybe they were burner line voltage.

See figure 7. ... toward bottom.

Here is a link that might be useful: R845A diagram