confusing thermostat

[John Arnold]

I can't figure out this thermostat. The system appears to be electric resistance only, and central AC. No heat pump. But what's with the dual mercury switches on the cool side?

[Jim Luttrall]

That indicates a dual stage cooling stat. Could be used for a two stage system just like having a primary and secondary heat source, there can be a primary and secondary cooling source. Pretty common on commercial, but rarely used on residential.

[John Arnold]

Jim - Thanks. This is commercial, which I do very little of. It's an optometrist office.

[Jon Randolph]

I may be wrong, but this is my interpretation of the thermostat:

It is just simply a dual thermostat. One side controls the heat and the other side controls the cooling. The bi-metalic coil springs are set to work exactly opposite. When heat is needed the both coils (working independantly) close the circuit for the heating side and opening the circuit on the cooling side via the mercury switches. The adjustment on the heating side allows you to fine tune the temperature swing (via increased or decreased electrical resistance, based on the position of the contact) from when the heat goes off and the cooling comes on and vice versa. To prevent both heating and cooling at the same time, there is a bar between the heat setting and the cool setting so that when you lower the cooling setting it also lowers the heat setting and an increased heat setting increases the cooling setting. The heat setting can be lowered independantly of the cooling and the cooling setting can be raised independantly of the heating.

[Jim Luttrall]

Jon, you have describe the function accurately. The question though was dealing with the dual switches on the cooling side.

But what's with the dual mercury switches on the cool side?
That is where the two-stage cooling comes in.

[Rick Cantrell]

Jon, you have describe the function accurately. The question though was dealing with the dual switches on the cooling side.

That is where the two-stage cooling comes in.

Jim
That is a polite way to correct someone.
Not critical, pointed or belittling.

[Kevin VanderWarf]

It is more common to hear of 2 stage heating. In this case, could it be that the cooling side is 2 stage?
You would really have to provide more detail about the cooling equipment.
I worked on a Trane or (Lennox maybe) years ago that had dual condensors, I seem to remember them coming on in separate stages.