Hi,
My 7 yrs old Lennox Elite Series heating system is having a problem. While the thermostat setting is set to 70 degree, the furnace is not shutting off. Rather, it keeps heating the house to the low 80s. If I turn the thermostat off, the furnace will shut off too.

Can you help?

Replace the thermostat. Or depending on the type of stat there may be a small adjustment screw to calibrate it near the mercury bulb.

I have separate honeywell brand digital thermostat (wireless) for 3 zones. I did check each one by process of elimination and the result was inconclusive. The system is still running full force.

Before you replace the thermostat, make sure it is the problem. With the red or white wire disconnected at the thermostat or its receiver, if the furnace still runs, the problem is not in the thermostat. Does the signal for the furnace to start come from the thermostat or the zone control?

Is this forced air or hot water?

The furnace will stop when the thermostat is in off position. The 3 zones are controlled by 3 separate and independent thermostats. If 2 are off and 1 is on, the furnace still runs. This tells me al 3 are operating fine except they are not controlling the furnace temperature, but can turn it on and off.

The zones complicate things. I doubt all 3 thermostats have failed the same way at the same time. I know in some hot water systems, the zone valve fires the boiler after it opens. If your system works that way, there could be a contact stuck at one of the zone controls.