Heating Thermostat Readings

[06Honda]

I have an infloor radiant heating system that is controlled by 2 manual non-programable Wirsbo Radiant dial type thermostats (small glass tube inside cover with small drop of mercury). 2 zones, 1 zone with the same thermostat is right on the money, but the 2nd thermostat reads quite a bit lower than the actual temp I set the dial to (most of the time 2 degrees). Last night it was pretty chilly here in southern ontario around -13F. This am when i got up the reading on the thermostat was 16.5 but the dial was set at 21. If I raise the temp higher on the unit it kicks in but never reaches the actual selected temperature. Thanks for any input or advice, much appreciated.

Q1: Can this unit just be dirty inside causing connections not to work properly?

Q2: Can an adjustment be made inside (calibrate) to bring the actual temp set on the dial to what the actual thermostat shows?

[HayZee518]

thermostat

thermostats like the honeywell all use a mercury switch to close a set of contacts. the heat anticipator is a variable resistance that's adjusted usually to the gas valve current, or the burner rating if its oil. the helical spring is the temperature sensing device. in the center is a small screw that you can rotate the whole spring to adjust for the set point.

[06Honda]

Thanks for the info and adjustment, I'll give that a try.

Paul