Hi there, I just bought RTH221b and install it by myself to replace my old thermostat with mercury. Problem is the heather is goes on for about an hour until the ambient temperature goes rise till 39 degree Celsius but my setting is just only 17 degree Celsius. Any help.

Sure you don't have your Celsius and Fahrenheit mixed up?

Nope! I'd change the Fahrenheit to Celsius before I run my heater on. Thanks for your time to answer my inquiry, anyway I fixed it already... its running good now. :-)