Have gas baseboard heat in 5 rooms in the house. We have one thermostat that controls all of the baseboard heat in the house. We noticed that the heat was coming on, but randomly heating up the house beyond the set temp. Heating professional replaced the thermostat. After using it for a couple of days the problem started again. House heats up past the set temp and the heat won't shut off. Turning off the thermostat doesn't help either.

Any suggestions on how to fix this?

I have been looking at gas baseboard heaters on line and one manufacturer states that the maximum number of heaters on one thermostat is 3, hope this helps you.