Quote Originally Posted by AzureN1ght
I think RuLyn's got it right. From what I understand about on/off thermostats (I use proportional), your thermostat will crank power to the heat source 100% until it hits a certain temperature (this is where the differential setting is important), then it shuts off. If your differential is set to 1--I think that means your T-stat can get up to 1 degree above your desired temp and 1 degree below before it shuts on or off. If it's set to 2, two degrees above or below your desired temp, etc.

Hope that's what the issue is!

EXACTLY!!! That's how they work...so the higher the diff. setting, the greater the range in temps before it either turns on, or off.