I'm using a Weil Mclain Ultra Series 2 boiler for my driveway melt. How do I adjust the thermostat?

After very cold nights (below 10 degrees) I have to reset my boiler. I'm afraid the glycol mix in the tubing is freezing and the boiler shows error codes. I'm hoping the solution is raising the low setting on the boiler, currently set at 38. Does this sound correct? If so, how do I do this?