Using the heater for the first time this year. Two related issues:
1. It makes a groaning noise when heating.
2. I got a HL 2 code twice, and the heater stopped heating for a few minutes, then resumed.
Looking at the troubleshooting guide in the manual, and other online sources, HL2 code is high limit--heater got too hot. Groaning means restricted flow through the heat exchanger. it looks like the Unitherm Governor (same an as automotive thermostat) was not opening. That means not enough water flow through the heat exchanger.
Removed it (unitherm), tested it by immersing in hot water, and sure enough, it was not opening when hot.
Can't get one until Monday. Question is--how bad is it to operate the heater without that part in place? Everything works fine, I'm just wondering what kind of risk I'm running.