Be VERY careful.
That's the first sign the thermostat is on its way out and the water will reach dangerously hot levels.
I had mine hit boiling temps and the relief valve did NOT open enough to relieve the hot pressure!
Almost burned up the entire side of my MH. And would have, had a neighbor not pointed out the dripping water. The side of the MH was so hot you could not place you hand on it! :E
Since that day I NO longer EVER leave that darn HWH on unless I am in the MH and using it period. And it gets shut off right after that.
BTW: Mobile Repair said 99% of the time the relief valve does NOT work. And he has seen some pretty bad damage to RV's.