New Ruud gas water heater won't maintain temp

[Mike from Brier]

We recently bought and installed a new Ruud P2-50F1 gas water heater. We have the thermostat set so the water temp is 120F when the tank is up to temp. We've noticed ever since it was installed (end of January) that if we don't put a demand on it for, say, 10 hours or so, that the temp of the water drops down to about 103F or so. We have to run about 3-4 gallons of hot water and then the burner fires up and about 30 minutes later we'll have 120 degree water again. I called Rheem technical service and they say this is normal. I have always been under the impression that water heaters were supposed to keep water at the temp you set the thermostat for.....seems I am mistaken? Our old Ruud gas WH didn't drop off temp like that until it was almost 20 years old, but it was so full of sediment that this didn't surprise me. Is this really the nature of new technology WH's?

[Dana]

All hot water tanks need some amount of hysteresis in the controls to keep from short-cycling, since longer burns increases average efficiency. But a hysteresis of 17F seems excessive for a 50 gallon tank- that's over 400lbs of water, quite a decent amount of thermal mass even for the 40KBTU/hr-output burner versions. (A 17 degree rise would be about a 10 minute, minimum burn.)

But if that's how they designed it, so be it. If you bumped the t-stat 130F it would only drop off to 113F before re-firing, which is still plenty hot for showers, even for filling cast-iron tubs.