You may have a holding relay in the circuit. If so, you have two t-stats: one to bring on heat and one to break connection to holding relay. If holding relay is bad the gas will cut off after the temp goes up about three degrees instead of working like it should by pulling in holding relay which keeps gas on after t-stat that starts the gas breaks power connection. Having two t-stats and a holding relay allows one t-stat to pull in relay at about 100 degrees F and relay keeps gas on until second t-stat breaks voltage to coil at about 160 degrees F. If the relay is bad the gas comes on at 100 and goes off at 105, which is not enough to get hot water. Replace relay or at least test it by ohming across coil connection. DrTom4444