I have a Hayward Pool and Spa heater (400K) Model H400FD. Natural Gas. It's used with an indoor Therapy Pool and the PT's and clients like the pool water temperature to be between 92-96 Degrees F. It has the typical Hayward red LED light that shows the temperature of the water running through the heater at all times. Pressing the + or - buttons change the desired setpoint which blinks for a few seconds and then goes back the the temperature readout. If the pool temperature falls below one degree of the setpoint, the green light comes on, the heater fires up and the water is heated until the setpoint is once again reached. Then it shuts off again and waits for the temperature to fall back again. This morning, I came into the pool room at 7:30 am and the pump was running as usual and the reading on the heater was 92. I do not keep the pool temperature high all night. I have the pump running at night but at a low RPM and the heater flow switch doesn't allow it to come on. OK, so the problem: This morning I turned the pump up and the heater came on. However. when it reached the setpoint, it did not turn off but kept heating the water. It crept up to 101 degrees and I then turned the pomp on low and turned the setpoint down to 94. The pool gradually cooled down again. This time, I set it to 95, the heater came on and switched itself off as usual when it reached the setpoint temperature. I am not sure, but I think this may have happened once before, on the Friday before Labor Day weekend. This heater is new, installed in June. The owner's manual indicates that the heater has 2 high limit switches. It states: "If the water temperature exceeds the limit set point, the gas valve will be closed, shutting off gas supply to the burners. " This setup does not have an external bypass. I am wondering if anyone on this forum has had a problem like this? The only reason why it got so hot is that I went out to get a bite to eat for breakfast at McDonalds. At that time, the setpoint was reached and the flame was still on, which is normal. It usually runs for a few minutes longer anyway. But this time, apparently, it stayed on. When the PT's arrived about 20-30 minutes later, is was on and heating, display said 101 but the setpoint was still 96. This heater was installed by qualified personnel and they did a professional job, IMO. I am going to carefully monitor it for a while before calling in the cavalry. Anyone ever have a similar problem?