Heater Won't Heat

        Heaters have a number of protection devices built in, High Limit Switches, Low Limit Switches, Thermostats, ... When one of these malfunctions, or when there is a problem with the heater, your heater will not function properly, or at all. Also most heaters require 220 v electric and, if gas, propane or natural, is involved, there are more safety devices concerning igniters, pilotless ignition sensors, gas valve release, ...
            Problems with temperature not rising to the proper level, or overheating the pool or spa are almost always related to thermostats. If you have a heater bypass valve in place, and you don't have it "mostly closed", even though most heaters are under pressure, telling the heater that there is water flow, there really isn't any. Heating water that isn't flowing will quickly trip the high limit switch. With this problem, the heater will fire for a few seconds and sound like everything is working fine, ... until the water heats up. Then it will shut off till it lowers below the trip limit (On some units), or until you shut off and restore the power to the heater (On others). Also, if you have valves to completely shut off water flow, you will never get a pressure reading to allow the heater to fire in the first place. Make sure those valves are open fully.
            There are many "other" reasons, including defects in these switches, bad gas valves, ... but only a professional trained in these, and who possess the proper manuals (In most cases), are qualified to do the diagnostics and repair on these.