Check and tighten the wiring. The pool heater user manual, listed in the References section, has a wiring diagram you can use to make sure no wires are crossed or missing. If you encounter problems with your heater shortly after installation, faulty wiring is often the culprit.
Check for leaks, and if you located any at the well, heat exchanger or at union nut connections, it is most likely a symptom of parts that need replacement. Before replacing, however, check the chlorine levels in your pool. Improperly maintained water chemistry can corrode parts, causing increased upkeep. Test kits are available at pool supply stores. If the chemical balance is correct, replace the O-rings, refractory, heat exchanger or well, whichever is the source of the leak.
Test the pool temperature. If the thermostat setting and pool temperature are not lining up, the first step is to clean the filter and burners, because clogs or grime can interfere with proper heating. Make sure the system is completely turned off before cleaning the burners or pilot orifice. Also reset the time clock and test the pressure switch. If everything is in working order, it's possible that your system or gas line is simply not big enough to effectively heat the pool. Consult a service professional about upgrading it.
Control your water flow rates. The Hayward H400 is designed for a maximum output of about 125 gallons per minute. Any flow heavier than that could cause the heat exchanger to soot. Add a manual bypass valve to control the flow rate, and be sure that the internal bypass is not damaged or stuck.
Check the fault indicators. A professional can use a diagnostic guide to read the codes and determine the problem with your system. Fault indicators will light if the problem is with the water pressure switch, the limit switch or ignition failure. Many of these problems require replacement parts and the skills of a technician.