My Hayward H250 pool and
The H250 pool heater has several normally closed safety switches, also called permissives. All permissives need to in the closed position before the controller can initiate ignition. If one or more of the permissives are open, the controller will not initiate ignition (the yellow light will be extinguished on some models, some models have no lights, some models use other displays to indicate faults).
CAUTION: Power, gas and the water pump should be turned off before attempting to repair your H250 pool heater.
CAUTION: The H250 pool heater is essentially a gas furnace. Only qualified technicians with the proper training and tools should attempt to repair your pool heater.
In most H250 models there are 5 permissives, at least one thermostat or temperature monitor/controller, and system on/off switch. Some models use municipal power, some do not. For a heater connected to a power source, of course the power source needs to be active before the system will operate. For those without a power source, the system is powered by a millivolt circuit that originates from the thermocouple on the pilot. The pilot flame generates enough energy via the thermocouple to operate the combustion safety and control circuit.
Now on to troubleshooting: Based on your comment of "mode light is no illuminated", I assume you have a system with a yellow and green LED indicator. This system is powered by municipal power and uses an ignitor/flame detector rather than a pilot. The yellow LED signifies the heater is powered and all permissives are active (ON), in other words, the heater is ready for operation. If the yellow LED is off, either municipal power has been turned off, or one or more of the permissives is/are inactive (OFF). The green LED signified the control system has opened the gas valve and combustion is active. The green LED cannot be ON unless the yellow LED is ON.
Based on your comments, I assume you have already checked your municipal power and it is ON. If indeed your yellow LED is off, at least one of your five permissives is inactive (OFF). These five devices are switches that monitor the operation of the heater. In normal operation, they are in closed position, allowing current to flow. If the pre-determined limit has been overcome, the switch will go to open, which will disconnect the entire control system and turn off the gas valve. Normally the device will return to it's normal state of ON once the limit is back under its pre-determined limit. There is one exception -- a manually resettable limit switch monitoring the combustion chamber for high temperature limit. If tripped, this switch has indicated temperature in the combustion chamber has exceeded the manufacturer's limit for the chamber. Even though this switch can be manually reset, one must take care to check the combustion system for the root cause. Once reset, the condition that caused the temperature overlimit may still exist, in which case there is a malfunction of the heater that needs critical attention. The resettable temperature switch is located behind the lower access panel, toward the right side of lower section. To reset, press the small button in the middle of the switch.
If the resettable temperature switch has not been tripped, then either one of the 5 permissive devices has malfunctioned or the wiring between them has become damaged or disconnected. Here is the list of the permissives:
2 - High water temperature limiters (located on front and rear of the water header)
2 - Combustion chamber temperature limiters (1 resettable as above, the other is not resettable. It is located on the left side.)
1 - Water pressure monitor (located in the water path, in the upper section of the heater). This switch should be ON when water is flowing through the heater.
Each of these devices can be checked with an OHM meter. When the heater is off, all the temperature switches should be in the ON state. When the pump is on, the water pressure monitor should be in the ON state. If any of these devices do not preform properly, it needs to be replaced.
Wiring for the permissive devices is in series -- that is to say, all devices must be ON to allow current to flow and the gas valve to open. The permissive wire string begins on the terminal block as a RED wire, passing through the first water temperature limit switch to a VIO wire, then through the second water temperature limit switch to a BLU wire, then through the water pressure switch to a VIO wire, then through the non-resettable combustion temperature switch to a BLK wire, then through the resettable combustion temperature switch to a PUR wire, than back to the terminal block. When the heater is powered, 24VAC hot-side of the transformer connects directly to the first permissive (RED wire). If 24VAC is not present, then the 24VAC transformer has malfunctioned and needs to be replaced (or there is a broken wire (GRY) coming from the transformer to the terminal block ).
When 24VAC if present and all the permissives are active, the yellow LED will illuminate. This is the signal to the controller that combustion can be initiated.
(It is possible that the yellow LED has malfunctioned, but this would not stop the controller from operating the combustion process. If the yellow LED is OFF and the combustion process will not initiate, the malfunction will likely be found in the permissive string.)
