Question about Hayward H-series Electronic Gas Pool Heater H400

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Temp on water boils in heater core

Temp on high on water boils in heater core h400 older model heater has temp dials

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This will be a water flow problem. If you have a heater by-pass it will need to be almost closed (80-90%) If the water has overheated you may need to remove the header going into the heater and replace the internal by-pass assembly.

Posted on Aug 02, 2010


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This sounds like a low water flow situation. The heater is telling you that the high limit sensor is curring heater operation. The noises are the heat exchangers boiling the water either due to a blockage in the exchanger , a bad mixing vlve, or most likely poor water flow. This is tough to troubleshoot through a posting like this, but 1st thing is 1st. Check your water flow and make sure it is minimum 50 Gallons per minute.

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H-series Electronic Gas Pool Heater H400: I have an H400 with digital temp control panel, th...

It is likely the keypad that has gone bad. Not difficult to replace. But you want to be sure thats what it is, as it is an electronic part, that is usually not returnable. The part number would depend on the model of the heater. Likely it is a h400idxl2 heater.

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2 Answers


Your heater core has no idea if coo or hot water is passing through it. Either it flows or it doesn't. Run the engine till it is at normal operating temp with the cap removed and the front end jacked up on the corner where the fill is. After it reaches operating temp, turn it off, wait about three minutes and start it again. Turn the heater on high. Add coolant till it's full and let it run. It may overflow a little a few times but should not overheat.Make sure that you are adding a 50/50 mix as straight antifreeze will boil at a lower temp than does water. If you have any bleed ports on the engine you can open them if necessary. If the radiator is flowing properly everything should return to normal.
A partially obstructed radiator can appear to flow normally as the coolant goes around the restricted passageways. turning the heat on will never cause an engine to overheat even if it was totally plugged. Be sure there are no external obstructions as well (bugs or leaves packed into the A/C condenser or between the condenser and the radiator. It is sometimes possible for a head gasket to allow compression gasses to enter the cooling system. it will first fill the heater core, then push water out of the cylinder head passages. You can have a hydrocarbon test done at any shop to find out if this is happening. Your original problem was an overheat. Even if it was due to another problem, it can cause a gasket to fail or a cylinder head to warp or crack.
Try what I said and if that fails have the test done.
Good Luck!!

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I think you could be right about there being air trapped in the system.
Or the heater core(radiator) is plugged.
Either way you need a pressurized radiator coolant pump to put the coolant in without air being trapped.
Although,i have seen it done by jacking the vehicle up high enough that the radiator was now higher than the heater core level, and then the coolant was put in.
But this is dangerous to try.

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