Hayward H400FDN "LO" Code

[Sgt1411]

Looking for some assistance with "LO" code that just wont go away.

H400FDN installed in 2018, this month installed brand new Heat Exchanger from Hayward ($967) due to a leak. While looking at the heater with the Exchanger out it was clear the water leak into the firebox had washed some of the insulation material all over the gas chamber tubes and they were plugged.
I pulled out all 8 gas tubes, cleaned them completely so they were 100% clear and re-installed them with a new Ignitor.

The most common cause of "LO" code is insufficient water pressure coming into the heater so I pulled the filter cartridges (Hayward SwimClear 8) and had them cleaned as well.

No changes to any plumbing same as before with 2" pipes coming out of the ground and one 90 degree elbow to meet the heater inlet and outlet pipes.

The heater fires perfectly, runs for about 3-4 minutes then shuts off and throws "LO" code.

Heater makes banging clanging noises which indicates low water in heat exchanger.

Talked to Hayward tech support they said to test water pressure switch, but it works fine, opens right away with filter pressure pushing water past it.

Hayward said to test Exhaust gas limit switch with meter and its good, ohms 0.01 by meter test. Also test Water pressure switch, vent pressure switch, high temp limits switches and they all test OK

Troubleshooting guide says its back pressure but exhaust side of heater is clear and unobstructed.

Kind of at a loss now, anyone have any ideas?

 

[Poolbreh]

LO is the limit string one of your sensors is being triggered, It sounds like a flow problem internal or external. Open external bypass, restrictions down stream. Internal there doesn’t appear to be any thermal governor but there is a internal bypass. “Bypass service cartridge” I would check

 

[ajw22]

The water pressure switch, the temperature limit switches, or if installed a Vent Pressure Switch, can open and throw the LO code.

If the heater is banging and clanging you have a water flow problem and not a sensor problem. The sensors are doing their job when the water in the heater begins boiling.

The bypass valve could be the problem if you have not replaced that.

 

[Sgt1411]

Thank you both for your suggestions, I will follow up on that.

I have not looked at the Bypass Service Cartridge, Its brand new as the entire header unit is brand new and comes from the factory already attached to the new Heat Exchanger.

 

[ajw22]

With all the supply chain issues I have heard of many defective parts coming from manufactures.

Quality control seems to be absent.