Hayward 400FDN starts but does not heat water

[Acarr203]

Looking for help!
I have an H400FDN that is a 4 years old and has always worked perfectly. The other day we turned it on as normal to enjoy a spring pool day. The heater turned on, lit the natural gas as normal. An hour or so later, we noticed the heater was running, but wasn’t heating the water. Water at all of the outlets was the same temperature as the water in the pool. The pool heater is not showing any error codes and appears to be running normally.
Does anyone have any ideas to begin trouble shooting?

 

[ajw22]

Welcome to TFP.

Service manual for your Hayward 400FDN heater is here - https://hayward-pool-assets.com/ass...uals/UHS-Service-Installation-011717-RevT.pdf

See Troubleshooting on page 49.

When you say "An hour or so later, we noticed the heater was running, but wasn’t heating the water. " what told you the heater was running? Was heat coming out of the exhaust? Any error codes on the heater control panel? You try turning the heater on and off at the heater?

Please create your signature with details of your pool?

Do you have automation controlling the heater?