Re: Hayward H400 heater won't relight.
Hi and welcome to the forum.
My guess would be a failing pressure regulator. Since the heater was working that says the pipe size is fine so an undersize feed pipe would not be the problem. You don't say if it's natural gas or propane. If it's nat gas then if you have other gas appliances check to see if the gas pressure is dropping on one of those when the heater tries to fire. I'm saying this because of what the guy told you. 'Loses gas supply'. There would be a main regulator somewhere for the house and they do fail. Infrequently but they do. If propane then there is a regulator at the tank. That could be failing. There is probably another stage of regulation in the heater itself and this is the one I would suspect. You could also have somehow gotten crud in the pipe which could be messing up the regulator. If you look at the plumbing, just before the pipe goes into the heater there is usually a TEE with a verticle pipe going down with a cap on it. That's a sediment trap to hopefully catch any heavy particulates that make it that far, before they go into the heater.
In a nutshell, I don't think you need a new heater if all else like the burners, heat exchanger, and other parts look OK.
Let's see where we go from here.
Hope this helps.
Al
16'x32' oval 22K gal IG vinyl pool; ; Hayward S244T sand filter; Hayward superpump 1 HP pump; hrs; K-2006; PF:5.5
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