Sta-Rite Max-E-Therm 333 heater starts heating but shuts down within seconds

[InFocus]

Greetings!
I'm trying to revive my spa heater, a Sta-Rite Max-E-Therm 333, which uses natural gas. The blower spins, burner ignites and there is hot air coming up out of the exhaust vent, but then the heater shuts down entirely after about 10 seconds of heating and the inlet temperature spikes high enough to prevent a restart. There is a light "thump" sound from somewhere when it stops heating. Still trying to narrow that down to a more specific location. The spa pump is not affected, so water is flowing throughout this cycle. I am not getting any error codes on the display or led lights under the control board. Any suggestions?
Thanks!

 

[ajw22]

@ps0303 @swamprat69 @Pool_Medic thoughts?

 

[ps0303]

Make sure you have good water flow to the heater first. If you have good flow and a clean filter then start checking parts on the heater. Then you should check the internal bypass. Remove the thermal regulator and check to make sure it isn't all crudded up. Then you can search inside where the regulator goes to see if the bypass is intack. It sits above the regulator. You can usally stick a finger or two inside to check it. BTW, if the thermal regulator has a hard pulling out, most likely the bypass is broken and hanging down in the way.

 

[InFocus]

Make sure you have good water flow to the heater first. If you have good flow and a clean filter then start checking parts on the heater. Then you should check the internal bypass. Remove the thermal regulator and check to make sure it isn't all crudded up. Then you can search inside where the regulator goes to see if the bypass is intack. It sits above the regulator. You can usally stick a finger or two inside to check it. BTW, if the thermal regulator has a hard pulling out, most likely the bypass is broken and hanging down in the way.

Thank you! It was the water flow. It turns out that the crew refinishing the pool turned some of the vlaves the wrong way and so the heater was not getting the flow it needed, though why it fired up without good pressure is curious. Working now, though.

 

[JamesW]

The heater uses a pressure switch and not a flow switch. So, you can have pressure but no flow.

What they really need is a flow switch, or a differential pressure switch that measures the difference between the inlet pressure and the outlet pressure.