Pentair MasterTemp 250 Nat. Gas doesn't turn on after cycle ends

[AeroAero]

Pentair MasterTemp 250 Nat. Gas

Unit is 6 months old (old unit lasted 2 years, cupro-nickel failed)
Had to replace igniter within 2 weeks of purchase
Water-flow is 68-70 GPM
No warranty option (direct replacement of old unit by self/not prof. installed)

Hi all,
This unit is used at a hotel and runs 24/7. It heats up to 85 just fine. When it comes time to turn back on, unit usually throws the service heater light. Unit will not come on until turned off and back on.

Manual (lots of help there) says I need a new control board. I swapped out the control board from the old unit. Same scenario, sometimes it works and sometimes it doesn't.

Thought maybe the diverter valve on the filter was drawing air since it was leaking. Just replaced that and no change.

Thank you for any suggestions, as I am just muddled at this point.

 

[racket]

If you look at the back of the board do you see any of the service lights lit?

 

[Keven]

Sounds like a bad thermal regulator.

 

[AeroAero]

No service lights on

 

[DJS]

I know this is an old thread, but I have an identical issue now. Heater brings water to selected temp, but does not come back on as temp drops, goes through start up sequence 3 times then service heater light comes on, no fail lights/codes. It lights when I shut power off and bring it back on. Seems like now I have a manual heater ;-) What ended up being the issue?