Mastertemp 400 - Ignites, then cycles, no error codes

[huggablejunk]

Hi TFP Crew,

Seems like a never-ending saga with these Mastertemp 400 units. I went outside to put my hot tub on and the heater turned on spun up the fans, ignited for about 3 seconds then ignition stops completely. You can smell a bit of the burned natural gas as expected coming out of the exhaust pipe. The heater backs off then tries the same process again 20-30 seconds later, ignites then no longer stays lite. I've opened the unit up and the Fenwal ignition board is giving no status/error lights and the underside of the control board is giving no status lights either. I've let this going on as long as 5 cycles and its not giving an error code. Seems like I can't maintain combustion for some reason. Unit is about 5-6 years old. The unit never gives the Service Required light.

1.) Checked the thermal regulator, looks fine, functions fine in hot water
2.) Checked the control board, no error lights (The control board is less than 1 year old)
3.) Checked the fennel, no error lights. (The fennel is less than 1 year old)
4.) I'm getting great flow and gas to the unit

If you recall, I had the whole heater rewired due to a mouse two years ago, the heater ran great for an entire year with no issue whatsoever (lit up every time) and it was even working fine all this current season until literally just now.

Do I maybe have a sensor that's gone bad? Any other ideas?

Thanks,
HuggableJunk

 

[MITBeta]

Bad flame sensor? That's usually what causes the flame to go out after 2-3 seconds.

Sometimes these can be cleaned.

 

[PaulStinson]

I have a MasterTemp 250, nine years old, in New Hampshire. This year, the unit would only fire for maybe ten seconds and then stop. It was burning, as I could feel the heat, but it shut off every time. To make a long story short, I fixed it by installing an alternate ground wire from the flame holder flange to one of the mounting bolts for the fan mounting adaptor. There is an article on this website, Heater Flame Sensing-Further Reading, that allowed me to figure this out. Originally, I was getting no error codes, but after working on the heater awhile, I started getting SFS code E-06, indicating a bad stack flue sensor. The sensor was indeed reading incorrectly, but the sensor was not bad. Apparently the lack of a good ground connection was causing the SFS to read incorrectly and also preventing a good reading on flame current. Once I connected the alternate ground, the heater runs continuously and the stack flue sensor now reads correctly. I have ordered rodent proofing kits to prevent mice, but I think this problem is a result of a poor design by Pentair. Their design sandwiches the flange of the flame holder between two gaskets, and it only gets grounded to the extent that the through bolts make contact with the flange. This alternate ground wire bypasses the gaskets.

I want to note that I noticed, before I added the alternate ground, that the plug for the ignitor was getting very tiny sparks (almost invisible) from one lead to the other while the ignitor was energized.

 

[huggablejunk]

Thanks for your advice @PaulStinson. It turns out I have a gas supply issue…again. Underground PE line converts to black pipe 2ft underground and the Georgia clay rotted the 2inch bell reducer and let water in. Had a plumber replace the bell and pipe and waterproof the heck out of it and I packed it tight with better top soil to hopefully prevent future water from sitting there.