MasterTemp 400 Shuts Off

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LED 11 is the service heater light on the boards other side. Does the heater run fine for a while then turns off at a certain temp each time and start to cycle as you note? If it (the water temp) is in the 80's, check the thermostatic by-pass
 
Is it U11? If so it should have the AFS letters next to it.

How old is this unit?

I would also check to see if there is water in the combustion chamber.
 
The unit itself is only a couple of years old. It will heat up to a normal temperature, but the unit will cut on and off throughout the heating process. What is the thermostatic bypass? I can't find any mention of it in the manual.

Thanks,

Justin
 
Sorry, Pentair calls it the "Thermal Regulator" It is between the inlet and outlet, has castellations on it that you can slide a screwdriver blade in to help unscrew it. It is held in place with a spring, so when you get it unscrewed to a point, get your fingers out of the way as the cap will snap out with a little force.
 
So just more for curiosity why would you think it would be the thermal regulator over the unsightly limit switch? Also can the thermal regulator be cleaned or repaired? I am wondering if may dirty or mineral deposits that would affect it? Oh and thank you so much for all of you help! :cheers:
 
When the thermal regulator is not regulating the the bypassing of water properly, it causes an overheating condition which trips the limit switch. The limit switch opening is (only) a symptom of the problem.
Unfortunately, the regulator is not serviceable.
 
Gunked up looking. Maybe rusted pretty bad. One way to test is after you remove the regulator, screw the cap back on and then turn the heater on. Hold the thermal regulator at the opposite end with a pair a pliers and hold the regulator part over the hot exhaust coming out. You should see the regulator open as it heats up. If in a few minutes it doesn't open, it's bad. Shut down heater.

Now a word of warning, while removing the regulator, if it doesn't slide out easily, seems like it hung up on something, then the bypass is broken and hanging down in the way. They you will need to replace that and possibly the regulator.
 

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ps0303 said:
Gunked up looking. Maybe rusted pretty bad. One way to test is after you remove the regulator, screw the cap back on and then turn the heater on. Hold the thermal regulator at the end of the opposite end with a pair a pliers and hold it over the hot exhaust coming out. You should see the regulator open as it heats up. If in a few minutes it doesn't open, it's bad. Shut down heater.

Now a word of warning, while removing the regulator, if it doesn't slide out easily, seems like it hung up on something, then the bypass is broken and hanging down in the way. They you will need to replace that and possibly the regulator.
Click to expand...

+1 agreed.

Heater would knock a little too if the by-pass was broken. Bad therm reg may still open, just not the proper amount. Even in some cases you really have to look hard to find the problem with a regulator, cracks sometimes do not show themselves that easily.

At least for me...

My eyesight is getting worse by the day, it seems. :hammer:
 
I've been fooled many times with a bad bypass on these units. Sometimes the bypass is broken but lodged in a way that it doesn't knock or bang when heating. The only way to find out for sure is to pull the thermal reg. The things you learn after banging you head on the wall many times.
 
Sorry I haven't responded in a while, I have been traveling for work. I took your advice and got the new thermal regulator kit, got the whole kit because there wasn't much price difference, installed it and everything seems to be back to normal. The hot tub heated up without incident. I would like to thank all of you for your knowledge and advice.

Regards,

Justin
:cheers:
 
Having same issue of the unit shutting/cycling off at 79 degrees (friends). On the board the PS light is lit so I assume pressure switch, I cleaned the terminals and it worked for 4 days. Could this be the cause or are we still looking at T. regulator? Unit is 5yrs old and propane. Also in the manual is say make sure pressure tap is not blocked, where is that.
 
PS is low pressure which the heater interprets as low flow, or a condition that would not let the water transfer enough heat away from the heater. So this pressure switch interrupts this before it is a problem.

Check for good flow by looking for a closed valve, dirty filter, clean strainer bskts, enough speed on the variable speed pump (if applicable), etc.

OR your pressure switch and or the associated wiring may have an issue.

If your getting the PS LED, all your heater knows (or cares) is that the switch is open. If you are getting intermittent operation you might be at the cusp of workable pressure (on again off again) flow.
 

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