Pentair MasterTemp 400 cutout problem

jgehlhaarNovember 27, 2009


I'm sure this has been answered a bunch of times here before so forgive me -- I did a search but did not turn up something specifically for this problem.

I have a 1 year old heater that only tonight started heating, then would cut out, light the service heater light, run a while more, then relight the burner.

All the usual suspects like not enough flow, temp setting below current temperature etc. were okay. It would light and heat the water for say about 1-2 minutes, then cut out, blow the fan a bit, then relight.

Any tips on what might be wrong? Have to get my pool builder out here to take a look, but wanted to see what the advice of the forum might be.


- Jeff

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Take a look at the t- stat if its stuck that will cause the symptom you describe as well as a bad gas block/valve.

Ive had two fail in two years.

Do you have a salt water pool by any chance?


    Bookmark   November 27, 2009 at 2:56PM
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Yes i have a salt water pool. Is this part of the problem?

How do I check for a stuck t-stat? Is this a diy repair?

Thanks for the info!

    Bookmark   November 27, 2009 at 3:25PM
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Possibly the thermal regulator stuck closed. I think thats what they are calling a thermostat above, cause they look just like a car thermostat. It is located behind the threaded plug between the inlet and outlet ports. You can twist open the cap with a screwdriver shaft. Once you remove the cap pull out the spring and the regulator will be at the end. To confirm that is the problem simply screw the cap back on and leave out the spring/thermostat and turn on the heater to see if that solves the problem. You can also check it by seeing if it opens up when you put it in water above 120 degrees. And yes we see more of these units fail with saltwater.

    Bookmark   November 29, 2009 at 11:39PM
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thanks Dapooltec - I wrote up but for some reason I must not have submitted a virtually identical answer.

If you dont have a one way trap between the heater and teh chlorinator you will corrode the "thermal regulator quickly"


    Bookmark   December 1, 2009 at 10:51AM
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need to have service person check flame sensor

    Bookmark   December 5, 2009 at 1:50PM
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In a master temp?

    Bookmark   December 6, 2009 at 12:01AM
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natural gas or propane? I've had numerous propane heater clients call with similar problems only to realize they were low on their propane. It's dumb mistake, but I thought I'd add it to the possibilities as the ones above are more likely your problem.

    Bookmark   November 15, 2012 at 8:54PM
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Heaters are not something a novice should be troubleshooting, given the complexities involved.

Having a salt water chlorine generator does not impact the thermal regulator. Having incorrectly kept chemistry can. So too, can a low flow condition, whatever the cause of that.

Without knowing the whole picture of how you particular system is configured, i.e. pump, filter, chems, etc..., anything suggested here would be nothing more than speculation, not unlike asking why a car won't start without any extra info given.

Please get it checked by a professional. There are too many things that can contribute to these symptoms.


    Bookmark   November 16, 2012 at 5:46AM
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