Question about Pentair Minimax NT Heater

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Minimax NT Pool Heater Problem cycling and knocking.

I have a NT Pool and Spa Heater that does not work. I have power and gas going to the heater and they work fine. I’m pretty sure the pressure switch works because anything over 10-12 PSI (on Pool Pump) and the pressure light lights and the display goes from off to three dashes. When I set either the pool or the spa temp the heater will start a cycle. First the thermostat light will come on and the blower at the same time. Then about 2 minutes later the heater light will come on and I can hear the gas ignite. Hot air in now coming from blower. Then after about 2 to 3 minutes the water tube coming out of the heater starts to shake and knock. The pressure switch light starts to blink ( and I see the pressure move below 10 PSI ) and the heater turns off and blower stays on for about three minutes more and then turns off, then they cycle starts over. I have a taken out the valve assembly and reseated it and after I did that it performed differently by getting too hot and the HI game on in the display. After it cooled down I tried it again and got the same cycle on, knock and shut down.
My first question what is the recommended PSI for normal operation? Second is there a way to test the valve assembly, I thought I saw something in here about putting it in hot water to see if it moves. If it not the valve assembly could it be the PSI or scaling on the heat exchanger. If it is scaling is there a way to clean it? Sorry for all the questions but would really like to get my heater going. Thank you in advance for any advice.

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  • imrvr Feb 04, 2010

    Thank you for answering my problem. The part that I reseated was the by-pass assembly because it had disk and two springs on it. I looked at the big and small spring at the other end and they were not broken but the disk was very tight and was not moving. When compressing the disk the little rod came out through the small spring tool a little to get straight again. This weekend I will pull it out and check it again and sand the shaft. Is the thermal governor at the end of the by-pass assembly at the end where the small spring is? I don’t really know if I have an external bypass but my guess is no. The pipe coming out of the heater goes directly to the salt chlorinator cell then into the ground. I do have a question about the water flow, how does the heater know to output to the spa or the pool if there is only one output pipe?

  • imrvr Feb 05, 2010

    Thank you again. I’m sure you are right about the heater
    only running the spa, that makes a lot more sense. My thinking was that if the chlorinator
    was in line with the heater output then the pool was also heated. I just bought the house and still leaning the
    pool system. I’m going to check the valves tomorrow (and only run the spa) and
    I will let you know.

  • imrvr Feb 14, 2010

    My Heater is working now J
    Sorry it took a couple of weeks to get back with you but had time this weekend
    to try and adjust the valves. Everything is working great now. Thank you so
    much for your help.

  • Anonymous Mar 11, 2014



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There are two valves inside the water manifold. The one you are talking about is the thermal govoner.This can be checked by putting it into boiling water and it will control how hot the water stays inside the heat exchanger. It is the piece that looks like the thermostat in a car and works the same way.
The other part is a by-pass assembly this mixes the hot and some cold water comming from the heater. One, to keep the water comming from the heater from being to hot and scalding. Two, it keeps the flow threw the heater from moving to fast to heat correctly. Their are two things to check on this. If the flow control disc is not moving free you will need to sand the shaft on this so it moves smoothly. The other is the spring that controls this and you just need to be sure that it is not broken,
Also you need to check if you have an external by-pass. This will be a valve bridging the inlet and outlet pipes going to the heater. If you have this on your system the valve needs to be shut down 80-90%. It will help to control the flow if your pump is moving too much water for the heater to control on it's own. You just need to adjust this pretty close as the one in the heater will fine tune the water flow. Let me know how you do and good luck.

Posted on Feb 03, 2010

  • Kieran Ruppert Feb 04, 2010

    I'm sorry wrong model, The one you describe is hard to check and probally you should just replace the whole unit. As I said before you should replace both itims at the same time, Well this confirms what I have always said. Both parts are built into one, So you will replace both at the same time. As far as sanding goes this unit has all stanless so you probally woun't find any scaling to sand off. You will probally just need to get a replacement part.

    Now, as for the pipe going into the ground. If you only have the one pipe then you don't have an external by-pass. If you only have one pipe then your heater only runs the spa. You could have a shared valve closer to the pool but where that would be is hard to say. You should check your suction pipes to see if their are more than one.

    If you don't have seperate equiptment then the heater may be beyond the mix valve and will only heat the spa. If this is the case before you do anything with the heater you need to make sure all the water is going to the spa. If not this can cause much the same symptoms as you have described. If you follow the inlet pipe back twards the filter you should run into this valve or set of valves. As I have said you will find another valve on the suction side of the pump that will mix water comming from the pool and spa. Both this and the return will both need to be set to spa only. If you can check for these valves soon, setting these right may fix the whole problem. Hope you can follow my drivel. Let me know.

  • Kieran Ruppert Feb 05, 2010

    They say you don't have to pay the landlord, but you have to pay attention. I let the whole chlorinator thing slip right by my the first time you said it. You are absloutly right if the chlor is past the heater then it is serving the pool and the spa. If you can't find any valves then you may have to get a pool company out to figgure the system out. The pipes may just go into the ground and come back out a few feet away just to make it look cleaner. If you have a bank of valves that all come from the groung then this is probally the case. If you have gate valves on these and one without any valve you have found the return manifold. If you have three way valves on the top of the header then you will have to play with them to see where you get water. Since you just moved in then it's probally a good idea to have someone in to help figure it out. Good luck and let me know how you do.


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There is an internal probe inside the pilot that actually stays heated. This probe can become dirty and/or corroded with soot and needs to be cleaned with a wire brush for it to keep the pool heated and lit.

Posted on Feb 03, 2010

  • Nate Stansfield
    Nate Stansfield Feb 14, 2010

    If heater is working, then try adjusting you valves. This should fix the problem, if by chance it doesn't help, the valves may need to be tested or replaced, but usually adjusting helps.


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