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Thread: Pentair Minimax Plus 400 Troubleshooting

  1. #1
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    Pentair Minimax Plus 400 Troubleshooting

    I have a nine year old 35k gallon pool with a Minimax Plus 400 heater that I am having problems with. Any and all help would be greatly appreciated. I have a number of questions.

    For starters, I live in Utah, where the temperature lately has been below freezing for a couple of weeks. I winterized the pool in October, and before closing it had issues where the high limits were open. I was able to bypass (jumper) one of them long enough to get the pool closed, and ordered new limit switches which i finally received.

    ***Question 1: It looks like these just screw right into the header. Are there any precautions or special things I need to take? (i.e. specific torque, heat sink compound, etc.?) [it appears they just thread in snug)

    Before closing the pool, I noticed some strange behavior where the heater stayed on, even when the pump was not running. I suspect the pressure switch. I want to check the one I have before ordering a new one

    *** Question 2: should the pressure switch be normally OPEN when there is no pressure (i.e. pump not running)?

    It also seemed (prior to closing the pool) that it was not getting the water very hot.

    *** Question 3; would this be related to either the limit or pressure switch issues above? (all LED's indicate normal operation)

    The Minimax Plus has a toggle switch on the front (Pool-Off-Spa). I believe I have been able to have it in either the Pool or Spa position (I only have a pool) and have it operate normally in the past. Now it only works on one setting.

    *** Question 4: Is this likely a problem with the thermostat, DPST switch, or control circuit board? Are there any troubleshooting steps available for checking each component?

    Non-Pentair question: Unfortunately, now that the pool is closed and winterized, I am now losing water. It is a vinyl lined pool, and I believe I can see the leak near the bottom corner of the deep end. I've been told the only way to fix it is to un-winterize the pool and warm it back up to about 80 degrees so it can be patched (it looks like about a 1/4 inch tear in the vinyl. I am losing about two to three inches of water per week (with it covered).

    *** Question 5: Is there a way to patch it without reheating the pool until spring? I am worried about the damage that the escaping water might be causing to the underlying soil and structure, so don't want to just keep adding water, but if I de-winterize it it will cost me a fortune to heat it up with the cold temperatures we are having.

    Thanks a ton in advance for all your help!!

  2. #2
    Mod Squad Bama Rambler's Avatar
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    Re: Pentair Minimax Plus 400 Troubleshooting

    1) They should just thread in. You should use a little teflon paste on the threads more for the ability to remove it in the future than anything else.

    2) Yes the pressure switch should be open with no pressure on it.

    3) If the cutout(s) are weak they'll open before the heater gets up to temp so it'll just get the water barely warm.

    4) I'd still think it's the high limit cutout causing your problem. Replace that and try it again. The Spa setting allows the water to get hotter than the pool setting. Since you don't have a spa it doesn't make a difference.

    5) This is the tricky one. If you can see the leak and you can get a small piece of thin rubber to seal over it you might be able to lay the piece of rubber over the crack and let it seal it till next year when it warms up. The weight of the water trying to escape the crack should be enough to hold it in place during the winter.
    Dave J. TFP Moderator
    24' x 52" Round AGP. 1.5hp Dynamo pump. 24" Pentair Sand Dollar Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
    Pool School ----- Pool Math ----- TF-Test Kit
    You can stop SLAMing your pool when you pass the OCLT (You lose 1ppm or less FC overnight, & You have .5ppm CC's or less) & your water is clear.

  3. #3
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    Re: Pentair Minimax Plus 400 Troubleshooting

    Thanks for the quick reply Dave! I'll get the new limits in tonight when I get home, and check out the pressure switch. If I can get the leak stopped (or at least slowed down), I'll wait to de-winterize when it warms up in the spring. I know I have at least one leak right in the corner of the deep end where a preexisting patch has come off. I am hoping that is the only one I need to deal with right now (fingers crossed!)

  4. #4
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    Re: Pentair Minimax Plus 400 Troubleshooting

    Still having issues, but (hopefully) making progress...

    OK - here's an update:
    1. I checked the Hi Limit switches, and they were in fact OPEN (when cold). Replaced both limit switches.
    2. Discovered the pressure switch was always CLOSED (not OPEN, like it should be when the pump is not running.) Replaced pressure switch.

    At this point, the heater appears to be working normally. All correct LED's are lit, ignitor is firing and burners are burning with a nice blue flame (yeah!!!)

    However - the pool does not seem to be heating very quickly at all. Because of leaking, I've had to plug both skimmers and the vacuum return line so the system does not suck air. I also had to bypass the Brominator with a straight pipe due to a leak there. (sheesh!) The line to the main drain is open, and water is running freely through the filter basket, pump, filter, heater and into the pool through the return lines. BUT, the water does not feel hot - it doesn't even feel much warmer than the water already in the pool. I've put a thermometer back in the pool, but am only gaining about 2 degrees F every 4 hours. I am still losing about 2 inches of water a day, so have culinary water running in through a garden hose. But it seems like it still ought to be heating more quickly. Is there a possibility there is still an issue with the heater? Any ideas of where to look???

  5. #5
    Mod Squad Bama Rambler's Avatar
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    Re: Pentair Minimax Plus 400 Troubleshooting

    Sounds like it's working as well as it can. What's your filter pressure?
    Dave J. TFP Moderator
    24' x 52" Round AGP. 1.5hp Dynamo pump. 24" Pentair Sand Dollar Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
    Pool School ----- Pool Math ----- TF-Test Kit
    You can stop SLAMing your pool when you pass the OCLT (You lose 1ppm or less FC overnight, & You have .5ppm CC's or less) & your water is clear.

  6. #6
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    Re: Pentair Minimax Plus 400 Troubleshooting

    Filter pressure is 12 psi. When I first turned it on, it was about 10 psi. I checked the temperature tonight (after running the heater continuously for the last 8 hours) and the temperature is only up to 54 degrees. (it was 50 degrees at noon). At this rate, to get into the 70's or near 80 will take another 48 hours or so. Is it possible that I have a problem with the heat exchanger? Clogged or something?

  7. #7
    Mod Squad Bama Rambler's Avatar
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    Re: Pentair Minimax Plus 400 Troubleshooting

    While it could be clogged. If it was the high limits would be taking it out or the flow switch would not allow it to fire I would think.

    Do you have prior experience with heating 35k gals of water from 50ยบ? I ask because to me that's a lot of water and that's a big delta T for a 400k heater to heat up very quickly at all.
    Dave J. TFP Moderator
    24' x 52" Round AGP. 1.5hp Dynamo pump. 24" Pentair Sand Dollar Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
    Pool School ----- Pool Math ----- TF-Test Kit
    You can stop SLAMing your pool when you pass the OCLT (You lose 1ppm or less FC overnight, & You have .5ppm CC's or less) & your water is clear.

  8. #8
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    Re: Pentair Minimax Plus 400 Troubleshooting

    Only in the spring when we typically open up for the season. But by then the air temperature is usually at least regularly above 60 if not 70 during the days. We are still in the 40's during the day and 20's at night... I guess I just need to be more patient It should be warm enough to get in to find leaks by Monday, so I'll post next week and let you know what we find. Thanks for all your help and advice so far, though!

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