Pentair MasterTemp 400 keeps cycling on & off

Pcma10019

Member
Jun 8, 2020
6
LangoYpaGmaYtimE
I have a MasterTemp 400 that I have been troubleshooting since last year. I've check different foruns and looks like a common issue, but I have yet to see a resolution. Perhaps you can help. As soon as I turn the heat on, I see a 75 reading. It 'heat' blinks to call for heat and then steady light. It Ignites, air coming out of the exhaust. After about 5 secs it shutts off and temp starts counting up fast to 124 and then goes back to 75 and do the same cycle offer and offer again. No error codes on the front and no LED indicator lights on the back.So far, I replaced the thermar regulator, thermistor, a broken and just before opening the pool for the season, I replaced the bupass manifold. I found a rock in fhe by pass manifold so I thought I have resolved the issue. I was wrong. I also checked the blower. It was clean. No nest in there. Wires are intact. I'm getting frustrated with this. I don't want to buy and replace the control board because it's expensive and it might not be the problem. Filter is clean since I just opened the pool. The pressure is 20-25 psi, pump is running at 3450 rpm about 85gpm.
 

JamesW

TFP Expert
Mar 2, 2011
19,783
It sounds like the exit of the heat exchanger is blocked causing the hot water to back up into the area where the temperature sensor is.
 

Pcma10019

Member
Jun 8, 2020
6
LangoYpaGmaYtimE
I'm trying to upload a 32mb mp4 video file, but it's too large. I actually strip the video from 600mb. Anyway, it cycles as soon as I turn the heat on. Burber ignites and try to heat water for a few seconds and then lights turns off, temp will count up fast from current temp 75 to about 124 and then count back down when it's getting close to 75, the "System Service" comes on for about 3 seconds and then it'll try to call heat again.
 

JamesW

TFP Expert
Mar 2, 2011
19,783
You can upload to Youtube and link.

It might be a bad circuit board, but I suspect that the water temperature around the thermistor is getting hot and causing the shutdown.

That would indicate a blockage in the heat exchanger.

If you have the old thermistor, try this:

Turn the heater off by pressing the Off button so that the heater won't try to start.

Then, disconnect the wires from the new thermistor and connect them to the old thermistor and see what the temperature reading is on the display.

Then, hold the old thermistor in your hand to warm it up to see what happens to the temperature on the display.

If the temperature rises smoothly to body temperature, the circuit board is probably ok.

Do not turn the heater on like this.

In fact, close the gas valve just outside the heater to make sure that the heater doesn't fire.
 

Pcma10019

Member
Jun 8, 2020
6
LangoYpaGmaYtimE
I did a bunch of troubleshooting today.

1. I removed detached the terminals from the termistor and got an E01 error code on the panel. I connected my old termistor and got a reading of 85 degrees. When I hold it, the temp slowly went up to 90. I then reconnected the terminal on the new terminstor and got 75 (I guess that's the water temp).

2. When problem started happening last early last year, one of the thing I did was replaced a broken Jandy check valve between the heater and chlorinator. I also noticed that the flap wasn't opening even at 3450rpm and 20-25psi. Although, there's strong water coming out of the 2 return lines. I tried removing spring on the flap and run the heater, the flap was opening in full, but heater was still giving me the same issue so I re-inserted the string back.

3. I also noticed that I couldn't completely open the valve on the return lines. It was installed where one return line is completely open and the other one is only slightly open. So, I opened the 3-port valve and adjusted it where the 2 return lines are fully open. I run the heater and this time it did something different. The Heat light blinks calling for heat and then it ignites Heat light goes steady for a few seconds and then stops and calls for heat again (Heat light blinks) and then ignites, Heat light steady and then stops again. It kept doing this several times and then sometimes the temp will start counting up rapidly and slowly counts down to the current temp and cycles again.

Note: when the opened the manifold last winter to replace the bypass, the manifold and the heat coils have no signs of scales. I only found 1 small stone stuck inside where the bypass is. So I thought that was causing the problem.

In case there is blockage in the heat exchanger, how do I fix that?

Thanks for your advise.

Here are the video links. 1st one is the initial issue. The 2nd is after modified the 3-port valve to the return lines.


 

JamesW

TFP Expert
Mar 2, 2011
19,783
The pressure is 20-25 psi, pump is running at 3450 rpm about 85gpm.
How are you getting the flow rate?

I think that there's a flow problem through the heater.

Possibly a plumbing or valve configuration issue or something clogging the heater somewhere in the heat exchanger or manifold.

When you removed the manifold, are you sure that everything got put back together correctly?

Is the thermistor in the correct place?


Are the thermal regulator and internal bypass working correctly?
 
Last edited:

JamesW

TFP Expert
Mar 2, 2011
19,783

Check over this reference. Are any LEDs on the circuit board coming on?

I suspect that there's some sort of water flow issue. Can you show a picture of the entire system?

If there's no obvious configuration problem, you might have to open the manifold and check everything.
 

Pcma10019

Member
Jun 8, 2020
6
LangoYpaGmaYtimE
I just approximated water flow based on the pump RPM. I opened the pool 3 weeks ako with freshly cleaned filter with psi running from 22-30 depending on the rpm. The pressure gauge is also less than a year old.

If I were to get a flow meter, where should I install it (before or after the heater)?

The manifold has been assembled back carefully. The new thermistor attached correctly as much I can tell. I removed the old one and place the new one.

Attached is a video of my setup and some pictures.
 

Attachments

JamesW

TFP Expert
Mar 2, 2011
19,783
It looks like the three way valve that controls water going into the heater is set backwards.

There should be a part of the handle that says "Closed" or "Off" to indicate where the internal gate is.

You want the gate to be blocking the bypass line.

Most of the water is going through the bypass.

You should rotate the valve so that the Off part is blocking the bypass.

If the flap on the check valve is not coming up most of the way, it indicates that the water flow is very low.

Maybe check the chlorinator to see if it's clogged.

There are unions on both sides of the tab feeder. If you remove the feeder, you can look through the water path to see if it's clogged. The hole is narrow to create a venturi effect. So, it can be blocked by debris.
 
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JamesW

TFP Expert
Mar 2, 2011
19,783
I opened the pool 3 weeks ako with freshly cleaned filter with psi running from 22-30 depending on the rpm.
What is the pump model and what is the filter pressure on different rpm settings?

30 gpm on that pump is likely to be close to maximum, which would mean limited flow.

Are you definitely feeling good flow from the returns?
 

Pcma10019

Member
Jun 8, 2020
6
LangoYpaGmaYtimE
James, you're truly a TFF expert. As soon as I read your reply about the 3-port valve going into the heater, I thought right away it all make sense. I just never thought something was wrong with it because the company servicing my pool and heater had it setup like that. Bought the house 2.5 years ago. I should've tried harder to learn how the pool works. This is a very rewarding learning experience for me. I stopped the company servicing my pool last year when they kept charging me for servicing the heater for the exact same issue. I just didn't trust them anymore. So I heed your advise although the flapper in the check valve still doesn't open completely, the heater started working again. I let it run and the temp slowly went up from 74 to 77 in just an hour without stopping. So I'm very happy. I cannot thank you enough for all your help. I'm so glad I didn't have to buy additional parts.
 
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JamesW

TFP Expert
Mar 2, 2011
19,783
Ok, glad that you got it working.

:goodjob:

Probably still need to look into the flow issue.

Maybe a filter cleaning will help.

Maybe check the tab feeder pipe to make sure that it's clear.