By-passing High Limit Switch on Pentair MasterTemp 400 Heater

wwtennant

Gold Supporter
Mar 21, 2015
32
Bellaire/TX
After reading several older but always helpful TFP posts, I was able to trouble shoot my MasterTemp 400 heater. I jumped / bypassed the High Limit Switch and the heater now works perfectly again. I did this only for diagnostic purposes and am not using the heater this way.

My question is simply this- all I need to replace now is the High Limit Switch since it works fine after jumping the High Limit Switch? I am pretty sure that is right but thought I'd ask. It won't be the Bypass Valve or the Thermal Regulator? Just wanted an expert opinion before spending the money. As always- THANKS TFP!!
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,741
Northern NJ
Did you draw some water off the heater drain and determine it was below the 135F the HLS should be opening at?

If the water is getting to 135 then the HLS is not at fault.
 

wwtennant

Gold Supporter
Mar 21, 2015
32
Bellaire/TX
Allen - I did not do this but I immediately see your point. So I should run the heater for a bit and then open the drain and see what temp the water is that is coming out at. If over 135, then it is not the HLS. If below, 135 then I replace the switch. If it is over 135, then what is my next step?? Thanks!!
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,741
Northern NJ
Allen - I did not do this but I immediately see your point. So I should run the heater for a bit and then open the drain and see what temp the water is that is coming out at. If over 135, then it is not the HLS. If below, 135 then I replace the switch. If it is over 135, then what is my next step?? Thanks!!
Run the heater until HLS shuts it down. Then check water temperature. If water is something like 80 then HLS is opening prematurely and should be replaced.

If HLS is opening around 135 then it is doing it job and you need to look elsewhere for why water is getting so hot. Don’t expect HLS to be precisely at 135. Just somewhere around there.
 

ps0303

TFP Expert
In The Industry
Jul 6, 2011
4,023
FL
Bypassing the high limit and the heater running doesn't always mean that is the culprit. Remember, the limit activates if the water gets too hot. Many times on the unit you have, that switch will activate actually because of a bad thermal regulator. So the limit is actually doing its job. So how does the thermal regulator look?
 

wwtennant

Gold Supporter
Mar 21, 2015
32
Bellaire/TX
Team TFP- I did what Allen suggest above and ran the heater until the HLS switch stopped it. I then checked the water coming out of the drain... it was 84 degrees. Thus, I conclude that it is the HLS and should replace it. However, i did not check the thermal regulator as ps0303 suggested. Is it possible at this point that it could the thermal regulator causing the HLS to trip when the water temp is only 84? It started at 83 and tripped shortly thereafter and the water was only up one degree to 84. Seems like it would not be the thermal regulator... One last check before I buy the new HLS. Thanks
 

JamesW

TFP Expert
Mar 2, 2011
16,587
I would suspect that it's a bad thermal regulator.

The water from the drain is not going to be the same temperature as the water exiting the heat exchanger.

If you want to verify the high limit, get some hot water and dip the high limit in it while checking the resistance of the high limit to see where it opens.

I would remove the thermal regulator and test it in hot water to see if it opens. If it looks corroded, replace it.

 
Last edited:

wwtennant

Gold Supporter
Mar 21, 2015
32
Bellaire/TX
I went ahead and pulled the thermal regulator and tested it. The video method didn't work as my water does not get hot enough. So I boiled some on the stove and the thermal regulator functioned properly. Thus, at this point, it must be the HLS which I will order. Am I missing anything?? Thanks
 

JamesW

TFP Expert
Mar 2, 2011
16,587
Try running the heater with the thermal regulator left out. If the high limit still trips, it's probably bad.

Typically, when the high limit goes, the heater won't even try to start. The high limit might be failing. You can remove the high limit and put it in hot water to see if opens too early. Maybe try 120 degrees.

Maybe just order both parts. They both fail fairly regularly and you will probably need both eventually.
 
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JamesW

TFP Expert
Mar 2, 2011
16,587
The thermal regulator blocks the flow of water through the heat exchanger until the water heats up. This prevents condensation from forming on the heat exchanger.

If the thermal regulator does not open enough when the water heats up, the water will overheat and trip the high limit.

With the thermal regulator removed, it can't block the flow of water through the heat exchanger.
 

wwtennant

Gold Supporter
Mar 21, 2015
32
Bellaire/TX
The thermal regulator blocks the flow of water through the heat exchanger until the water heats up. This prevents condensation from forming on the heat exchanger.

If the thermal regulator does not open enough when the water heats up, the water will overheat and trip the high limit.

With the thermal regulator removed, it can't block the flow of water through the heat exchanger.
That makes sense. Thank you!!
 

wwtennant

Gold Supporter
Mar 21, 2015
32
Bellaire/TX
So I ran the heater without the thermal regulator and it still had the same issue of cycling again and again. So, I replaced the HLS and it is still doing the same thing. It just keeps cycling... it runs for about 5 to 10 minutes at first and then just cycles and cycles. It will ultimately get up to the correct temperature but it needs to cycle about 20 times to do it.

Given that i have replaced the HLS and ran it without the thermal regulator, what else could be causing it?? I am replacing the thermal regulator simply because i have an extra. The flow of the water into the spa seems strong... not like it is being blocked and the pressure gauge on the filter appears normal. Thoughts for me??
 

wwtennant

Gold Supporter
Mar 21, 2015
32
Bellaire/TX
So I ran the heater without the thermal regulator and it still had the same issue of cycling again and again. So, I replaced the HLS and it is still doing the same thing. It just keeps cycling... it runs for about 5 to 10 minutes at first and then just cycles and cycles. It will ultimately get up to the correct temperature but it needs to cycle about 20 times to do it.

Given that i have replaced the HLS and ran it without the thermal regulator, what else could be causing it?? I am replacing the thermal regulator simply because i have an extra. The flow of the water into the spa seems strong... not like it is being blocked and the pressure gauge on the filter appears normal. Thoughts for me??
And I just ran it with the new thermal regulator and new HLS both installed... same cycling... Thoughts??
 

wwtennant

Gold Supporter
Mar 21, 2015
32
Bellaire/TX
I would check for scale or corrosion in the heat exchanger.

How old is the heater?
The heater is 7 years old now but i have never had a scale problem in the salt cell that sits right outside of the heater... Also, would it still run properly with good flow and heat when the HLS is bypassed if it was scale or corrosion?? Could it be the thermal bypass valve??
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,741
Northern NJ
If HLS is functioning properly then it says water is getting to 135. That means you have inadequate water flow through the hear exchanger and what little water is flowing through is over heating.

Lack of scale on SWG has no correlation to scale in the heater. If heat exchanger tubes are restricting water flow then water will overheat in heater.
 

JamesW

TFP Expert
Mar 2, 2011
16,587
Do you have an external bypass?

Maybe it's the internal bypass. If you look in the Inlet, you should be able to see the half of the internal bypass with the spring. If you look in the hole where the thermal regulator goes, you should be able to see the other part of the internal bypass with round disk.

Do you have a picture of the system?