Mastertemp 400 won't heat

mattske

Member
Sep 19, 2021
16
So Cal
Heater stopped heating two weeks ago and made a rattle sound near the manifold before shutting itself off. I replaced the igniter, thermal regulator and thermistor. Rattle is gone. Heater turns on, hot air blows out for 4-5 seconds and then luke warm air comes out. This process repeats itself. At times, the temp display spikes to around 115 degrees before it settles back down to the pool temp of around 76. I also replaced the hi limit switch with the same results. Wiring and connections all appear fine. After much reading, I'm thinking the mother board may need to be replaced. Thoughts?
 

ps0303

TFP Expert
In The Industry
Jul 6, 2011
4,128
FL
The rattle sound is actually the small amount of water being pushed into the unit boiling and thus sounds like a rattle because the bypass is broken. When you removed the thermal regulator, the plastic piece might have been fallen out and down inside the heater. Remove the thermal regulator again and place your finger inside and feel up towards the top. You should feel something like the shape of a large button. If you don't feel it, then it's missing and that is the issue. If you do feel it, it should be firmly in place. If it moves around, it's broken. #8 in this image is the bypass part. You can see how it hangs down. Many times if it's broken, it hangs down in the way of the thermal regulator spring and makes it hard to pull the spring out.

1632074409786.png
 

mattske

Member
Sep 19, 2021
16
So Cal
Thanks. I have the part on the way. I won't be able to get to it until next weekend. In the meantime, I've noticed a small water leak on the bottom of the manifold. Small as in a drop every few minutes. I will figure that out when I pull that sucker off. Gonna have to upgrade from a six pack to a twelve pack for when I'm done.
 

mattske

Member
Sep 19, 2021
16
So Cal
Alright, I removed the manifold and the manifold bypass part. The old part was intact and seemed to decompress properly. I replaced it with the new part anyway since the manifold was removed and taken apart already. And, no more rattle, but heater still doesn't work. It fires up when I turn it on, I hear the whoosh and heated air comes out of the exhaust. The entire unit shutters slightly and the heated air turns to luke warm air and the temp spikes to about 124-125 before the temp returns to the displayed water temp, the unit shuts down, and starts the process again. I let it run for about 5 minutes doing this cycle before I turned it off. I've read other places maybe the control board isn't working properly, but I don't get a service heater light, and no warning lights on the back of the panel or on the fenwal panel. So far I've replaced the igniter, thermal regulator, hi limit switch and mainfold bypass valve. I may be upgrading my 12 pack to a case. Any other ideas?
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
29,153
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Please create your signature with details of your pool and equipment.

What filter do you have?

What pump do you have?

What is your flow rate into the heater?

What you describe can be caused by a low flow rate. The MasterTemp 400 requires a minimum flow of 40 GPM and more like 60 GPM.

Do you have a VS pump?

When did you last clean your filter?
 

mattske

Member
Sep 19, 2021
16
So Cal
I added the following: Intelliflo VS-3050 Pump, IC 40 Salt/Chlorine Generator, Pentair FNS Plus Filter, Pentair MasterTemp 400 heater, Appx, 17,000 gallons.
Flow shows good/green on the salt generator. Last filter clean was in early or mid-June. I had a backwash last week.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
29,153
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
The SWG requires little flow compared to the heater. The fact that the SWG flow light is green is meaningless to the question of adequate flow through your heater.

What RPM is your VS pump running with the heater?

What is your clean filter PSI?

What is your current filter PSI at what pump RPM?
 

mattske

Member
Sep 19, 2021
16
So Cal
Filter PSI is 21 and clean filter starts at 22 (my pool guy is aware of my issue and he already stated a filter cleaning is due soon). I'm guessing I'm looking at this correctly...spa shows 3300 RPM's, high speed 2950, pool 2800 and pool heater 2600 (I've never messed with these). Those numbers are from the settings/IntelliFlo/pump on my control panel and remote.
 

mattske

Member
Sep 19, 2021
16
So Cal
Filter clean was completed yesterday and I set the RPM's to 3000. Heater turns on and heats for exactly 30 seconds, then the luke warm air comes out for 30 seconds, then it heats again for 30 seconds, luke warm air for 30 seconds, etc. No shudder, no shut down. I have to admit I never paid much attention to how it worked in the past, does that sound correct?
 

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Poolbreh

In The Industry
Jul 31, 2021
272
San Diego
I would pull the thermal regulator and place it boiling water just to verify it’s opening. I know it’s new, but there is an issue with the flow in the manifold if it’s spiking to 115-125f which would shut the heater down before it triggers hi limit 135f. Which wouldn’t give you fault. Next I would check for a clog in the heat exchanger. Since bypass has been replaced.
 

JamesW

TFP Expert
Mar 2, 2011
28,263
You probably have a flow problem.

Hot water is backing up into the inlet, which means that there is almost no flow through the exchanger.

Maybe an external bypass is open or the heat exchanger is clogged.

Do you have pictures of everything?

Did you take pictures when you had the manifold off?
 

mattske

Member
Sep 19, 2021
16
So Cal
No, the temp display shows the water temp. It no longer spikes. I tested the new thermal regulator before I installed it and it worked. I also tested the old one and it did not work.

No photos and here is where I'm at:

Pool mode and heater on: 52 seconds of heat/hot exhaust followed by thirty seconds of no heat and luke warm air/exhaust.
Spa mode and heater on: 42 to 52 seconds of heat/hot exhaust followed by 30 to 40 seconds of no heat and luke warm air/exhaust. The spa temp did increase from 75 to 78 over the course of about the 8-9 minutes I had it running, but I shut it off since it didn't seem to be working correctly.
 

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
4,285
Palm City/FL
Pool Size
12800
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-30 Plus
Filter clean was completed yesterday and I set the RPM's to 3000. Heater turns on and heats for exactly 30 seconds, then the luke warm air comes out for 30 seconds, then it heats again for 30 seconds, luke warm air for 30 seconds, etc. No shudder, no shut down. I have to admit I never paid much attention to how it worked in the past, does that sound correct?
I was leaning toward Jame's analysis until you said it doesn't shudder. I'm guessing that means no weird flow sounds. They're pretty loud and unmistakable. If you had a bad sensor in the safety loop it would fail and shut down the heater before you get a flame. Next thing on my list would be to check the temp sensor that tells the heater what temp the water is being heated to. I've seen these go bad from time to time and they usually fail by reading the temp higher than it is so it turns the heater off. They're also pretty inexpensive. If it were me that's what I'd try next. You can test then with an ohmmeter but it's kind of a pain since you have to measure the resistance right when it kicks off the heater.

Chris
 

mattske

Member
Sep 19, 2021
16
So Cal
Hard liquor it is. So, same deal as before. There is something of a high pitched sound at various points during the cycle. It's not overwhelming, but I can hear it. However, I am finally getting a "service heater" light at the end of the cycle, and the "HLS" light is displayed on the control board. I find this ironic since I had never had this before and the hi limit switch is brand new. The wiring looks good. It is not frayed, loose or damaged that I can see. On a whim, I put the old hi limit switch back in on the possibility the new one is not working, and I get the same thing. Service heater light and HLS light on the back. I double and triple checked that I purchased the correct part and as far as I can tell, I did so.

I attached a couple photos, but the videos won't load because it says the file does not have an allowed extension.
 

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ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
29,153
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
All the symptoms say the water in your heater is overheating including triggering the HLS.

There is no indication that the electronics or the sensors are the root cause of your problem. You have an internal water flow and heat transfer problem creating the overheating.
 
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