Pentair mastertemp 250 very involved problem

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Brerspidur

Member
May 9, 2025
7
virginia
I have a pentair mastertemp 250 that I have a very in depth issue. it has the new 6 button control pad and corresponding fenwal. At first I was having sporadic err ignition issues. The heater would fire and heat to temp, but then after losing temp and attempting to re-fire it would give me err ign. I could restart, and it would work at first until getting to temp and then would eventually fail again. I also noticed that it was running HOT, not hot enough for it to overheat, but around 350-400. eventually it overheated (scorched some wires that were touching the furnace) and melted the cover on the fenwal box, also burned out the transformer. And now to the current issue, .. .
I have replaced the transformer (it is putting out good voltage)
I have fixed wiring that was scorched and triple checked all other wiring
I have replaced the fenwal (dont know for sure the if last one was dead, but the casing melted enough and there was some scorch on the board that I felt it should be replaced)
I feel the control board is good, but also have a replacement that I can testing with.

Upon startup, i get error AFS. The blower will not come on. I checked values at F1, and F2 and everything seems to be good, but the relay is not closing. If I jump F1/F2 then the blower DOES operate. The control board is getting 120 and 24vac. everything looks good on J7 coming in but i did notice it doesnt appear to have 24 AC going from the control board to fenwal via J1.
Am I correct in assuming that is 24ac from control board TO the fenwal?
If this is the case, then something is causing control board (2 separate ones) from sending that 24AC. we have replaced the ignitor, and SFS, and thermistor. and have bypassed the AGS, pressure switch and hi limit switch.
the airflow switch seems to be working because if I jump f1/f2 for the blower to come in, then I get err IGN and not err AFS.
lastly I have not looked into the gas valve yet.


TLDR summary - replaced fenwal, control board, transformer, and most sensors, and also double checked all wiring and grounds but still get ERR AFS on startup, blower will not turn on.

Can anyone provide any input to point me in the right direction?
Thanks in advance
 
Welcome to TFPO.

Did you use all genuine Pentair parts?

Post pictures of your gas meter showing all data plates on it. Do you know what size gas meter your house has?

The gas valve will not come into play until the Fenwal is powered.

The PCB sends 24V AC to the FENWAL on IND as a call for heat. If the PCB is not sending the call for heat then the PCB is seeing a safety open.

The WPS is in the circuit for the FENWAL IND to get power.

Getting the AFS errors says the PCB requested heat from the Fenwal.

Check all the wires connected to the blower.

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@swamprat69 may have ideas.
 
@ajw22 Thankyou for that info. to answer some of your questions, yes I am using Genuine parts. I am attaching pic of the gas meter, we upgraded it when we put the pool it, and when I last checked static and dynamic pressure with a manometer they were in the normal range. we also double checked the wiring on the blower. Given that we can jump f1/f2 and the blower runs fine , I think that is not the problem. we DO have 24v at IND which tells me that the PCB is calling for heat. so that leads me to believe that although we are getting signal to fenwal for heat, but the fenwal is not responding. the one wire that was bothering us was the yellow 24v into the fenwal. That was not getting 24V from PCB. On a hunch (and running out of things to test, we jumped 24v to that wire and everything fired right up. and heated for a while and even maintained 275F at the stack flu temperature rather than the ~400 I was seeing before.

So now my follow up question is why is the control board not sending 24V to the yellow wire in the fenwal. (it is acting the same with 2 different control boards. it must mean that the control board does not like something and is not powering the fenwal. I have bypassed AGS, Hi Limit Switch, and pressure switch. and have swapped out air flow switch, stack flue sensor (which seems to be giving accurate readings and tried a second SFS), and swapped out the thermistor which seems to be giving correct values as well.

the only thing that we have not tested is the gas valve, and i cant find info on that, but it looks like it might only be an output. from the PCB.

Any thoughts ?

attached photos of crispy fenwal that we replaced (although it melted the housing and looks scorched, I dont know for sure its bad. and of gas meter plate, and lastly of the jumper I put on PCB to force 24V to fenwal input.

thank you
 

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Brerspidur said:
Update, it appears the issue running hot is NOT solved, it had sat at 275 for a while, but going back out to check it again, it is not getting up to 400
Click to expand...
What is getting up to 400? Your exhaust temperature?

Have you pulled off the manifold and checked for a scaled heat exchanger?

The heat has to go someplace. It either goes into the water or up the exhaust. If the exhaust is too hot than the heat exchanger is not working correctly.

The SFS should be shutting down the heater with high exhaust temperatures before anything melts.

The gas valve is controlled by the Fenwal, not the PCB.
 
I would check the pins and continuity of the yellow wire since it supplies basic 24V power from the PCB. I think it is not logic switched and powers the Fenwal first before any other control signals.
 
ajw22 said:
What is getting up to 400? Your exhaust temperature?

Have you pulled off the manifold and checked for a scaled heat exchanger?

The heat has to go someplace. It either goes into the water or up the exhaust. If the exhaust is too hot than the heat exchanger is not working correctly.

The SFS should be shutting down the heater with high exhaust temperatures before anything melts.

The gas valve is controlled by the Fenwal, not the PCB.
Click to expand...


@ajw22
SFS is getting to 400, I checked the thermal regulator and it is not operating properly. will replace that tomorrow. I guess that could have an impact on something. The SFS was getting high, but not enough to shut down (480) i think. I will pull the manifold and take a look, what should I look for that will show scaling? I will also take a look at the bypass valve.
Thank you again for the responses
 
Removing the manifold will let you see if you have white calcium scale in the tubes.

You need to open up the burner can to see if you have scaling on the outside of the heat exchanger.

 
@ajw22 got the manifold off and things looked pretty good in my opinion, the bypass seemed ok, but the tip broke off when I removed it, so i will replace that. DIdnt notice any build-up anwyhere, but copper did have some green spots.

different question, how does the bypass valve work? I notice there are no wires to it, so seems that it is just there and working, but I have a menu option to turn bypass on or off in my control board. Is this just for additional functionality that I do not have the hardware for?
 

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Bypass valve is a temperature controlled spring.

Your heat exchanger looks in good shape.
 
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