MasterTemp AFS Error

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randdusing

Bronze Supporter
Feb 7, 2021
35
Conroe TX
Howdy,

I turned on the heater this weekend, and everything seemed to work fine for about 5 minutes. Blower turned on, flame ignited, hot exhausted was blowing hard and hot water was coming out of the return lines. Then the unit's Service Heater led started flashing and the AFS led was lit up. I thought the air flow switch itself or the tubing may have gone bad, so I replaced it all earlier tonight. Heater seemed to work fine for about 10 minutes, but then the same AFS led lit up on the control board. I also checked the Fenwal this time, and it was flashing a single LED to indicate an air flow issue as well.

I reviewed Pentair MasterTemp Heaters - Further Reading, but I'm pretty sure the switch and blower are fine.

Should I try partially blocking some of the vents as indicated in this thread? Mastertemp 400 heater AFS light I'm just confused on what to actually block.

Thanks,
Rand
 
randdusing said:
Howdy,

I turned on the heater this weekend, and everything seemed to work fine for about 5 minutes. Blower turned on, flame ignited, hot exhausted was blowing hard and hot water was coming out of the return lines. Then the unit's Service Heater led started flashing and the AFS led was lit up. I thought the air flow switch itself or the tubing may have gone bad, so I replaced it all earlier tonight. Heater seemed to work fine for about 10 minutes, but then the same AFS led lit up on the control board. I also checked the Fenwal this time, and it was flashing a single LED to indicate an air flow issue as well.

I reviewed Pentair MasterTemp Heaters - Further Reading, but I'm pretty sure the switch and blower are fine.

Should I try partially blocking some of the vents as indicated in this thread? Mastertemp 400 heater AFS light I'm just confused on what to actually block.

Thanks,
Rand
Click to expand...
It won't hurt to try.
 
There are only 2 choices when you have an airflow switch problem. Either you are developing the correct pressure differential to make the switch close and it isn't closing (failed airflow switch) or you are not developing the correct pressure differential to close the switch. A differential manometer teed into the 2 sensing lines to the AFS should give you the answer. I believe the set point on your airflow switch is 1.25" WC. From what you are saying it seems that your AFS is making (closing) on startup, then dropping out. If you monitor the pressure when the system is running and see that developed pressure is greater than 1.25"WC initially, but drops below 1.25"WC when the systems stops after 10 minutes you know that the problem is not the AFS itself. Having the correct test instruments is always helpful in diagnosing a problem.
 
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I didn't add any tape but adjusted the air flow "screen" to partially block the vent hole. Whenever I initially started diagnosing the the issue, the screen was actually out of place, and I was hoping that was the cause of the issue. Unfortunately the heater followed the same pattern: it ran about 10 minutes the first time and then only about 3 minutes the second time. Is it possible for the blower motor to be overheating and stopping itself? The motor spins freely and continues to make several rotations after the AFS led switches on. I also noticed the "molex" connector seemed a bit loose, but the issue still persisted after fulling pressing the connectors together. I stuck a camera down the flue stack and the inside looked pretty clean as well.

@swamprat69, do you know how quickly the AFS will turn off the motor? I'm wondering if it will be hard to determine whether the AFS opened and thus turned off the motor, or if the motor stopped and turned off the AFS? Instead of investing in a manometer, could I also measure the resistance of the switch while the heater is turned on?
 
If your heater is running the AFS contacts are part of an electrical circuit and you would be measuring voltage not resistance. If the blower is not creating adequate differential pressure either to close the AFS contacts or keep the AFS contacts closed, there can be numerous causes for that.
 
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Update:
I temporarily gave up and hired a pool repair company to take a look. They wanted to replace the AFS and possibly Fenwal for about $1,000 and warned that other parts may require replacing if that didn't fix the issue.

Thus I did a little more research including reading some Pentair documentation that essentially said if the AFS is working and there are no blockages, try replacing the blower. I decided to buy a used blower off eBay for around $200 shipped, painstakingly replaced it and the heater is back in action!
 
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