Jandy JXI 400 High Limit Fault (already replaced limit switches)

[alwhea78]

I have a Jandy Jxi 400n heater that is ~4 years old. I have had issues with a high limit fault since the end of last season. At the start of this season, we changed the limits and checked all of the flow and vents to confirm no obstructions. It seemed to work ok for awhile and then the fault started again. This time we checked the wiring and it appeared that one of the temp sensor black wires was loose, again seemed to work ok almost all summer after fixing that. Now this week it's starting again. The fault occurs almost immediately (within minutes) of starting up the heater and then clears itself within 30 seconds, the heater fires again, faults again, etc. I can't imagine that the water even has time to get hot enough to actually be a true fault based upon how quickly it occurs. I can see on the circuit board that when it faults, the light next to "D2" goes out and then comes back on when it clears. Is this telling me anything? Could it be the water temp sensor itself? The temperatures seem to be reading accurately, at least from what I can tell. What else can I check or try?

 

[1poolman1]

I have a Jandy Jxi 400n heater that is ~4 years old. I have had issues with a high limit fault since the end of last season. At the start of this season, we changed the limits and checked all of the flow and vents to confirm no obstructions. It seemed to work ok for awhile and then the fault started again. This time we checked the wiring and it appeared that one of the temp sensor black wires was loose, again seemed to work ok almost all summer after fixing that. Now this week it's starting again. The fault occurs almost immediately (within minutes) of starting up the heater and then clears itself within 30 seconds, the heater fires again, faults again, etc. I can't imagine that the water even has time to get hot enough to actually be a true fault based upon how quickly it occurs. I can see on the circuit board that when it faults, the light next to "D2" goes out and then comes back on when it clears. Is this telling me anything? Could it be the water temp sensor itself? The temperatures seem to be reading accurately, at least from what I can tell. What else can I check or try?

For some reason, everyone blames the safety device, hi-limit in this case, when it does its job. Most likely the internal bypass is "bypassing" too much water, allowing the water in the exchanger to get too hot, and tripping to let you know that there is a problem, and not sending scalding water into the pool or spa. When it trips the flame goes out, cooler water goes through the system, allowing the hi-limit to "close," and the heater re-lights and you go around in circles with what you see.

 

[alwhea78]

For some reason, everyone blames the safety device, hi-limit in this case, when it does its job. Most likely the internal bypass is "bypassing" too much water, allowing the water in the exchanger to get too hot, and tripping to let you know that there is a problem, and not sending scalding water into the pool or spa. When it trips the flame goes out, cooler water goes through the system, allowing the hi-limit to "close," and the heater re-lights and you go around in circles with what you see.

So how would I fix that? The only reason I question the limits is because from a cold startup (heater off for 24 hours+), it trips within a minute - is that enough time for the exchanger to even get hot enough to heat the water to a temp that would trip the limit? And this problem started very suddenly - was working fine all day one day and then I shut it off and then back on minutes later and this started happening and now has been doing it for a week, within a minute every time it starts up. How can I check to see if too much water is bypassing?

 

[alwhea78]

So how would I fix that? The only reason I question the limits is because from a cold startup (heater off for 24 hours+), it trips within a minute - is that enough time for the exchanger to even get hot enough to heat the water to a temp that would trip the limit? And this problem started very suddenly - was working fine all day one day and then I shut it off and then back on minutes later and this started happening and now has been doing it for a week, within a minute every time it starts up. How can I check to see if too much water is bypassing?

I should also add that I have my versaflow disabled currently.

 

[1poolman1]

So how would I fix that? The only reason I question the limits is because from a cold startup (heater off for 24 hours+), it trips within a minute - is that enough time for the exchanger to even get hot enough to heat the water to a temp that would trip the limit? And this problem started very suddenly - was working fine all day one day and then I shut it off and then back on minutes later and this started happening and now has been doing it for a week, within a minute every time it starts up. How can I check to see if too much water is bypassing?

Firebox temperature is an almost instant 2000 (yes, two thousand) degrees. Without water going through the exchanger the heater will, quite literally, burn itself up in short order. If not enough water is actually going through the exchanger, and not just the manifold, it heats incredibly fast. Haven't seen it in a JXi, but in both models of Pentair, the water will flash boil, create a lot of steam pressure, and the heater bounce off the ground by an inch or so just before the safety shuts it off.

Need to visually inspect the bypass to determine if it is damaged. The limit switches are very simple and seldom fail.