Raypak 336A Temp Sensor Reading Too High

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orangegreyhound

New member
Mar 27, 2025
3
Las Vegas, NV
Hello! I am stumped trying to figure out an issue with my heater. Got a sensor fail code, so I swapped in a new thermistor. Easy peasy, right? I also cleaned the contacts on the board with QD Cleaner before I installed the sensor. Here's basically what I got: https://www.amazon.com/Flngr-009577F-Temperature-Compatible-Heaters/dp/B0B7H8SVJJ

However, now, when I go to turn the heater on, within ten seconds or so I can watch the "Water Temperature" readout on the LED screen climb rapidly and smoothly, all the way up to 190ish degrees. It stays there, and slowly comes back down to temperature once the heater turns itself off due to the apparently-high temperature.

I have tried turning it off/on, waiting a while, resetting the heater to factory defaults... nothing seems to be working to fix this problem. The heater behaves normally other than this incorrect water temperature reading, so I'm inclined to think it's still some problem with the thermistor. I really don't think I ordered an incorrect part. I don't know of any way to calibrate this, or have any idea on next steps forward to troubleshoot. It's not a very complicated machine, so I'm wondering just what might be causing the actual problem. Any suggestions would be much appreciated. Thanks!
 
Well, there was indeed a water flow problem within the heater. I pulled the internal bypass and governor devices, and verified that they are working correctly. Inside the manifold was also clean and free of scale buildup, which was surprising to me.

While I was standing there scratching my head, I looked over and had a eureka moment -- one of those "nobody ever touches this one" valves had, in fact, been touched. It was the bypass valve for the heater. My hypothesis is that someone accidentally opened this valve instead of the correct one while trying to heat up the spa... perhaps it was me after eight of Colorado's finest, perhaps it was "someone else." I closed the valve to divert all the water to the heater, and the problem is fixed.

I appreciate all of the help in just those two responses, I am glad I took a deeper look into the internals and am much more confident to work on this again, should another problem arise. Thanks again!
 
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orangegreyhound said:
Well, there was indeed a water flow problem within the heater. I pulled the internal bypass and governor devices, and verified that they are working correctly. Inside the manifold was also clean and free of scale buildup, which was surprising to me.

While I was standing there scratching my head, I looked over and had a eureka moment -- one of those "nobody ever touches this one" valves had, in fact, been touched. It was the bypass valve for the heater. My hypothesis is that someone accidentally opened this valve instead of the correct one while trying to heat up the spa... perhaps it was me after eight of Colorado's finest, perhaps it was "someone else." I closed the valve to divert all the water to the heater, and the problem is fixed.

I appreciate all of the help in just those two responses, I am glad I took a deeper look into the internals and am much more confident to work on this again, should another problem arise. Thanks again!
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Should anyone read this, at every heater school I have ever attended, the first thing they teach is that 95% or more (literally as I have found out) of all service calls on pool heaters have nothing to do with the heater, it is a waterflow problem. Most often it is a dirty filter that was "just cleaned." Then clogged pump basket, clogged valve (spas don't usually have baskets and the main drains let a lot of stuff through), clogged impeller, valve in the wrong position. Occasionally a clogged skimmer or suction line, but there are not a lot of heated pools in my area, it costs too much.
 
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1poolman1 said:
the first thing they teach is that 95% or more (literally as I have found out) of all service calls on pool heaters have nothing to do with the heater, it is a waterflow problem.
Click to expand...
I agree with this.

So, I find it incomprehensible that the manufacturers don't include a flow switch on the heater.

50 years ago, the system was simple with a single speed pump and no bypass.

So, a pressure switch was a reliable way to detect flow for most applications.

However, with variable speed pumps and complex systems with bypasses, it is foolish to make a heater without an included flow switch.
 
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