Pentair MasterTemp 400 HLS intermittent error

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ndakotaninja

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Nov 26, 2013
122
Elkridge, MD
I read through a bunch of posts including the massive "Further Reading" page about Pentair heaters and I'm still stuck. Our heater is plumbed for both our pool and in-ground spa, so we went to last night and I turned on the heater to get into the hot tub, and ran into an odd issue. The heater would turn on, fire, heat up for ~5 minutes, then something would trip and it would cut off and show the "Service Heater" light. After about 60 seconds, the light would go away, the heater would fire up again, and it would heat for another ~5 minutes. The hot tub eventually got up to temp, but obviously something is wrong.

First thing I did was swap out my dirty "opening" cartridge filters with brand new filters, cleaned out my pump basket, and checked for any other potential flow issues. Tried the heater again, high RPM (I think 2900 for the spa setting). Same thing...after a few minutes, it trips, waits a minute, clears, and starts up again. I took the heater apart and see it's the HLS light. The reading indicated the issue could be flow-related (taken care of), and could be the temperature regulator. I pulled it out, tossed it in hot water ~140 degrees and it opened up just fine. It was a little bit corroded/rusted, but looked just like the ones in all the youtube videos i searched.

From other posts, it seems if it was the HLS sensor itself that was bad, it wouldn't even fire up. I don't know if that's true in every case, but I don't know how to test if the sensor is bad prior to buying a replacement. So I figured I would ask here and see what I've missed that I could try out. I had to replace the AGS sensor last year, and saw someone said usually if one goes bad the other is close behind, but again, would the heater even kick on if the sensor was bad? Could it be "going bad" and this is the behavior you'd see?
 
The thermal regulator should open at 120F and not 140F. Replace the thermal regulator as a start.

The HLS sensor should be normally closed. You can test it with a multimeter. If it is open the heater will not start.
 
I may not have been clear. The regulator seemed to open fine. My hot water is only 110, so I had to boil some water to add to test it. At 125 it opened which could just be a thermometer discrepancy. At 130 it clearly was opened all the way. If you think I should have seen some movement before 120, then I can replace it, but I assumed it should only start to move at 120 or above. I can always take it back out and do another test or send pictures, but the regulator looks fine and seems to act fine.

I don't have a multimeter to test, but as you mention, if it were a bad sensor, it would therefore be open and sending current, which would prevent the heater from starting? So if my heater is turning on firing up, and heating properly, but tripping every ~5 minutes, it doesn't seem like a sensor issue either.
 
I ordered a new regulator that came in today. I tested it and it did not open until 140 degrees. It also did not have a rating stamped on it. It is supposedly a pentair original part but a lot of people have complained that they aren't the correct part, so that wasn't a surprise. I took the opportunity to re-test my current regulator and verified it opens right at 120 degrees as rated.

So I'm still stuck. It seems like the next place to check would be "External bypass open" or "internal bypass broken". I'm guessing the internal bypass just means if you feel the thermal regulator get stuck, or if you don't feel the 'button'? Not sure what the "external bypass open" means. But if trying to fix either involved taking apart the entire manifold, I guess I'm stuck calling a repair guy because I don't want to do that.

If the bypass it's referring to is the bypass valve, it says it can short cycle and/or make noise. Mine doesn't make any noise, but if it isn't the HLS sensor or the thermal regulator, I would assume this is the issue. Any way to check/test it without taking the entire system apart?
 
Yeah if that is the only next option (reading the Further Reading, that was all it suggested), then I'm going to call a service company. I saw a thread with pics about taking apart the heater manifold and not really something I want to get into, especially if that's still not the issue.

Is it odd for this type of thing to fail in the first 5 years? It's inside a pool shed and still in great condition overall. Anyone know what the standard warranty (# of years) for these heaters are?
 
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