MT400 HLS error @ 100 degrees

[mrsol]

Hello. I have a master temp 400 about .w years old. It powers our pool and spa in the summer and only the spa in the winter.

This week it started throwing an HLS error as soon as it gets to 100 degrees. At which point it shuts off the gas/heat, seems to reset, try again, heat for 30-60 seconds, then repeat.

The googling I’ve done says it could be water flow, however it will heat from 70-100 without problem. It’s just when it his 100 it starts shutting off.

Other results said it could be insufficient gas. But again, it heats 70-100 just fine.

Could the high temp switch really be bad after only 2 years?

Anything else I could test/try?

Thanks.

 

[ajw22]

Check the thermal regulator and that it opened at 120F.

Check the internal bypass in the manifold.

Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com

The HLS is often caused by insufficient water flow through the heat exchanger. This can be due to several things:

• Thermal regulator not opening.
• External bypass open.
• Internal bypass broken.
• Pump too small or on too low rpm.
• Clogged filter.
• Scale in heat exchanger.

 

[mrsol]

Thank you @ajw22 - I’ll search here or google to see how to do inspect those parts.

Is this common after only 2 years? We do have extremely hard water in the area so I imagine it wears things out much faster.

These two area you recommend, they align with the condition of failing after a certain temperature is reached?

Thank you again.

 

[wireform]

The thermal regulator is by far the easier one to check so I wouldn't wait.