Pentair mastertemp 400 works fine to 90 then cycles

[120d]

Hi, I have a 6 year old master temp 400 heating a pool/spa combo. It has been working fine until recently. Now it heats the spa from 60 to 90 degrees fine... But then consistently starts to cycle above 90 degrees where the display shows the temp climb rapidly (15-30 seconds) from 90 to the 104 set point. Heater shuts off. Temp goes back to 90 immediately. Heater kicks on and repeats. No errors. Will do this as long as I let it... And gets the spa over 100 eventually.

I checked the thermal regulator. A bit of scale and was opening around 130. Replaced with new that I checked (opens at 120). Also replaced thermistor and high cut out switch. No affect at all. Ran briefly with no thermal regulator. Same thing happened but appeared to climb and cycle more slowly.

Any ideas?

 

[ajw22]

Welcome to TFP.

Check the bypass valve in the manifold.

Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com

 

[1poolman1]

Hi, I have a 6 year old master temp 400 heating a pool/spa combo. It has been working fine until recently. Now it heats the spa from 60 to 90 degrees fine... But then consistently starts to cycle above 90 degrees where the display shows the temp climb rapidly (15-30 seconds) from 90 to the 104 set point. Heater shuts off. Temp goes back to 90 immediately. Heater kicks on and repeats. No errors. Will do this as long as I let it... And gets the spa over 100 eventually.

I checked the thermal regulator. A bit of scale and was opening around 130. Replaced with new that I checked (opens at 120). Also replaced thermistor and high cut out switch. No affect at all. Ran briefly with no thermal regulator. Same thing happened but appeared to climb and cycle more slowly.

Any ideas?

Anytime you replace the thermal regulator the internal bypass should be replaced as well. They are both in the same water and tend to be worn out at the same time.
Its just harder to replace the bypass, but after doing a couple repairs just replacing the thermal regulator then having to go back and replace the bypass for free (because I do guarantee my work and told them the heater was repaired), I won't do one without the other even if I'm asked to with the "we won't blame you" effort to get me to change my mind.
No, I'm not trying to pad the bill, but I can't do a lot of work for free either.

 

[120d]

Thanks... Looks like I'll try the bypass valve replacement. Interesting I noticed the heater immediately heats water +14 degrees when it kicks on. That explains why it cycles at 90 (it reads 104). I suppose water getting trapped or slow flow could cause that? Thermistor was replaced and same 14 warmup. Runs fine from 60 to 90 degrees

 

[120d]

Update: Well, that's frustrating! I was looking at how hard it would be to free the manifold to replace bypass valve when I noticed the heater appeared to be plumbed backwards. Outlet plumbing connected to clearly market "inlet" on manifold. The fairly large, somewhat respected local pool company had done some fixes to leaks in the plumbing, and connected it back up backwards! I had a old picture of the setup... clearly done backwards. Filter/pumps have run like that for almost six months. House is under construction, so I wasn't using heater and didn't know there was a problem. Any idea if this would have caused any damage? Here's the old and new pics.

 

[ajw22]

Only moving part in the MasterTemp water flow is the bypass valve.

You can flip the pipes around and see how the heater runs.

Or while you have the pipes disconnected open the manifold and check the condition of the bypass valve.