Pentair MasterTemp 400 starts cycling when water is 88 degrees

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cryptyk

Active member
Mar 5, 2022
29
San Diego
Hi - I have a MasterTemp 400. When I turn on the Spa, the heater runs constantly (maybe 10 minutes) until the water temp is about 88 degrees. Then I get an HLS led and the heater starts cycling. Off for about a minute, then on for about 2 minutes. The water eventually gets to 104, but it takes longer than it could if it just kept running all the way up until the set point.
I have the pump running at 3400 RPM to make sure there was enough flow and still get the same result.

Is this as-designed, or do I have a problem?
 
You have a problem with short cycling due to the heater overheating.

The HLS is often caused by lack of sufficient 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.

Start by checking the thermal regulator or the bypass valve in the header.

What type of filter do you have and when did you last clean it? Even with your pump at 3400 rpm a clogged filter can give you low flow.

What is your filter PSI?

Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com www.troublefreepool.com
 
Remove the thermal regulator to see if the problem goes away.

When you have the thermal regulator plug out, look to see if the internal bypass plunger is in place.

Look into the heater inlet to see the internal bypass spring.

Check the stack flue temperature.

Turn the heater on.

The fan will come on for about 30 seconds, then you will hear a click, and the heater should fire.

At that moment, which ever thermostat you are using (POOL or SPA) hold that button down for ~10 seconds.

Display should change from water temp to exhaust temp. Normal temps should be in the range of 290-350.


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Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com www.troublefreepool.com

While running, press and hold the On button to display the exhaust temperature (Older Models).

The new models use the Menu to get the SFS.

Here is what the normal exhaust temperature should be:

  • Below 250 degrees...very low
  • 250 to 290 ..................low
  • 290 to 350.................acceptable
  • 350 to 480 ..................high
  • Above 480...................error/shutdown.
 
Thanks everyone. I removed the thermal regulator and the heater doesn't cycle anymore. Ordered a new one and will replace it.
The bypass valve seems to be in good shape. It was closed when I felt up there, but it's intact.

Thanks!
 
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Check the stack flue temperature.

Don't leave the thermal regulator out too long as it helps prevent condensation from forming on the heat exchanger.

If you have cool water going through, you might produce condensation, which can drip into the combustion chamber and the water can build up.
 
JamesW said:
Check the stack flue temperature.

Don't leave the thermal regulator out too long as it helps prevent condensation from forming on the heat exchanger.

If you have cool water going through, you might produce condensation, which can drip into the combustion chamber and the water can build up.
Click to expand...
I wasnt able to get the heater to show the exhaust temperature.
I turned it on from the easy touch, waited for the blower, then waited for ignition and held down the spa heat button on the heater for 10 seconds.
It just continued showing the water temp.
 

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If the heater is set to "Pool Mode" then you have to hold the "Pool On" button, but If the heater is set to "Spa Mode" then you have to hold the "Spa On" button.

If that does not switch to the stack flue temperature, then maybe the On button is defective.
 
None of the buttons on the heater do anything. The heater seems to only be controllable via the EasyTouch.

Here's what happens:
The heater is normally without power. The screen is blank.
When I turn on the spa heater or the pool heater from the EasyTouch, it supplies power to the heater and the panel comes alive.
The panel says 128, then r=5, then the Spa On light comes on, the blower comes on, and the heating light starts blinking. Note that the Spa On light comes on regardless of whether I'm heating the pool or the spa.
After a few seconds, the panel starts to show the water temperature.
A few seconds later, the ignition kicks on and the heating light turns solid.

At no point do any of the buttons do anything, regardless of whether I press them or hold them.
 
JamesW said:
Ok, the heater is really old, so the buttons are probably defective.

What is the date code on the barcode label?
Click to expand...
I don't see a sticker/barcode anywhere, but the circuit board is 42002-0007 REV A. The pool came with the house, so I don't know much about the history of it.
In any case, I have a thermal regulator showing up next week and I just won't run the heater until then.
 
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