Pool/Spa Heater (cycling on and off every 45 seconds). Takes 1.5 hours to heat spa


New member
May 3, 2021
Waxahachie, Texas
I am new to this site and I pay a professional pool service to take care of my pool.

Our Setup
PentAir Master Temp 400 - the pool was installed in Sept 2020.
PentAir Water Pool and Spa Automation.
Salt Water Pool.

Back in Dec 2020, we set our spa (which we weren't using very often) to 101 and when we went to get in, it was at 90.
We could hear the heater cycling on and off, hot water in for a little while, then colder water.

Now May 2021:

The pool/spa heater is still not working properly.
The heater itself is cycling on and off about every 55 seconds. I timed it. Consistently on and off.

The spa is taking 1.5 hours to heat up to 100 degrees. It used to take about 20 minutes.
So far replaced at 3 different service calls.

The thermal regulator $94.68 (Dec 2020)
Temp Sensor $85 (Feb 2021 – before freeze)
Replaced Air and Water Temp Sensors $98.06 (Apr 2021) *is this the same Temp Sensor?
Max-E-Therm Hi Limit Switch $80.94 (Apr 2021)
Jandy Valve Oring Kit $14.72 (Apr 2021)

Can it be water flow (or lack of) or something in circuit boards?

Any ideas?


Well-known member
Mar 30, 2019
Las Vegas, NV
Why are you showing part prices for a heater that is only about 7 months old? Hopefully you are not paying for the parts! The service technician needs to determine the reason that the heater is short cycling. How long does the heater actually run with the flame established. Are there any fault codes showing on the Fenwal ignition control? Are there any diagnostic LEDs showing on the control board? It seems that parts are being replaced in the heater randomly in an attempt to solve the problem without correctly diagnosing the root cause of the problem. If possible, observe the technician while he is working on the heater. Ask questions regarding what he is doing and why. Read and refer to the "Installation and Users Guide" that you should have for the heater.


Gold Supporter
Dec 26, 2019
NE Oklahoma
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
I had this issue last weekend. Short cycling on the heater. Heater was installed 1 year ago. Pulled the panels to see what lights were on. None. Only LED10 and LED8 which are circuit on and heating/call for heat. No error LEDs, no service heater LED on the front panel, nothing.

Couldn't determine anything was wrong. Stack temperatures were ~270. When I pressed the up arrow button on the keypad, it was showing a set temp of 94. My Intellicenter setpoint was 88. I raised that up to 104 and the issue seemed to go away. Still don't understand it. Not sure that fixed anything but I am going to keep an eye on it for the next few weeks.

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