Pentair MasterTemp 400 hot but not heating


New member
Sep 12, 2015
The Woodlands/TX
Greetings all,

Our Pentair is on and is blowing hot air out of the vent, but doesn't seem to be actually heating the water. I have the pool temp set to 84 and the board is currently showing the temp at 74. I had a E05 error, I replaced the flu sensor which cleared the service light and error code. Now on the surface it seems to be working but is just faking me out. I checked the back of the board, the LED11 light is on, and I did see the HLS light on but when I got my phone out to take a picture it went off. Is the LED11 light telling me something? I couldn't find anything specific on the world wide web, everything related to the LED11 light also showed the service light on, but that doesn't properly explain my issue.

Thank you


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
How many gallons are in your pool? Please create your signature with details of your pool and equipment.

If the heater is on and blowing heat out of the exhaust then the heat needs to be going someplace. If the heat is not going into the water the heater will overheat and trip the SFS or HLS overheat safeties.

LED11 is the Service Heater light.

Heat not getting into the water could be caused by a bad thermal regulator, bypass valve, or scaled heat exchange.

How old is the heater?



New member
May 10, 2020

Same exact thing is happening to me. Heater is running fine (blowing hot air). I removed the Thermal Regulator per Pentair Tech Support and no hot water. I also pulled the manifold and the bypass looked and behaved fine. I have to imagine now I have to take a peek at the coils. I kind of hit my limit and calling home warranty to deal with it.

Hope your solution is much more clearer than mine.
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