Pentair Mastertemp 400 help

mackerman22

New member
Jul 21, 2020
1
Flower Mound, TX
I have a Pentair Mastertemp 400 that we use to heat our spa. Last year, if you set it to heat the water to 100F, it would do it without any issue. This week we tried using it and if you set it to 100F, it will raise the water temp to 90F and the heater keeps running but the water temp does not rise. No warning lights or codes on the display. Any ideas what the issue could be?
 
Last edited:

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
21,219
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
This week we tried using it and if you set it to 100F, it will raise the water temp to 90F and the heater keeps running but the water temp does not rise.
Explain more what keeps running? The exhaust fan? Is the exhaust hot?

If the heater is running with hot exhaust then the heat generated needs to be going someplace.

Are you sure your pool/spa return valve is closing fully and you are not losing some heated water back to the pool?
 

the_sphynx

Well-known member
Feb 19, 2019
79
North San Diego County
Also, I just had a similar thing although mine had the "Service Heater" light on. Swapped out the "Thermal Regulator" and that seems to have fixed it.
Mine would get to 95 and then take forever and a day to get to 100. The thermal regulator that got pulled out showed some signs of corrosion but not like completely crusted over or anything. When running the old one under a faucet of hot water the thingy never lifted like it's supposed to do. I found a YouTube video describing it.