Help me diagnose: Low flow after installation of new pool heater


Jul 13, 2008
I just replaced my pool heater that was about 10 years old. It was rusted beyond repair and I replaced it with a smaller, more efficient model. I had a Pentair 400,000 BTU heater that hadnot worked for almost a year. I had a pool company install a Pentair MasterTemp Low NOx Pool Heater - Electronic Ignition - Natural Gas - 400000 BTU heater. Heater works fine but my flow to the spa and pool is now much lower at the same pump speed.

I have a Hayward Echostar variable speed pump and run my pool at around 1200 RPM on low speed. This is normally just fine to get good flow to the pool, the spa overflow and the returns. Now, after the the new heater has been installed, I have to turn the speed up pretty significantly to get the same flow.

Can anyone help me diagnose the problem? The installer I used did not seem very knowledgeable so there may be something that was incorrectly installed.

The cartridge filter is relatively clean and the flow was fine just yesterday (with the old heater), so I can probably rule out a dirty filter.

Do any experts have thoughts as to what might be going on?


Jul 13, 2008
At higher speeds, the pressure has gone up, but I don't thing the pressure has increased at the cartridge filter at the same speed I used to run the pump.


Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
Bedford, TX

I would make sure the installer has the Input and Output connected correctly...

Jim R.
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