Mastertemp 400 turns on and heats even though flow is only 21 GPM

[GrandLSU]

As I understand it, the minimum acceptable water flow for a Pentair Mastertemp 400 heater is 40 GPM. My understanding is that there is a flow switch (or pressure switch or something) in the heater that will not allow the heater to turn on if the flow is lower than 40 GPM.

My heater however will turn on and heat the water when the flow is only 21 GPM (21 GPM is the normal amount of flow that I get when in Pool mode with my pump set to 1,600 RPM).

Is something wrong with a sensor on my heater, since the heater will turn on even though the flow is only 21 GPM?

 

[mas985]

Most heaters, including yours, actually have pressure sensors and not flow sensors. So as long as the pressure at the heater is sufficient it will turn on. They use a pressure sensor as a proxy to a flow rate sensor although it is not always a very good proxy.

See page 10:

https://www.pentair.com/content/dam/extranet/nam/pentair-pool/residential/heaters/mastertemp/manual/Manual-MasterTemp-472592-English.pdf

In reality, if the flow rate is too low, the water temperature in the heat exchanger will rise and the temperature sensor (HLS) should shut off the unit before damage. The AGS and SFS both act as backups to shut down the unit if things get too hot. So there are several mechanisms to protect the heater.