Is the heater flow switch necessary?

Jun 30, 2012
I have just fitted a variable speed drive to my filter pump. My problem is that I can only reduce the pump speed slightly before the heater flow switch operates. I was wondering whether the flow switch is necessary and what it's purpose is?

My pool heater consists of an oil fired boiler pumping hot water through the primary side of a heat exchanger with the filter pump pushing the pool water through the secondary side of the heat exchanger.

I cannot see any problem if the pool water was flowing very slowly (or even static) in the heat exchanger because the boiler would just cut out on its own thermostat.

Can anyone see any potential problem with bypassing the flow switch?


TFP Expert
Apr 29, 2016
Quaker Hill, CT
You should never defeat the flow switch on a pool heater. You can damage both the heater and the heat exchanger. You can also create a dangerous potential for scalding water to be pumped to the pool which could damage the PVC plumbing or seriously hurt someone.


Well-known member
Aug 30, 2016
Riverside, CA
Not only that, If flow is too low, it won't end up heating the pool. Just crank up the flow when in heat mode and then turn it back down otherwise.

The way I look at using a VSP is that I'm going to get the cost savings for normal filtration, cleaning and salt generation but am going to run it at higher flow rates when I am actually using the pool for fun. This will still result in a tremendous savings in electricity and money because the pump runs way more for the former than it does for the later; it runs every day of the year between 4 and 6 hours right now burning 2.3kW for the former and that will be reduced to less than 10% of that.