Different Filter Pressures Depending on Valve Position?


Silver Supporter
Nov 7, 2018
Oklahoma City, OK
I have one main drain and one skimmer. As a matter of habit, I leave the main drain closed and use only the skimmer. Pressure has held at a steady 11.5 since installing new cartridges a few months ago. In anticipation of going on a trip for a few days, I went ahead and opened the main drain so there would be an additional source of water if the skimmer were to get blocked. Anyway, I noticed this AM that the pressure was running at about 15. I returned the valve to the "skimmer only" position and it went back to 11.5. Selecting "MX Drain only" resulted in about the same reading. There is no cavitation, flow seems normal and there are no bubbles in the pump housing under any configuration. Is the higher pressure an expected result? I would think (which I know is dangerous!) that if anything the pressure would be lower? :unsure:


Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
Tucson, AZ
That is expected. When you opened the valve, there is less head loss on the suction side which allows the pump to move more water, increasing the pressure at the filter.

If the valve was a return valve after the filter, opening it would likely reduce the filter pressure.


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

Thinking too much, by almost any engineer, is almost never a good thing.. :mrgreen:

Not being an engineer, I don't care, but would "assume" that when both the skimmer and the main drain are open, it allows the pump to move slightly more water through the filter, increasing the filter pressure.


Jim R.


LifeTime Supporter
TFP Guide
Oct 25, 2015
Jim's right. We over-think things too often. Centrifugal pumps just work according to their curve. Raise the suction pressure (by opening the drain and allowing more flow with parallel piping) and you raise the discharge pressure with the delta P provided by the pump. While we're over-thinking this (sorry Jim but you know we can't help it), it's amazing to me that this smart Swiss guy named Bernouilli figured all this out in 1700's:


  • Like
Reactions: HeyEng