Short answer: Pressure that drops right after a filter goes in usually means a clogged or wrong cartridge, a valve that is not fully open, a housing too small for the home's water use, or an older pressure problem the filter exposed. A quick bypass test tells you whether the filter is the cause.
- Who this is for: homes with a new or newly changed whole-house or under-sink filter, on a well or on city water.
- Can wait: a slight drop right after install, if it improves after the system is flushed.
- Act soon: pressure that is much lower, or that sinks more each week, because a clogging cartridge can also strain a well pump.
- Call or text Ryan if: a new cartridge and the bypass test do not clear it.
Start With a Bypass Test
The fastest way to know if the filter is the cause is to take it out of the loop. Many whole-house filters have a bypass valve set that lets water go around the housing.
- Turn on a tap that feels weak and note the flow.
- Put the filter into bypass, or close the valves on each side of the housing and open the bypass valve between them.
- Check the same tap again.
If the pressure comes back, the filter or its cartridge is the problem. If it is still weak, the filter was not the cause, and the problem is somewhere else, such as a pressure tank, the pump, or a clogged pipe. In that case read low water pressure. If you do not have a bypass, text Ryan before you start closing valves.
Filter installed and pressure fell?
Text Ryan a photo of the filter, valves and cartridge. He is a licensed Master Plumber and answers his own phone, day or night.
A Clogged or Wrong Cartridge
The cartridge is the part that does the filtering, and it is the first thing to check.
- It is clogged. On a well, a new filter can fill with sediment quickly, especially right after a heavy rain or a change in the well. If the old cartridge is dark or slimy, replace it and see if pressure recovers. If a fresh cartridge clogs within days, the water has more sediment than the filter can handle, and the cause upstream needs attention. See cloudy or sandy well water.
- It is the wrong type. A very fine cartridge slows the flow more than a coarser one. A carbon block cartridge slows flow more than a simple sediment cartridge. A cartridge that is too fine for the job can cause low pressure on its own.
- It was installed wrong. Some cartridges have a top and a bottom, or need the o-ring seated correctly. Water can bypass a badly seated cartridge, which causes leaks and not low pressure, but a cartridge pushed in wrong can also block the outlet.
A Housing That Is Too Small
Every filter has a flow rating, the amount of water it can pass at a time without a big loss of pressure. If the filter is sized for a smaller home than yours, or sized for a single tap, it will choke the house when two showers run, or when the dishwasher and washer run at the same time.
The same is true of the pipe size feeding the filter. A small fitting or a long run of narrow tubing slows the flow even when the cartridge is clean. Houses with several bathrooms need a system built for that load. That is a sizing question, not a cartridge question, and it is where having the filter chosen and installed by a plumber pays off. Ryan installs whole-home filtration.
"They installed our whole-house water filtration system perfectly. Ryan went above and beyond by carefully drilling through our brick and even patching the hole with masonry cement. Who does that? His attention to detail and professionalism were top-notch."
Valves, Air and Other Simple Causes
- A valve that is not fully open. Check the shutoff valves on both sides of the housing. A ball valve is fully open when the handle lines up with the pipe.
- Air in the system. After a cartridge change, open a tap and let it run until the sputtering stops. Most housings also have a small button on top to release trapped air and pressure.
- A bypass valve left partly closed. If the bypass is not set back to normal flow after a change, the filter can be starved.
- A pressure regulator or well pressure that was already marginal. A filter adds a little resistance. If the pressure was borderline, the filter made it noticeable. On a well, the pressure tank and the switch settings matter. See well pump short cycling for tank trouble signs.
What to Do Today
- Try the bypass test above.
- Check the valves and release air at the housing.
- Look at the cartridge. If it is dirty, swap it for a clean one of the type your filter calls for.
- Note whether the problem is at every tap or only one. One tap points to that faucet's aerator, not the filter.
- If pressure is still low, text Ryan a photo of the filter setup, any gauge you can see, and the cartridge label.