Why Does Water Pressure Drop When More Than One Fixture Is Running?

The short answer

Every fixture in your house is fed from the same incoming water line. The plumbing code sizes that piping so fixtures still get enough flow “under conditions of peak demand,” when several are running at once (2021 International Plumbing Code, Sec. 604.3). So opening a second fixture can lower the flow at the first one a little. A drop big enough to make a shower unusable, one that is new, or one that keeps getting worse is worth a closer look.

Possible causes include a valve that is not fully open, buildup inside older pipe, a pressure-reducing valve that is set wrong or not working, filter or treatment equipment, a leak, or a problem on the supply side. Several of these look the same from the faucet. The pattern you notice helps narrow the list. Pinning down the cause takes someone looking at the system.

One fixture or several?

Start here, because it splits the problem in two. Ogden City’s water utility suggests running both hot and cold water through all faucets to see whether low pressure affects one fixture or the entire property (Ogden City).

Only one fixture is weak, even when nothing else is running. That points to the fixture itself. The same utility says to check for a clogged aerator or a partially closed supply valve. That is usually a repair at one spot.

Several fixtures drop when they run together, or the whole house feels weak. That points upstream, to something the fixtures share. This is the pattern this article is about.

Only the hot side is weak. If cold water is fine everywhere and hot water is weak everywhere, Ogden’s utility says to check the water heater. Spangler’s water heater page covers that equipment.

What the drop can mean

These are possibilities, not a diagnosis. More than one can be going on at the same time.

• A main shutoff that is not fully open. Bremerton, Washington’s water utility notes that “a partially closed valve can restrict flow and effect water pressure” and advises making sure the on-site shutoff is in the full open position (City of Bremerton).
• Buildup inside older pipe. Denver Water explains that minerals and corrosion accumulate inside lead and galvanized service lines, which “decreases the interior pipe diameter” and reduces the area water can flow through (Denver Water). Bremerton adds that rust from old galvanized lines can flake off and clog aerators, showerheads, cartridges, and fixture shutoff valves. Spangler’s guide to older Wilmington homes walks through what that pipe looks like.
• Piping that no longer matches the house. Piping is sized for the fixtures it serves. If bathrooms, fixtures, or appliances were added later, the original lines may be working harder than they were designed for. Only a plumber looking at the pipe sizes and layout can tell you whether that is a factor.
• A pressure-reducing valve. Some homes have one on the main supply line. Bremerton’s utility says that if there is low pressure at every fixture and a pressure-reducing valve is installed, it is worth verifying that the valve is set appropriately and still working.
• Filter or treatment equipment. If the house has a whole-house filter, softener, or other treatment on the main line, note it and when it was last serviced. A plumber can check whether it is restricting flow.
• A leak. Water escaping from a hidden supply leak is water that is not reaching your fixtures. The meter check below helps rule one in or out.
• Supply-side trouble. If neighbors lose pressure at the same time, or the drop started right after crews worked on the street, the cause may be outside your house. Spangler’s guide to who owns the water line to your house covers where your responsibility starts.
• A well system. On a private well, the pump, pressure tank, and pressure switch set your pressure. See Spangler’s well water guide for how those behave.

The shower that changes when a toilet flushes

When a toilet refills while someone is showering, it draws water from the same supply, and the shower can feel the change. Shower valves built for this are called pressure-balancing valves. Symmons, a shower-valve manufacturer, says they “react to changing water pressure to prevent you from scalding should someone flush the toilet or run cold water elsewhere” (Symmons).

If your shower turns noticeably hotter or weaker when someone flushes, mention it when you call. A plumber can tell you what type of shower valve you have and whether it is working as designed. A sudden jump in temperature is a scald risk, especially with children or older adults in the house.

What to write down before you call

You do not need tools. A notepad, your phone, and ten minutes are enough.

• Which fixtures were running when the drop happened, and which room each is in.
• Hot, cold, or both. Run each side separately at a sink and note whether one is weaker.
• One fixture or several. Check whether the weak fixture is also weak when it runs alone.
• When it started. Sudden, gradual over months, or after a specific event like a repair, a new appliance, or work on the street.
• Whether it is getting worse.
• Anything you can see or hear. Wet spots, stains on a ceiling or wall, hissing in a wall, banging when a valve closes, or discolored water.
• Whether neighbors have the same problem.
• Whether the water meter moves when nothing is running. The EPA suggests reading the meter, using no water for two hours, and reading it again; if it changed at all, you probably have a leak (EPA WaterSense).

Safe things to look at yourself

Look; do not take anything apart.

• The main shutoff. Find it and note whether the handle looks fully open. Do not force a stiff or corroded valve. An old valve can start leaking when it is forced.
• A whole-house filter, if you have one. Note the date on the cartridge or when it was last changed.
• The area around the water heater and main line. Look for drips, corrosion, or wet floor.
• Faucet aerators and showerheads on the weak fixtures. Bremerton’s utility suggests checking aerator screens for rust, debris, or scale.

Leave adjusting a pressure-reducing valve, cutting into a wall, or opening anything under pressure to a plumber who can measure what is happening.

When to call right away

Stop troubleshooting and call promptly if:

• pressure dropped suddenly and severely across the house,
• you see water where it should not be, or the meter moves with everything off,
• you hear water running in a wall or under a floor when nothing is on, or
• the water turned discolored along with the pressure loss.

For an active leak, shut off the main if you can do it safely, and call Spangler’s emergency plumbing line.

What a plumber checks that you cannot

A symptom pattern narrows the list. It does not confirm which valve has failed, how narrow a pipe has become, or where a hidden leak is. A plumber can measure the actual pressure at the house, check the main shutoff and any pressure-reducing valve, look at the pipe material and where old pipe meets newer pipe, and isolate the problem to one branch or the whole house before recommending a repair. That is the difference between replacing the right part and replacing parts until it gets better.

Frequently asked questions

Is it normal for water pressure to drop when two faucets are on?

Some change can happen, because the faucets share one supply. Plumbing is sized so fixtures still get adequate flow at peak demand (2021 IPC, Sec. 604.3). A drop big enough to make a shower unusable, or one that is new or worsening, is worth having checked.

Why does my shower change when someone flushes the toilet?

The toilet refills from the same supply the shower is using. Pressure-balancing shower valves are designed to respond to that change and protect you from scalding (Symmons). If your shower turns hotter or weaker when someone flushes, mention it when you call so a plumber can check the valve.

Can old pipes cause low water pressure?

Yes. Denver Water explains that buildup inside older lead and galvanized lines narrows the pipe and reduces flow (Denver Water). A plumber can tell you how much older pipe is left in your house.

Could low pressure be my water company’s problem?

Sometimes. If neighbors lose pressure at the same time, or it started after work on the street, call your water provider. If the drop is only in your house, the cause is more likely inside it.

Should I adjust my pressure-reducing valve?

Not without measuring. A plumber can test the pressure and tell you whether the valve is working, needs adjusting, or needs replacing.

Tell us what you are seeing

Spangler Plumbing handles leak and pipe repairs for homeowners in Middletown and across New Castle County. If the pressure drop is new, affects more than one fixture, or is getting worse, tell us which fixtures were running and whether hot, cold, or both were affected. We will look at the system, narrow down the cause, and give you a straight answer on the fix. See general plumbing repair or all plumbing services, request service, or call 302-293-8008. For an active leak, use our 24/7 emergency line. Delaware Master Plumber #UT1488.