Quick Answer:
It’s usually normal for a shower to lose some pressure when another fixture (toilet, sink, washing machine) runs. Most homes share the same supply lines, so opening another tap reduces available flow and causes a temporary pressure drop. Small checks at the shutoffs, a quick cold-vs-hot test, and a few simple workarounds will tell you whether this is normal or a sign of a bigger problem.
Why This Happens
Two main plumbing realities cause a shower to weaken when another faucet is used:
- Shared supply and competition for flow — the pipes feeding bathrooms and kitchens are connected. When two fixtures demand water at once, each gets less.
- Restrictions and controls — partially closed shutoffs, a failing pressure regulator, clogged aerators or a shower cartridge can reduce flow on one or both lines.
If the drop affects the whole house, you’re more likely looking at supply or regulator issues. If it’s limited to one bathroom or one fixture it may be local hardware or internal blockages. Watch for symptoms like sputtering, banging, or steady low pressure — those are more than simple demand effects and can point to corrosion, debris, or a failing regulator causing overall low water pressure.
Step-by-Step What to Do
Step 1 — Reproduce the problem safely
Run the shower at a normal temperature and, while watching flow, flush a toilet or turn on the sink tap. Note whether the shower pressure drops only while the other fixture runs, and how severe the drop is.
Step 2 — Test cold vs. hot
Try the same test using only cold water and then only hot water. If only the hot side drops, you may have an issue with the water heater supply or a dedicated hot valve. If only cold drops, the cold branch may have a restriction or partially closed valve.
Step 3 — Check nearby shutoff valves and aerators
- Make sure the shower’s inlet shutoffs (angle stops) and the main house shutoff are fully open.
- Unscrew the shower head and check for scale or debris. Clean the screen or aerator on the sink that’s interacting with the shower.
Step 4 — See whether the problem is limited to one fixture
Test other faucets simultaneously. If the loss only happens when a particular fixture runs, that suggests that fixture is over-consuming or has a faulty valve. If multiple fixtures drop at once, it points to a shared-supply or regulator issue. If you find the issue affects just a single outlet, you’ve found the likely culprit: **only one faucet** behaving badly.
Step 5 — Consider the pressure regulator and supply pressure
If the housewide pressure is low or fluctuates, the pressure-reducing valve (PRV) or the municipal supply could be the cause. Signs include sudden changes in pressure at multiple fixtures, or the pressure being poor even when nothing else is running. Replacing or adjusting a PRV is a job for a pro unless you have plumbing experience and the right tools.
Step 6 — Practical workarounds
- Stagger water-using appliances (start the washer before showering or vice versa).
- Temporarily turn off other fixtures while showering if practical.
- Clean or replace a restrictive shower head or clogged aerator.
- Consider a pressure-balancing or thermostatic shower valve if pressure swings cause temperature changes — have a plumber install these.
What Not to Do
- Do not crank the shower temperature up to compensate for low pressure — that risks scalding.
- Do not assume it’s always the shower valve without checking other fixtures and shutoffs first.
- Do not ignore sudden new behavior across the house — a new, widespread drop can signal a serious issue.
When to Call a Professional
- If the pressure drop affects the whole house or appeared suddenly without explanation.
- If you find leaks, water hammer, sputtering, or visible corrosion in pipes.
- If the pressure regulator (PRV) may be failing or if you’re uncomfortable adjusting or replacing valves.
- If cleaning aerators and checking shutoffs don’t solve the problem.
Safety Notes
- Turn off the main water before working on supply lines or replacing cartridges. If you can’t locate the shutoff or aren’t sure how, call a plumber.
- Be careful with hot water — test slowly after any repair to avoid scalds.
- Don’t attempt complex PRV or gas-water heater adjustments unless you are qualified; those can create hazards or code issues.
Common Homeowner Questions
- Is this normal? — A small temporary drop is normal when two fixtures run at once.
- Can I fix it myself? — You can check shutoffs, clean aerators, and test cold vs hot; more complex fixes should be handled by a pro.
- Could this mean a major problem? — If the drop is sudden, affects the whole house, or comes with noise or leaks, it could signal a bigger issue that needs a plumber.