Part of our guide to Tankless vs Tank Water Heater — The Real Trade-off
Start with one question, because it splits the possible causes into two lists that share almost nothing.
Is it every fixture, or one?
One fixture — the cause is at that fixture. Aerator, cartridge, or its own shut-off. Usually fixable in minutes with no tools worth mentioning.
Every fixture — the cause is shared. Main shut-off, pressure reducing valve, corroded supply pipe, or the utility. Bigger, but still narrowable in about ten minutes.
If it is one fixture
Check the aerator first. The small screen in the end of the tap catches sediment and mineral scale. Unscrew it — usually by hand, sometimes with pliers and a cloth — rinse it, soak it in vinegar if it is crusted, refit it. This is the single most common cause of a slow tap, and it costs nothing.
Shower heads clog the same way. Same treatment: unscrew, soak, refit.
Check that fixture's own shut-off valve under the sink or behind the toilet. These get knocked, or left partly closed after work, and a half-open valve looks exactly like low pressure.
If only hot water is affected, the fixture is not the problem — go to the water heater section below.
If it persists after all that, the tap's cartridge or mixing valve is likely worn. That is a part replacement, not a plumbing emergency.
If it is every fixture
Work outward from your house.
1. Ask a neighbour. If their pressure changed too, it is a supply-side issue and nothing inside your house will fix it. Call the water company. This takes two minutes and saves the rest of the list.
2. Check the main shut-off valve where the supply enters the house. It should be fully open. Valves get left partly closed after plumbing work, and a partly closed main throttles the entire house. Check any meter valve too.
3. Check the pressure reducing valve. Most homes on municipal supply have one near the main entry — a bell-shaped brass fitting with an adjustment screw. Its job is to knock street pressure down to a safe household level, and they fail with age. A failing PRV is one of the most common causes of a sudden whole-house drop. They are replaceable.
4. Consider a water softener or whole-house filter, if you have one. Both restrict flow when the media is exhausted or a filter cartridge is overdue. Bypassing it temporarily is a quick diagnostic.
5. Consider the pipes themselves. Older galvanised steel supply lines corrode internally, narrowing over decades. This is the classic gradual decline — if pressure has faded slowly over many years in an old house, this is the likely answer, and it is a significant job.
People use the words interchangeably and it sends them down the wrong path.
Pressure is force, measured with a gauge at a hose bib with nothing running. Flow is volume actually delivered over time.
A partly blocked pipe can show normal pressure at rest — the gauge reads fine — while flow collapses the moment anything is running, because the restriction only bites when water moves. If your gauge says the pressure is acceptable but showers are still weak, you have a flow restriction, not a pressure problem, and you should be looking for blockage rather than adjusting a valve.
An inexpensive screw-on pressure gauge settles which one you have, and is worth owning before calling anyone.
If only the hot water is weak
That narrows it to the hot side, which is useful.
Sediment builds in the bottom of a tank water heater and can restrict the outlet. Flushing the tank is routine maintenance that many households never do.
The heater's own shut-off valve may be partly closed. And on older installations, the short flexible connectors at the heater can corrode internally.
If you have a tankless unit and hot water is weak only when several fixtures run, that is not a fault — it is the flow rate limit working as designed. See tankless vs tank.
What you can do yourself, and what you cannot
Yourself: clean aerators and shower heads, verify valves are fully open, bypass a softener as a test, read a pressure gauge, flush a water heater.
Call someone: replacing a pressure reducing valve, diagnosing a suspected leak, anything involving corroded supply piping, or a whole-house drop you cannot trace after the checks above.
Call promptly if pressure dropped suddenly and you see damp, hear running water with everything off, or your bill jumped — those together suggest a leak in the supply line, which does not improve on its own.
The order to work through
- One fixture or all of them?
- If one — aerator, then its shut-off, then its cartridge
- If all — ask a neighbour first
- Then the main valve, then the PRV
- Then the softener or filter
- Then consider the age and material of the pipes
Most household cases end at step 2 or step 4.
This is general information, not professional advice — see our disclaimer.
Frequently asked questions
What is the first thing to check?
Whether it affects every fixture or just one. One fixture points to that fixture — an aerator or cartridge. Every fixture points to something shared, such as the main valve or a pressure reducing valve.
Why did it drop suddenly rather than gradually?
Sudden change suggests a single event — a failed pressure reducing valve, a partly closed main valve after work, or a supply problem. Gradual decline over years suggests scale or corrosion building inside pipes.
Could it be the water company?
Yes, and it is worth ruling out early. Ask a neighbour whether theirs changed too. If it did, the problem is upstream of your house and not yours to fix.
What is a pressure reducing valve?
A valve near where the main enters the house that lowers incoming street pressure to a safe level. They fail with age, and a failing one is a common cause of a whole-house drop.
Is low pressure the same as low flow?
No, and confusing them wastes time. Pressure is force; flow is volume delivered. A partly blocked pipe can leave pressure normal at rest while flow is poor whenever water moves.
Sources
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