Low water pressure in the house: causes and what a plumber checks

Low water pressure in house taps and showers comes from a short list of causes, and you can rule a lot of them in or out yourself. The trick is working out where the pressure is being lost: at one tap, on the hot side only, or across the whole property. Below are the safe checks, what each result tends to mean, and what a plumber does from there.

Quick answer: Work out where the pressure is being lost. One weak tap or shower points to that fixture, most often a clogged aerator or shower head. Weak hot with good cold points to the hot side. Weak everywhere points to the supply or your own pipework, and the garden tap by the meter tells you which.

Want it looked at? Call 0475 407 670. Mon-Fri 7-5, Sat 8-1. Check we cover your area on the service area page.

Water bill jumped as well? A leak can drop the pressure, so run the meter test in our guide to finding a water leak in your house first.

Low water pressure in house plumbing tends to follow a pattern, and the pattern points at a short list of parts. Work through these three questions before you ring anyone.

1. One tap, every tap, or only the hot?

Turn on the cold at the kitchen sink, then a bathroom basin, the shower and the laundry, one at a time. Then try the hot at the same taps. Finish at the garden tap nearest the water meter, which is normally at the front of the property, with the main stop tap fully open. That tap shows what the supply is like before the rest of the house gets a say.

• One fixture weak, the others fine. The problem is at or near that fixture.
• Hot weak everywhere, cold fine. The problem is on the hot side, somewhere between the hot water unit and the taps.
• Weak everywhere, even at the garden tap by the meter. The problem is at the meter end or further out in the street. The next question and the bucket test help sort out which.
• Strong at the tap by the meter, but weak at every tap inside. The problem is somewhere past that tap, on your side of the meter, and that is a plumber's job.
• Fine with one tap running, poor with two. That points to pipes that have narrowed or are too small for the house, or to a supply that is weak overall.

2. Is it just your place, or the whole street?

Ask a neighbour whether their pressure has dropped too, and check your water retailer's website for faults and planned works. In Melbourne's east and south-east that is mostly Yarra Valley Water or South East Water, and your retailer's name is on your water bill. If the whole street is affected, it's the retailer's to sort out, and a plumber can't fix it from your side of the meter.

Yarra Valley Water also publishes a bucket test on its website. With the main stop tap fully open, you time how long the tap closest to the meter takes to fill a bucket. If it is slow, the problem is before the meter and Yarra Valley Water wants a call. If it is fine, the problem is after the meter and needs a plumber. If you are with South East Water, ring the faults number on your bill instead.

3. Did it drop suddenly, or fade over months?

A sudden drop normally means something changed. A slow fade over months or years points to build-up or wear. The lists further down go through both.

If the cold runs well and the hot is weak at every tap, the restriction is on the hot side. These are the places to look:

• The tempering valve. Many hot water systems have a tempering valve near the unit, which mixes in cold water so the bathroom taps can't scald. Small strainers on the inlets catch grit, and the valve itself can wear and hold the flow back. Cleaning the strainers or replacing a tempering valve is licensed work, and the valve has to be set and tested afterwards. If you're not sure which valve your house has, our tempering valve and TMV comparison explains the difference.
• The valves on the hot water unit. The cold pipe into a mains pressure unit normally has an isolation valve, often combined with a non-return valve and sometimes a pressure limiting valve, and some of these have strainers too. If that valve is partly closed or blocked, every hot tap suffers. Check the cold valve going into the unit is fully open. Anything past that is for a plumber, and our hot water repairs page covers the unit itself.
• A low-pressure hot water system. Some older homes still have a hot water system that runs at a lower pressure than the cold, either fed from a tank in the roof space or fitted with a valve that holds the pressure down. On those, a weaker hot flow is how the system was built rather than a fault. A plumber can tell you which kind you have.
• Continuous flow units. Instantaneous units have an inlet filter that can block with grit, and they need a minimum flow before they fire. A clogged shower head or aerator can stop the unit firing properly, so clean those first. Anything inside the case is licensed work: a gasfitter for a gas unit, an electrician for an electric one.

One thing not to do: don't turn down the thermostat on the hot water unit, or adjust the tempering valve, to change how the hot water feels. A storage tank has to keep its water at 60°C or more to stop bacteria growing in it, and the tempering valve is what brings it down to a safe temperature at the taps. Both are set by a plumber for good reason.

Start at the meter. If the garden tap there is weak too and the neighbours are fine, the fault is at the meter end: a part-closed stop tap, the meter itself, a valve fitted right beside it, or the supply coming in. If that tap runs strongly but the house is weak, the fault is on the property side, between the meter and the taps inside. The retailer is responsible up to the meter, and the property owner from there into the house.

The main stop tap isn't fully open

The main stop tap sits beside the water meter. If it was turned off for a repair, or the meter was changed, it may not have been opened all the way again. If it has a round handle, turn it gently anticlockwise until it stops. If it has a lever, it is fully open when the lever sits in line with the pipe. That is fine to do yourself. If it is seized or leaking, don't force it. Call a plumber, or the retailer if the fault is in the meter itself.

If the stop tap is fully open and the tap by the meter is still weak, do the retailer's bucket test or give them a call before you book a plumber. The causes below are the ones that leave the meter tap strong and the house weak.

A pressure limiting valve that has failed

The plumbing standard limits the pressure at the outlets inside a home to 500 kPa. Where the street supply runs higher, a pressure limiting valve is fitted near the meter to bring it down. These valves wear. Some fail by letting too much pressure through, which shows up as banging pipes and a dripping hot water relief valve, and our guide to fixing water hammer covers that side. Others fail the opposite way: the valve sticks or its strainer clogs, and every tap past it runs weak. Testing and replacing a pressure limiting valve is licensed work.

Old galvanised steel pipe

Some older homes still have galvanised steel water pipe, in at least part of the house. Over the years the inside rusts and builds up, and the space for the water gets narrower. The drop is slow, and it is often worst with two taps running at once. Brown or rusty water first thing in the morning is another hint. Narrowed steel pipe can't be cleaned out, only replaced, and a plumber can tell you how much of it is left.

A hidden leak on the supply pipe

A leak between the meter and the house, or under the floor, takes water away before it reaches the taps. Low pressure on its own doesn't prove a leak. Low pressure together with a higher water bill, a wet patch in the yard or the sound of running water is worth chasing. Our post on underground water leak signs goes through what to look for, and the meter test in how to find a water leak in your house shows whether water is going somewhere it shouldn't. If the meter says there is a leak and you can't find it, that is a job for water leak detection.

The supply in the street

If the neighbours are affected too, report it to the retailer even if their website shows nothing yet. Once their work is finished, check your aerators and shower heads, because grit stirred up in the mains often ends up in them.

How fast the pressure went is the third clue.

If it dropped suddenly

• Work or a burst main in the street.
• A stop tap or isolation valve left partly closed after other work.
• Grit in aerators and strainers after work on the mains, or a pressure limiting valve that has stuck.
• A pipe that has burst or started leaking. If you can see or hear water where it shouldn't be, turn the water off at the main stop tap and get help. Our burst and leaking pipes page covers what happens next.

If it faded over months

• Scale in aerators and shower heads, or a mixer cartridge or tempering valve wearing out.
• Rust building up inside old steel pipe, or a pressure limiting valve wearing slowly.
• A small leak that is getting bigger.

A plumber works through the same questions, with gauges in place of guesswork. What gets tested depends on the pattern, but a visit for low pressure will often include:

• A pressure test at the meter. A gauge on the tap nearest the meter shows the pressure coming in, both with nothing running and with water flowing. A big drop once the water is flowing points to a restriction.
• Tests at the problem fixtures. Comparing the readings at the weak taps with the reading at the meter shows roughly where the pressure is being lost.
• A flow test. Timing how long a tap takes to fill a container shows how much water actually gets through, which can tell you more than pressure alone.
• The valves and strainers. The stop tap, the pressure limiting valve, the valves on the hot water unit and the tempering valve are checked, and strainers are cleaned where they are fitted.
• Leak checks. If the meter suggests water is going missing, the supply is isolated in sections, with leak detection gear where needed.
• The pipework. Where pipe can be seen, under the house or near the meter, the plumber checks what it is made of and its condition.

Sometimes the answer is clear from the first test, and sometimes it takes a process of elimination, especially where old pipe and a tired valve are both playing a part. Either way, ask what was found and what the options are before agreeing to a repair.