Why your water pressure might be low. Part 1

One of the biggest complaints I come across regarding plumbing is the lack of decent water pressure or what people really mean is the flow rate of water through their taps and showers.

How quickly does the bath take to fill? How powerful is the shower? Why does the hot water dribble out of the bathroom tap?

The first thing to consider is the incoming mains water pipe. This can be lead or iron(if you are unlucky) copper or plastic. Lead is rapidly being removed over the last few years but is still present in some older properties. It looks big on the outside but the internal bore is very small due to the softness of the material.

Likewise iron pipes are rapidly being replaced. Water companies will replace free of charge the lead or iron pipe up to their stop tap (see more details here) you are responsible for all pipework afterwards.

Copper superseded lead and iron and is usually 15mm  (1/2″ in old money) diameter.

New properties are fed with 25mm / 1″ MDPE/plastic pipe to the stop-tap which then drops down usually to 15mm copper but can be 22mm for larger properties with multiple bathrooms.

An exception to this is if the ground is contaminated with hydrocarbons (petrol, oil, diesel), then proprietary barrier (foil wrapped) PE pipe or PVC-wrapped table Y copper must be used.

From April 2008 the Guaranteed Standards Scheme (GSS) came into force in England & Wales which requires water companies to maintain a minimum pressure of water to a domestic property of 7 metres static head(pressure required to lift the water in a pipe 7 metres) or 0.7 bar. This is the pressure to the service stop tap outside, anything else is your responsibility. If the mains water is high pressure then a pressure regulator is fitted immediately after the main stop tap to limit the incoming pressure so as not to damage plumbing fittings, boilers etc.

Thames Water Service Level

Our service standard level of mains water pressure is ten metres/head (or one bar). This means there is enough force/pressure to push the water to a height of ten metres. This is measured at the point where the water leaves our pipework and enters yours (usually the outside stop valve or property boundary). As a guide, if you have a suitable single service pipe, the first tap in the home (this is usually the kitchen tap) should be able to fill a 4.5 litre (one gallon) bucket in 30 seconds, with all other taps and appliances turned off.

So, assuming that you have a decent water supply to your property, the first thing to check is if the mains stop tap outside is fully open. If you have a water meter fitted, it will be in the same chamber, if not, it will be situated in an inspection chamber usually in the pavement outside your home. These often get filled with silt and dirt due to rainwater run-off.

The stop tap can be either a traditional cruciform head, a square, a slot or a plastic screw down type. The type varies depending which part of the country you are in.

All will be around 600mm or more deep to protect from frost. Many require an extension tool to turn, many will be stiff. People have turned them with just sticking their arm down, or using various spanners.

I’ve had some where I spent an hour digging out all the silt just to find the tap. Remember if the stop tap in your house fails, this is the only thing between you and the mains supply.

Many people find they can’t access the pavement service tap in an emergency. Some of the old cast iron covers tend to rust solid because they’re rarely opened. Have a look, if you can’t see it, contact your water company to clean the chamber out, or call a plumber.

In part 2, we are going to examine the potential problems within your property that can cause low water pressure.


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