Low Water Pressure After a New Combi Boiler

Why a new combi can make the water feel weaker

Many London homes that switch to a combi were previously on a gravity system: a cold tank in the loft, a hot water cylinder in the airing cupboard and a boiler that only did the heating. On that system the hot taps were fed by gravity from the loft, and the bath filled from a 22mm pipe with a store of hot water behind it. The kitchen cold came off the mains.

A combi changes all of that. Hot water is now heated as it passes through the boiler, so everything depends on two things: how much water your mains can supply, and how fast the boiler can heat it. A combi rated at 30kW for hot water can raise the temperature of roughly 12 litres a minute by 35 degrees. On a cold January morning, when the mains comes in at around 5 degrees, that is about as much as it can manage at a comfortable temperature. Ask it for more and the water goes lukewarm, so many combis fit a flow restrictor to hold the output to what they can heat.

So there are two different complaints that people describe the same way. Weak hot water at every tap while the cold is fine usually means the combi is doing what it was sized to do. Weak cold and hot water together means the mains supply itself is the limit, and the new boiler has exposed it.

Checks to do first, in this order

Work through these before you call the installer back. Each one takes a few minutes and none needs tools beyond a bucket and a watch.

• Find your internal stopcock and make sure it is fully open. It is often turned down during installation and left half shut. Turn it anticlockwise until it stops, then back a quarter turn so it does not seize.
• Check the isolating valve on the cold inlet under the boiler. It is usually a small valve with a screwdriver slot. The slot should run along the pipe, not across it.
• Do a bucket test at the cold kitchen tap. Time how long it takes to fill a bucket of known size with the tap fully open, then work out litres per minute.
• Run the kitchen cold and a hot tap together and watch whether both drop badly. A big drop means the supply pipe is too small for the demand.
• Check whether the problem is at every tap or one. A single weak tap or shower is usually a blocked aerator, shower head or valve, not the boiler.

Ofwat’s long-standing reference level for adequate supply is 10 metres head (about 1 bar) with a flow of 9 litres a minute at the boundary stop tap. The legal guaranteed minimum is lower. If your bucket test shows less than 9 litres a minute at the kitchen tap with nothing else running, the supply is weak before it even reaches the boiler.

The inlet filter nobody mentions

Most combis have a small mesh filter on the cold water inlet, where the mains connects underneath the boiler. On a new installation it catches whatever was loosened in the pipework during the work: flux, solder, grit and scale from the old system. A filter that is half blocked in the first few weeks is common and gives exactly the symptom people describe, with hot water that starts fine and then fades.

Cleaning it is an engineer’s job. On some models the filter sits in the cold inlet valve under the boiler; on others it is behind the casing, and nothing behind the casing is a householder check. Either way the supply has to be isolated and the connection opened without damaging a seal. If your installer is still within their workmanship guarantee, ring them first.

If the boiler also struggles to reach a steady hot water temperature, mentions a low flow fault or cycles on and off while a tap is running, see our guide to a combi that will not deliver hot water.

When the supply pipe is the real limit

A combi was never going to give strong hot water from an old half-inch lead or iron supply pipe. Many Victorian and Edwardian houses in London still have one, often running from the pavement under the front garden. Iron supply pipes also narrow inside over time as rust builds up.

Signs the supply pipe is the problem: the bucket test is poor even at the kitchen tap, pressure is fine late at night but drops at breakfast time, and neighbours with newer supply pipes do not have the same issue. Before fitting a combi, a good installer measures the mains flow and pressure, and the Benchmark commissioning checklist in the back of the boiler manual records the hot water flow rate achieved on the day. Look there and ask the installer for their mains readings. If the supply was already weak when they measured it, the boiler was fitted to a supply that could not support it.

The fix is usually a new supply pipe in modern plastic, typically 25mm, from the boundary to the house. That part of the pipe is the owner’s responsibility, not the water company’s. Our guide to who is responsible for the mains water pipe explains where that line falls. Once pressure is the issue, low water pressure repair covers what an engineer tests on a visit.

Pumps, pressure reducing valves and what not to fit

It is tempting to fit a booster pump. Under regulation 5 of the Water Supply (Water Fittings) Regulations 1999, a pump or booster drawing more than 12 litres a minute from a supply pipe has to be notified to the water company before it is installed, and the company can refuse or attach conditions. Pumps also cannot create flow that the pipe cannot supply, and they can starve the neighbours. Our piece on whether a water pressure pump is worth it goes through the few cases where one makes sense.

Some properties, especially flats and houses on low ground, have a pressure reducing valve fitted on the incoming main. If the installer adjusted or replaced it during the job, or it has stuck, the pressure after it will be lower than the mains. It is usually a brass body with an adjustment cap and sometimes a small gauge. See pressure reducing valve problems for how to tell if one is stuck.

Do not try to remove a combi’s flow restrictor to get more flow. It is there to match the flow to the boiler’s heating power, and taking it out gives you more water at a lower temperature.

What the engineer checks on a visit

On a call-out for low pressure after a new combi, an engineer should measure static pressure with a gauge on the kitchen tap or a washing machine valve, measure dynamic pressure and flow with a tap running, and compare them with the figures recorded at commissioning. That quickly separates a supply problem from a boiler problem. They then check the stopcock and inlet valves, clean the cold inlet filter and look for a stuck pressure reducing valve.

If the supply is fine and the combi is simply undersized for the house, the options are a larger combi, a system boiler with an unvented cylinder, or accepting one outlet at a time. If the supply pipe is the limit, the answer is a new supply pipe, and nothing done at the boiler will fix it. Visits are the £60 call-out, and the repair price is agreed with you on site before any work starts. If the boiler itself shows a fault, see boiler repairs.