Low mains water pressure and flow is common in the UK, and more so in London. Smaller homes often have a combi. A combi heats the tap water from the cold main as the tap runs, so a weak incoming flow means weak hot water.
What a combi needs
The hot-water rate on the boiler badge is the rate it can deliver when the cold main can supply that much. A 24 kW combi is usually about 9 litres a minute. A 37 kW combi is usually about 15 litres a minute. If the main cannot supply that, the boiler cannot reach the figure on the badge.
A home with more than one bathroom, or an unvented hot-water cylinder, usually needs 20 litres a minute or more at the cylinder, plus the manufacturer’s minimum pressure. A bigger combi does not create that flow. The pipe has to provide it.
What the water company must supply
You will often see “9 litres a minute at 1 bar” quoted as the requirement. The standard is lower than that. Ofwat’s guaranteed standards say the water company must keep 7 metres static head (0.7 bar) in the communication pipe, the company pipe that ends at the outside stop valve. Many companies aim for about 1 bar (10 metres head) at that point. They do not have to hold that pressure inside the house.
The private supply pipe from the outside stop valve to the boiler belongs to the home. A long run, a shared pipe, a narrow or lead pipe, or a lot of bends can drop the flow before it reaches the kitchen tap.
A practical check is the kitchen cold tap, with every other outlet shut. Filling a 4.5 litre bucket in about 30 seconds is 9 litres a minute. That suits a smaller combi. It is short of what a larger combi or an unvented cylinder needs. WaterSafe and the Consumer Council for Water describe the same boundary between the company’s pipe and yours.
If the kitchen tap is already slow
A larger boiler will not fix a slow cold tap. Ask the water company to check the pressure at their stop valve. A plumber checks the private pipe. Replacing that pipe is a plumbing job. Choosing a boiler or an unvented cylinder that matches the flow you actually have is a heating job. Boiler size is covered on what size boiler. For a visit, get a quote.
This is incoming mains pressure. It is separate from the sealed heating system, which should sit at about 1.2 bar when cold. That gauge is on the boiler pressure page.