What mains water flow rate do you need for a combi boiler?

A combi boiler has no hot water tank. It heats the water as it passes through, straight off the mains, which means the amount of hot water you get is capped by the amount of cold water your house can pull in. No boiler can give you more water than the street gives it.

That is why the number to check before you buy a combi is your mains flow rate, measured in litres a minute. It is a different number from water pressure, it is the one that decides whether your shower is any good, and you can measure it yourself in about two minutes with a bucket.

Do it before you get quotes, not after. A combi fitted on a supply that cannot feed it is the single most common cause of somebody being disappointed with a brand new boiler.

How to measure it, properly

Use the kitchen cold tap. In most houses that is the first tap after the incoming main, so it is the closest thing you have to a clean reading. Turn off every other tap, and do not run the washing machine or the shower while you are testing.

Take a container you know the volume of, open the tap fully, and time how long it takes to fill. A ten litre bucket filled in one minute is ten litres a minute. If your bucket is smaller, time it and scale up: five litres in thirty seconds is also ten litres a minute.

WaterSafe give the same test as the rough benchmark for an acceptable supply: on a suitable single service pipe, the first tap in the house should fill a 4.5 litre bucket in thirty seconds with everything else off. That works out at nine litres a minute.

Do the test twice, once in the morning and once around six in the evening. Flow drops when the whole street is using water, and the evening figure is the one that matters, because that is when you want a shower.

Source: WaterSafe, minimum water pressure a supplier must provide

What the number needs to be

Compare your measured figure with the hot water flow rate the manufacturer publishes for the boiler being quoted. If your mains delivers less than the boiler is rated for, the boiler cannot make up the difference. You have bought the bigger model and you will get the smaller model's performance.

As a working guide from what we see on real jobs: under about ten litres a minute and a combi will feel weak, and a bigger combi will not fix it. Between roughly ten and fifteen, a correctly sized combi works well for one bathroom. Above about fifteen you have the headroom for a larger combi and a genuinely strong shower.

None of that is a legal standard. It is the practical band, and it is why an honest installer measures before recommending rather than after.

The temperature rise trap nobody explains

Here is the part that catches people out, and it is the reason two boilers can look very different on paper and perform identically in the house.

A hot water flow rate is only meaningful alongside the temperature rise it was measured at. Worcester quote the Greenstar 4000 range at up to 12.3 litres a minute at a 35 degree rise, and they quote the 25kW model at 9 litres a minute at a 40 degree rise. Same range, and the headline number moves a long way depending on which rise is being used.

The reason is simple. A boiler has a fixed amount of heat to give away. Ask it to lift the water by 35 degrees and it can heat a certain number of litres a minute. Ask it to lift the same water by 40 degrees and there are fewer litres to go round.

That matters in January. Incoming mains water is colder in winter than in summer, so getting to the same shower temperature needs a bigger rise, so the litres a minute fall. A boiler advertised at a 35 degree rise will feel slower in the middle of winter than the brochure suggests, and there is nothing wrong with it.

So when you compare two quotes, check both flow rates were quoted at the same rise. If they were not, you are not comparing anything.

Source: Worcester Bosch Greenstar 4000 technical data

Pressure is a second number, and it also has a minimum

Flow is how much water arrives. Pressure is how hard it is pushing. A combi needs enough of both, and manufacturers publish a minimum inlet pressure as well as a flow rate. Worcester list a minimum inlet pressure of 1.0 bar for the Greenstar 4000.

Now put that next to what your water company actually owes you. Their guaranteed standard of service is a minimum of 7 metres static head, which is about 0.7 bar, measured where their pipework meets yours. Most aim for at least 10 metres head, about 1.0 bar, but the guaranteed floor is lower than that.

Read those two figures together and the point is uncomfortable but true: a supply that is entirely within your water company's obligations can still sit below the minimum inlet pressure a common combi is specified for. Your supplier is not doing anything wrong. The combi is simply the wrong appliance for that supply.

This is also why nobody should promise you a flow rate over the phone. It has to be measured at the house.

Source: WaterSafe, guaranteed standard of service and static head

Where the flow goes missing

A poor reading at the kitchen tap is not always the street's fault. More often than not the loss is inside the boundary, on pipework the homeowner owns, and some of it is fixable.

• An internal stop tap that has been part closed for years, or one that has seized and never opens fully. This is the cheapest possible fix and it is worth checking first.
• An old narrow supply pipe. Older stock was commonly run in 15mm, and lead or steel that has corroded internally is narrower again. A modern supply is usually 25mm MDPE.
• A shared supply pipe serving more than one property, which is common in older terraced streets. Your flow then depends partly on your neighbours.
• Scale and debris in the pipework and at the tap, particularly in hard water areas.
• Long runs and lots of bends between the stop tap and the boiler position, which is worth knowing about if you are also thinking of moving the boiler.

The local picture around Northampton and Milton Keynes

The two ends of our patch behave very differently, and it is worth knowing which one you are in before you set your expectations.

The Victorian and Edwardian terraces around Abington, Semilong and the older parts of Northampton are where we most often find narrow, old or shared supply pipes. A combi can still be the right answer there, but the size of it has to be set by the measured flow, not by the number of bedrooms.

The estate stock across Milton Keynes and the newer Northampton developments generally has a modern supply into the house and tests well. The constraint there is usually not the mains at all, it is the internal pipework and the original boiler position.

If your flow rate is low, what are the options?

Three honest routes, and any installer who offers you only the first is not giving you the full picture.

First, upgrade the supply. Replacing an old or undersized supply pipe from the boundary into the house can transform the reading. It is disruptive and it is a real job, but where the pipe is the problem it is the only fix that addresses the cause.

Second, size the combi to the supply you have. A smaller combi on a modest flow will be perfectly satisfying. A large combi on the same flow will not be better, and you will have paid for output you cannot use.

Third, store the hot water instead of heating it on demand. A system or regular boiler with a cylinder fills up slowly over time and then delivers quickly, so it is far less sensitive to a mediocre mains flow. Note that an unvented cylinder has mains requirements of its own, so it is not automatically the answer on a weak supply either.

What to do with your number

Write it down, note what time of day you took it, and give it to every installer who quotes. Then ask each of them the same question: what hot water flow rate will this boiler give at my measured mains, and at what temperature rise?

A quote that answers that properly is a quote from somebody who has thought about your house. A quote that just names a kilowatt figure has not.

If you want that done properly at the survey rather than argued about afterwards, we can arrange quotes from Gas Safe registered installers covering Northampton and Milton Keynes.