The short version
- Pressure is how hard the water pushes. Flow rate is how much water arrives in a minute. A combi depends on both, but flow matters most.
- A combi can only heat the water your mains supplies. A bigger combi can’t make up for low flow.
- A simple timed test with a container of known size at the kitchen cold tap gives you a rough flow rate.
- If flow is low, a system boiler with a cylinder may suit you better. We measure pressure and flow on the survey.
The short answer
A combi heats water as it passes through, straight from the cold mains. It has no tank of hot water to draw on, so it can only deliver as much hot water as your mains supply gives it. If the mains flow is good, a combi gives you strong showers on demand. If it’s low, the showers will be weak however powerful the boiler is.
That’s why we measure your mains pressure and flow before recommending a combi boiler installation. It’s one of the most important checks on the survey.
On the survey we measure both pressure and flow at the mains, with the water running, and look at the size and condition of the supply pipe. Together with the number of bathrooms and how your household uses hot water, that tells us whether a combi will do the job and what size it should be.
Pressure and flow are not the same thing
People often say “water pressure” when they mean both, but they’re different:
| Pressure | Flow rate | |
|---|---|---|
| What it is | How hard the water is pushed through the pipes | How much water arrives in a set time |
| Measured in | Bar | Litres per minute |
| What you notice | How forcefully water leaves the tap | How quickly a bath or bucket fills |
You can have reasonable pressure but poor flow, for example when water is pushed through a narrow or partly blocked supply pipe. The water seems fine from a single tap, then drops away when a second tap is opened. For a combi, flow is the figure that decides how much hot water you get.
Don’t confuse either of these with the pressure gauge on the front of a combi. That shows the pressure in your heating system, which is separate from the mains water supply.
Why flow matters so much for a combi
A combi’s hot water output depends on 2 things: how much heat the boiler can put into the water, and how much water is coming in. A larger combi can heat more water, but only if the mains supplies it. Fit a large combi on a low-flow supply and it can’t use its extra power.
Flow also shapes what happens when 2 outlets run at once. Open a kitchen tap while someone is in the shower and the flow is shared between them. On a strong supply that’s fine. On a weak one, the shower can drop to a trickle or swing in temperature.
As a rough guide, combis around 24 to 28kW suit many flats and 1 to 2 bed homes, and 28 to 32kW many 3 bed homes. Larger combis around 32 to 38kW can suit 3 to 4 bed homes with an en-suite, but only if mains flow allows. The survey confirms the right size.
Get a combi quote with flow checked firstTwo minutes, no obligation, a Gas Safe engineer replies.
Start my quoteA simple test you can do at home
You can get a rough idea of your flow rate with a container of known size and a timer. It won’t be as accurate as an engineer’s gauge, but it tells you whether you have a lot of water or a little.
- 1Make sure no other taps, showers, washing machines or dishwashers are running in the house.
- 2Use the kitchen cold tap. In most homes it comes straight from the mains, unlike some bathroom taps that may be fed from a loft tank.
- 3Use a bucket or container with a known volume, such as a marked bucket or a jug you can fill several times. Turn the tap fully on and time how many seconds it takes to fill.
- 4Work out litres per minute: divide the litres by the seconds, then multiply by 60. For example, 10 litres in 30 seconds is 20 litres per minute.
Try it at a different time of day too, because flow can change with demand on the local supply. Note your result in the quote form. It helps us, though we’ll still measure it properly on the survey.
What affects your mains flow
- The supply pipe into your home. Older homes can have a narrow or ageing supply pipe from the street.
- Shared supplies. Flats and converted houses sometimes share a supply, so flow drops when neighbours use water.
- Your stopcock. A stopcock that isn’t fully open, or is partly seized, restricts flow.
- Height and distance. Upper floors and long pipe runs get less than a ground-floor kitchen.
- Time of day. Demand on the local network changes through the day.
Some of these can be improved, for example by opening a stopcock fully or replacing an old supply pipe. Work on the supply outside your property involves your water company, so it isn’t a quick fix.
If you’re not sure where your stopcock is, it’s worth finding it anyway. It’s often under the kitchen sink, and you’ll need it quickly if a pipe ever leaks. Turn it gently: an old stopcock that hasn’t moved in years can be stiff, and forcing it can damage it.
If your flow is low
Low flow doesn’t mean you can’t have good hot water. It means a combi may disappoint, and another type of boiler may suit your home better.
- A system boiler with a hot water cylinder stores hot water ready to use, so it isn’t limited by how fast the boiler can heat water on demand.
- An unvented cylinder delivers stored hot water at mains pressure. It still relies on a reasonable mains supply, so we check yours first. It must be fitted by someone with an unvented hot water qualification (G3), and we’ll confirm on your quote.
- A vented cylinder fed from a tank in the loft doesn’t depend on mains flow in the same way, though its pressure depends on the height of the tank.
See system boiler installation for how a stored hot water setup works, or boiler replacement if you want us to recommend the right type for your home.