When a combi boiler is the wrong choice (and what to fit instead)

Combi boilers now make up the large majority of domestic installations in Britain, and for most homes that is the correct outcome. They are cheaper to fit, they free up the airing cupboard, they remove loft tanks entirely and they never run out of hot water.

But "most" is not "all", and the minority is larger than the industry admits. This guide is about how to work out, before you spend the money, whether you are in it.

Why the combi became the default

Three reasons, and only one of them is about your house.

The first is genuine: combis suit the typical British home, which has one bathroom and a reasonable mains supply. The second is that they are cheaper and quicker to install, which suits the installer as much as the customer. The third is momentum — once a type becomes the default, recommending it requires no thought and no explanation.

The result is that a lot of people are recommended a combi by someone who never asked the questions that would reveal they shouldn't have one.

1. Your mains flow rate is too low

This is the big one, and it is the single most common reason a combi disappoints.

A combi heats water as it passes through. It cannot store, and it cannot boost. Whatever the street delivers is the ceiling on what your shower gets. If your incoming main gives ten litres a minute, a combi rated at sixteen delivers ten.

Below about ten litres a minute, a combi will never give you a shower you are happy with. Between ten and fourteen, you get one decent shower and nothing else running. Fifteen and above and you have room to work with.

The reason this catches people is that low flow often has nothing to do with the property. An old lead or narrow steel supply pipe from the main, a shared supply serving several houses, or simply being at the end of a run can all cap it. You cannot see any of that from inside the house, and the only way to know is to measure.

The test: run the cold kitchen tap fully open into a bucket for exactly a minute, with nothing else running, and measure the litres. It is crude but it tells you which bracket you are in.

2. Two outlets run at the same time

A combi produces hot water at one rate, and every open outlet shares it. Turn on the kitchen tap while somebody is showering and the shower gets roughly half of what it had.

This is why the complaint is always so specific: the shower is fine until somebody else uses water. It is not a fault, it is arithmetic, and no amount of servicing will change it.

Be honest about your household's actual behaviour rather than its bathroom count. A four-bedroom house where everyone leaves at different times is easier to serve than a three-bedroom house where three people shower between seven and half past. Peak simultaneous demand is what matters.

If two showers regularly run at once, a combi is the wrong appliance. Not a compromise — the wrong appliance.

3. The house is large, or the pipe runs are long

Two separate problems here, and large houses often have both.

The first is heating output. A combi's hot water figure and its heating figure are different numbers. A house with twenty-plus radiators needs enough heating output to lift them all on a cold morning, and a combi chosen for its hot water flow may not have it.

The second is the wait. In a large house the run from boiler to the furthest bathroom can be long, and every time you open that tap you first push out the cold water sitting in the pipe. In a big property that can be an uncomfortable wait and a genuinely wasteful volume of water, every time.

A cylinder positioned sensibly, or a secondary return, solves what a combi cannot.

4. Very hard water and a demanding household

Most of Surrey sits on chalk, and the water is hard. In a combi that matters more than in any other boiler type, because the plate heat exchanger which produces hot water has very narrow passages, and it sees fresh mains water every single time you open a tap.

A cylinder system heats the same stored water repeatedly, so the scale-forming minerals are largely used up after the first heating. A combi is continuously fed new hard water, which is why scaled plate heat exchangers are such a common failure in hard water areas.

This does not rule a combi out on its own — a scale reducer on the incoming main deals with it well. But in a hard water area with heavy hot water use and no water treatment, a combi will have a shorter life than it should, and that is worth knowing before you choose.

5. There is nowhere sensible for the flue or the boiler

This is a practical constraint rather than a performance one, and it decides more installations than people expect.

Flue terminals have minimum distances from windows, doors, boundaries and anything above them. In a narrow terrace, a flat with a single external wall, or a listed building where nothing may appear on a visible elevation, the available positions can be very limited.

Sometimes the answer is a vertical flue or a relocation. Sometimes the honest answer is that the existing cylinder arrangement, in the position it already occupies, is the path of least resistance and the least expense.

6. You want a genuine power shower

A high-flow or multi-head shower can demand more hot water than any domestic combi can produce. People fit an expensive shower, find it underwhelming, and blame the shower.

Genuine high-flow showering needs stored hot water at mains pressure — an unvented cylinder — or a pumped system from a vented cylinder. If a luxurious shower is the point of the project, decide that first and let it drive the boiler choice, not the other way round.

What to fit instead

A system boiler with an unvented cylinder

The usual answer. Hot water is stored at mains pressure, so several outlets can run at once without any of them noticing. Showers behave the same upstairs as down.

The trade-offs are honest ones: the cylinder occupies a cupboard, the store is finite so very heavy back-to-back use can empty it, and because it is a pressure vessel it legally requires annual servicing. In exchange you get hot water that behaves predictably, which in a busy household is worth a great deal.

A regular boiler, kept and modernised

If you already have a loft tank and a vented cylinder, the system works, and you have no appetite for a two-day conversion, replacing the boiler alone is a perfectly respectable choice. Add a modern cylinder and good controls and you have a system that will serve you well for years.

Installers often push to remove regular systems. Sometimes that is right. Sometimes it is simply the bigger job.

A combi with an accumulator

Where flow is the only problem and there is space, an accumulator stores mains water under pressure and releases it during peak demand, effectively boosting flow for the length of a shower. It is not a universal fix and it takes room, but in the right property it allows a combi where the raw mains would not.

How to check before you commit

1. Measure the flow rate. Bucket and a minute, or ask the installer to do it properly with a gauge. If they will not, that tells you something.
2. Count your real simultaneous demand. Baths and separate shower cubicles, and how often two are genuinely used at once.
3. Count your radiators. More than fifteen and heating output needs checking as a separate question from hot water.
4. Time the wait. How long does hot water currently take to reach your furthest tap? A combi will not improve that.
5. Ask where the flue goes. Before the quote, not on the morning of the installation.
6. Ask what happens when two taps run. A good installer will give you a number in litres a minute. A poor one will say "it'll be fine".

When a combi is exactly right

None of this is an argument against combis. In a one or two bathroom home with a mains supply above twelve litres a minute, where showers rarely coincide, a correctly sized combi is the best answer available: cheaper to fit, more efficient in use because nothing sits in a tank losing heat, and it gives you back a cupboard.

That describes the majority of Surrey homes, and in those homes we fit combis without hesitation.

The point is simply that it should be a decision rather than a default — and that the four minutes it takes to measure a flow rate is what separates the two.

Related reading

• Combi, system or regular boiler?
• Unvented cylinders explained
• Get an instant boiler price

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