Combi, system or regular boiler: which one does your home actually need?

If you ring three heating companies and describe your house, there is a good chance all three will recommend a combi boiler. Combis are the default in Britain now, and for the great majority of homes that default is correct. They are cheaper to fit, they free up the airing cupboard, and they heat water on demand so nothing sits in a tank losing heat overnight.

But the default is wrong often enough to matter. We are called out several times a year to homes where a combi was fitted two or three years ago into a property that was never suited to one, and the owner has spent every day since living with showers that go cold when somebody runs a tap. That is not a faulty boiler. It is the wrong boiler, correctly installed.

This guide explains the difference between the three types, and more importantly how to work out which one your own house needs. It is the same reasoning we go through at a survey.

The three types, in plain English

Forget the technical names for a moment. The three types differ in one respect: where your hot water comes from.

A combi boiler makes hot water the instant you turn a tap on, by firing the burner and passing mains water through a heat exchanger. There is no store. Hot water is made to order, and it never runs out.

A system boiler makes hot water in advance and keeps it in an insulated cylinder, usually in a cupboard. When you turn the tap on, you are drawing from that store at mains pressure. When the store is empty, you wait for it to reheat.

A regular boiler — also called heat-only, conventional or traditional — also stores hot water in a cylinder, but the system is fed from tanks in the loft rather than directly from the mains. Gravity does the work, which is why the water pressure in an old house with a regular system is often disappointing upstairs.

Everything else follows from that one difference.

Combi boilers: what they do well, and badly

What they do well

A combi is compact, and in a small home that matters more than people expect. No cylinder means an airing cupboard you can actually use, and no loft tanks means nothing to freeze, leak through a ceiling, or need looking at every few years.

Hot water is unlimited in the sense that it never runs out. You can run a bath, then another bath, then a shower, and the boiler will keep making hot water indefinitely. That is genuinely useful.

They are also the cheapest type to install in a house that already has one, because the swap is straightforward and there is no cylinder to accommodate.

What they do badly

A combi produces hot water at a fixed rate, measured in litres per minute. That rate is the whole story. If your boiler delivers twelve litres a minute and your shower wants ten, everything is fine — until somebody turns on the kitchen tap and now there are twelve litres to share between two outlets.

This is why the complaint is so specific and so consistent: the shower is fine until somebody else uses water. It is not a fault. It is arithmetic.

The second limitation is that a combi is only as good as your incoming mains. It cannot give you more water than the street gives it. A house with a weak or shared supply will have a weak combi, whatever is written on the boiler.

System boilers and unvented cylinders

A system boiler paired with an unvented cylinder is the answer for homes with genuine simultaneous demand. The cylinder holds a store of hot water at mains pressure, so two showers can run at once without either of them noticing.

The trade-offs are real and worth understanding before you commit.

First, the cylinder takes up space, usually a cupboard. Second, the store is finite — a big family that runs three showers back to back can empty a 210 litre cylinder, and then everyone waits. Third, an unvented cylinder is a pressure vessel and legally requires an annual service, which a combi does not.

What you get in return is hot water that behaves the same regardless of how many taps are open. In a house with three bathrooms in daily use, that is worth a great deal.

Sizing the cylinder matters as much as sizing the boiler. Too small and you run out; too large and you spend every day heating water nobody uses. As a rough guide, allow something in the region of 45 litres per person, then adjust for how your household actually behaves — a family that all shower between seven and eight in the morning needs more storage than the same number of people spread across the day.

Regular boilers, and why they still exist

Regular boilers look old-fashioned, and installers often push to remove them. Sometimes that is right. Sometimes it is not.

They remain the sensible answer in three situations. Large period properties with original pipework, where converting the whole system would mean lifting floors that nobody wants lifted. Homes with genuinely poor mains pressure, where the loft tank is doing useful work by providing a buffer. And houses where the existing system is sound and the owner simply wants a new, efficient boiler rather than a project.

If your regular system works and you are happy with it, a straight swap for a modern condensing regular boiler is a perfectly respectable choice. Anyone who tells you otherwise without asking about your pipework and your water pressure is selling, not advising.

The four questions that actually decide it

Everything above comes down to four questions. Answer these honestly and the choice usually makes itself.

1. How many bathrooms and showers do you have, and how often are two used at once?

Not how many bathrooms exist — how many get used simultaneously. A four-bedroom house where everyone leaves at different times has different needs from a three-bedroom house where three people shower between seven and half past.

One bathroom: a combi is almost certainly right. Two bathrooms used at different times: a combi is usually fine, sized properly. Two or more used at the same time: you want a cylinder.

2. What is your incoming mains flow rate?

This is the single most important number and the one most rarely measured. More on it below.

3. How many radiators are you heating?

This decides the heating output, which is a different number from the hot water output. A large house with twenty radiators needs a boiler that can heat them all on a cold morning, regardless of what you have decided about hot water.

4. What have you got now, and what would converting involve?

A like-for-like swap is a day's work. A conversion is two days and several hundred pounds more, because tanks come out, pipework is re-run and ceilings sometimes need making good. That does not make conversion wrong, but it should be a decision rather than an assumption.

Mains flow rate: the number nobody measures

Your mains flow rate is how many litres a minute your incoming supply can deliver. It is measured with a weir gauge or, at a pinch, a bucket and a stopwatch. It takes two minutes and it determines whether a combi will work in your house.

As a rough framework:

• Under 10 litres a minute. A combi will disappoint you. This is the territory where a cylinder system is the honest recommendation, whatever you have been told.
• 10 to 14 litres a minute. A combi will run one good shower. It will not run a shower and a tap well at the same time.
• 15 litres a minute and above. A combi can support two outlets reasonably, if the boiler is sized to match.

Here is the part that catches people out: a boiler advertised at sixteen litres a minute cannot deliver sixteen litres a minute if your mains only brings in eleven. The boiler figure is a ceiling, not a promise. Fitting a bigger combi to solve a weak-shower problem is one of the most common and most expensive mistakes in domestic heating, because the bigger boiler changes nothing at all.

If an installer quotes you for a combi without measuring your flow rate, ask why. There is no good answer.

What converting actually involves

Cylinder system to combi

The cylinder comes out, the loft tanks come out, and the pipework that fed them is either removed or capped. New pipework runs to the boiler position. Where the tanks were, you usually gain a cupboard and lose a bit of ceiling, which has to be made good.

It typically adds around £900 to a straight swap and turns a one-day job into two. It is worth it if the space matters to you and your flow rate supports it. It is not worth it purely because somebody said combis are modern.

Combi to cylinder system

Less common but increasingly requested, usually after an extension has added a bathroom. This is the more expensive direction, because you are adding a cylinder that does not currently exist, along with the pipework and controls to run it. Expect the cylinder alone to be a four-figure item on the quote.

Back boiler removal

If you have a gas fire with a boiler hidden behind it, that is a back boiler. Removing one is a bigger job than any straight swap: the fire comes out, the chimney is swept and sealed, the builder's opening is made good, and the pipework is re-run to wherever the new boiler is going. Budget around £1,250 on top of the boiler, and two days.

Sizing: why bigger is not better

There is a widespread belief that a bigger boiler is a safer choice. It is not, and the reasoning is worth understanding.

A boiler that is substantially oversized for its house will heat the radiators quickly, reach temperature, and shut down. A few minutes later the temperature drops and it fires again. This is called short cycling, and it is the central heating equivalent of driving in stop-start traffic. Every ignition is a moment of inefficiency, the components wear faster, and the boiler never settles into the steady low-output condensing mode where it is at its most efficient.

The result is a boiler that costs more to buy, more to run, and fails earlier. Oversizing is common because it is the safe choice for the installer — nobody complains that their house is too warm — but it is not the right choice for the person paying the bills.

Correct sizing means a heat loss calculation, or at minimum a sensible assessment of radiator count, property size and insulation. It takes a few minutes and it is the difference between a boiler that lasts fifteen years and one that limps to eight.

The five mistakes we see most

1. Choosing a combi because it is cheaper, in a house that needs a cylinder

The saving is a few hundred pounds. The consequence lasts as long as the boiler does.

2. Fitting a bigger boiler to fix weak water pressure

If the mains cannot supply it, the boiler cannot deliver it. This is the single most expensive misunderstanding in the trade.

3. Skipping the system flush

A new boiler connected to a system full of rust and sludge will have that debris pumped through its heat exchanger from day one. Most manufacturer warranties require a flush and a filter. Skipping it saves a few hundred pounds and can cost you the warranty.

4. Keeping radiators sized for the old system

Modern condensing boilers work best at lower flow temperatures, which means radiators often need to be larger than they were. Fitting a new boiler and keeping undersized radiators is why some homes feel colder after an upgrade.

5. Not registering the warranty

A twelve-year warranty is worth nothing if nobody registered it within the manufacturer's window. A good installer does it for you on the day and gives you the paperwork. Ask.

So which one do you need?

For most homes in Surrey, with one or two bathrooms and a reasonable mains supply, a correctly sized combi is the right answer and everything above is reassurance rather than warning.

If you have three or more bathrooms in daily use, a weak incoming main, or a household that showers at the same time each morning, a system boiler with an unvented cylinder will make you considerably happier, and the extra cost is worth it.

If you have a large period property on sound original pipework and no appetite for upheaval, a modern regular boiler is a perfectly sensible choice that nobody should talk you out of.

The one thing that should never happen is a recommendation made without anybody asking about your bathrooms, measuring your mains and counting your radiators.

Related reading

• New boilers and what they cost
• Unvented cylinders explained
• Get an instant boiler price

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