Combi Boiler vs System Boiler: Which Is Right for Your Home?

A combi boiler heats water the moment you open a tap, with no cylinder and no tanks, which makes it the right choice for most homes with one bathroom. A system boiler heats a cylinder of stored hot water instead, so it can feed two or three showers at once without anyone going cold, which is why bigger and busier households usually end up with one.

That is the short answer. The longer one depends on how many people use hot water at the same time, what your mains supply can actually deliver, and what is already installed. Those are the questions our installers work through on every boiler survey across Deal, Dover, Sandwich, Canterbury and Thanet, and this guide walks through them so you can get most of the way to an answer before we arrive.

What is the difference between a combi and a system boiler?

Both are gas boilers that run your radiators in exactly the same way. The difference is entirely about hot water.

A combi boiler is a single unit on the wall. Cold water from the mains runs through it and comes out hot, on demand, for as long as the tap is open. There is nothing to store, nothing in the loft, and nothing to run out of. The catch is that one boiler can only heat so much water per minute, and the mains has to push that water through it.

A system boiler heats central heating water and sends some of it through a coil inside a hot water cylinder, usually in an airing cupboard. The cylinder fills with hot water over an hour or so and holds it, so when you open a tap you are drawing from a tank that is already hot. Modern system boilers are paired with an unvented cylinder, which runs at mains pressure with no cold tank in the loft. Our guide to unvented versus vented cylinders covers that side in detail.

A regular (heat-only) boiler is the older arrangement with a cylinder and a cold water tank in the loft. Most of the homes we visit with a regular boiler are candidates for either of the two above when it is replaced.

Is a combi or system boiler better?

Neither is better in the abstract. One is better for your house, and the way to find out is to answer the same questions our installers ask.

How many bathrooms are in use at the same time?

This is the question that decides most surveys. A combi serves one outlet properly at a time. Run the kitchen tap while someone is in the shower and the shower cools or loses flow. In a home with one bathroom and one or two people, that almost never matters. In a four-bedroom house with an en-suite and a family bathroom, with two teenagers getting ready for school at the same time, it matters every morning.

As a rule, one bathroom points to a combi. Two bathrooms that are regularly used at once, or a large bath that gets filled often, point to a system boiler and a cylinder.

What can your mains supply deliver?

A combi can only heat the water the mains gives it. We measure the incoming flow rate and pressure at the stopcock on every survey, and across East Kent the results vary a lot more than people expect. Older streets in Deal, Walmer and Ramsgate with a long run of narrow lead or galvanised supply pipe sometimes give us well under ten litres a minute, and no combi on the market can make a good shower out of that.

If the mains is weak, a system boiler with an unvented cylinder does not solve the problem on its own, because an unvented cylinder also relies on mains pressure. In those houses the honest options are a new supply pipe from the street, which is sometimes the best money spent on the whole job, or a cylinder with a pumped arrangement. We will tell you which applies rather than fitting a combi that disappoints.

What is already installed?

If you already have a cylinder and the airing cupboard space for it, a system boiler is usually the simpler swap. The cylinder may be worth keeping if it is unvented and in good condition, or it may be due for replacement at the same time.

If you have a combi already and it has served the household well, there is rarely a reason to change type. Most of our next-day boiler installations are a like-for-like combi swap for exactly that reason.

Do you want the space back?

Taking out a cylinder and loft tanks frees up an airing cupboard and clears the loft. For a smaller house or a flat, that is often what tips a borderline decision towards a combi.

Can you replace a system boiler with a combi?

Yes, and it is one of the most common conversions we do. The cylinder and any loft tanks come out, the hot and cold pipework is re-routed to the new boiler, and the old tank feeds are capped off. It is a bigger job than a like-for-like swap, usually two days rather than one, and it only makes sense where the mains flow is good enough and the household is not going to miss stored hot water.

The conversion that makes us pause is a large family home going from a cylinder to a combi to save space. The space saving is real, but so is the cold shower when two outlets are open. Where our measurements on the day make that likely, we say so.

Going the other way, from a combi to a system boiler and cylinder, is also possible and is the right move for a home that has grown, added a bathroom, or extended into the loft. It needs somewhere to put the cylinder, and we check the floor and the discharge route for an unvented unit before quoting.

What size house needs a system boiler?

Think in bathrooms and people rather than bedrooms. A three-bedroom house with one bathroom and a family of three is combi territory. A three-bedroom house with a bathroom, an en-suite and four people showering between seven and eight in the morning is a system boiler house, whatever the floor area.

The other thing that pushes a home towards a system boiler is a high hot water demand in a short window. Large baths, body-jet showers, or two showers running together all draw more water per minute than a combi will comfortably supply, and a cylinder shrugs that off.

What does each one cost?

Because this is a guide rather than a quote, these are broad ranges only. Across East Kent, a straight combi swap is typically £1,900 to £2,800, depending on the boiler chosen and the state of the existing pipework. Converting from a cylinder system to a combi is typically £3,000 to £4,200, depending on the property. A system boiler with a new unvented cylinder is typically £3,600 to £5,000, depending on cylinder size and the controls specified.

The real figure for your house comes from our instant quote, which asks the survey questions above and prices the job properly, with one price for everyone. Boiler installation is one of the two things we do at a fixed price, so the figure you are given is the figure you pay.

A few things that apply whichever you choose

• Every boiler we fit is installed by a Gas Safe registered engineer. David Osborne Heating is Gas Safe registered, number 2836, and gas work is never a job for anyone who is not.
• A system filter and a proper flush go in with every install. Sludge from an old system will shorten the life of a new boiler of either type, and it is the most common cause of the fault codes we are called to on newer boilers, including the ones in our Glow-worm fault code guide.
• An annual service keeps the warranty valid. Ours is £90+VAT, and the BOYLA HomeCare plans from £9 a month include it.
• Our full guide to new gas boiler installation covers what happens on the day, from survey to commissioning.

Still not sure which to choose?

Most people are not, and that is what the survey is for. Tell us how many bathrooms you have, how many people use them and roughly when, and we will measure the rest at the stopcock. Call 01304 381999 and a real person will answer, or start with the instant quote and we will take it from there. You can read more about how we approach a new boiler installation across Deal, Dover, Sandwich, Canterbury and Thanet before you decide.