Combi or System Boiler with a Cylinder: Which Is Right for Your London Home?

What actually separates a combi from a system boiler

A combi heats your hot water in the moment you ask for it and stores nothing. A system boiler heats a cylinder full of water and hours later you draw it off. Everything else about the two, the size, the running cost, the way they fail, comes out of that one difference.

Inside a combi there is a second heat exchanger, usually a plate exchanger about the size of a paperback, which puts boiler heat into the cold main as it passes through. A diverter valve decides which of the two jobs the boiler is doing at any moment, heating the radiators or heating the tap. That is why a combi cannot do both at once: open a tap and the heating stops until you close it, which nobody notices in a flat and everybody notices in a cold house with four people in it.

A system boiler has the pump and the sealed system components built into the case, and its only water job is to push hot primary water around a coil inside the cylinder. The cylinder does the delivering. On an unvented cylinder that water reaches the taps at close to mains pressure, which is what most people actually mean when they say they want better pressure. The boiler is then just a heat source, and it can be a modest one, because it has all night to fill the store if it needs it.

There is a third arrangement, the old regular boiler with a cold tank in the loft and a vented copper cylinder. It is common in Victorian conversions and it still works, but almost nobody specifies one new. The practical choice today is combi or system boiler with a cylinder.

• Combi: no cylinder, no loft tank, hot water on demand, heating pauses while a tap runs.
• System boiler: cylinder stores the hot water, heating and hot water can run together, needs a cupboard.
• Regular boiler: loft tank and vented cylinder, still repairable, rarely chosen for a new installation.

How many bathrooms can a combi actually serve?

One bathroom and a kitchen, comfortably. A second shower running at the same time is where a combi stops being convincing, and a second bathroom that gets used every morning is where it stops being the right appliance.

The reason is arithmetic rather than opinion. A combi has a fixed heat output and it has to lift the incoming cold water to a usable temperature as it flows through. The data badge states a hot water flow rate at a given temperature rise, normally thirty five degrees, and that figure is the honest one. Open two outlets and the same heat has to cover twice the water, so either the flow at each outlet halves or the temperature drops. In practice a thermostatic shower reacts to the drop by closing down to protect you, and the person in the second shower gets a dribble.

Winter makes this worse and nobody mentions it when selling. Incoming mains water is colder in January than in July, so the same boiler has to work harder to reach the same temperature at the tap and the flow falls away. A combi that felt adequate in August feels marginal in February, and that is not a fault anyone can fix.

A cylinder does not care. Two showers running together simply empty the store faster, and the store has already been heated. What limits you there is capacity and the incoming main, not the boiler output, which is why sizing the cylinder to the household matters more than the badge on the boiler.

If a quote solves your two bathroom problem by fitting a larger combi, ask what flow rate it gives at thirty five degrees rise and compare that to what your main actually delivers. A big combi on a weak main is an expensive way to buy the same shower you already have.

Test your incoming main before you choose anything

Do the bucket test first. It takes two minutes, it costs nothing, and it decides more of this than any brochure does.

Take a bucket whose volume you know, go to the kitchen cold tap, which is normally fed straight off the main, make sure nothing else in the property is running, open it fully and time the fill. Convert to litres a minute. Twenty litres a minute or better and you have a genuine choice between the two systems. Mid teens and a combi will feel ordinary with any second demand, and an unvented cylinder will be merely good rather than excellent. Around ten litres a minute and the supply is the problem, not the appliance, and spending money on either without improving it is money thrown away.

In older London housing a poor result usually traces back to the supply pipe itself, often a long run of old lead or narrow pipe from the stopcock in the street. In a converted flat it can also be a shared rising main. Run the test again in the evening if you are in a block, because the answer changes with what the neighbours are doing, and the worst case is the one you design around.

• Kitchen cold tap, everything else off, timed fill of a known volume.
• Repeat at the busiest time of day if you share a rising main.
• A weak main is a supply job on its own terms, not a reason to choose one boiler over the other.

What it means when a combi fails

You lose everything at once. No heating, no hot water, no way to warm a bath, and nothing to fall back on until an engineer has been and the part has been sourced. A combi is a single point of failure for the whole house, and that is the real cost of the cupboard it saved you.

The faults themselves are not exotic. The plate heat exchanger is the one London sees most, because it has narrow passages, it sees raw mains water on one side every time you open a tap, and hard water bakes scale into exactly those passages. The symptom is familiar: the heating is fine, the hot water runs hot then goes lukewarm, and the shower swings between too hot and too cold as the flow changes. The diverter valve is the other common one, and when it sticks on the heating side you get plenty of radiator heat and tepid taps, while stuck the other way you get hot water and cold radiators.

Then there is the part nobody thinks about until it happens. A combi crams a lot into a small case, so an engineer works inside a sealed appliance. Availability of parts for that specific model, on that day, decides whether you are cold for an afternoon or for a week. Serviceability is the question worth asking before you buy.

What it means when a system boiler or cylinder fails

Usually you still have hot water. That single fact is the strongest argument for a cylinder in a family home, and it is the one that never appears in the sales comparison.

If the boiler stops on a system with a cylinder, the immersion heater is still there. Switch it on and the cylinder heats electrically, expensively but reliably, and the household showers while you wait for the repair. If the immersion element fails instead, the boiler still heats the cylinder and you have lost nothing but the backup. Two independent heat sources into one store is genuine redundancy, and it is the reason we rarely regret a cylinder in a house with children in it.

The trade is that there are more components and some of them are safety components. An unvented cylinder carries a pressure reducing valve, a check valve, an expansion relief valve and a temperature and pressure relief valve, plus the expansion arrangement, and manufacturers specify a yearly check because those valves are only proven by being operated. Skip that for years and what surfaces is water running through the tundish, which is a visible sign rather than a hidden one. That is a fair comparison to make honestly: a cylinder asks for a short visit once a year, and a combi asks for nothing until the day it asks for everything.

Failure on the cylinder side is also more gradual. Scale builds on the coil and reheats get longer, an element burns out, a valve starts to pass. You get warning. Combi faults tend to arrive as an event.

Space: where each one can physically live

A combi buys you the airing cupboard and the loft. A cylinder costs you a cupboard and gives you back a system that does not fall over. In a one bedroom flat that trade is obvious. In a three bedroom house it is not.

What people underestimate is the pipework, not the appliance. Moving a Victorian conversion from a regular boiler and loft tank to a combi normally means the gas supply pipe has to be checked and often upsized, because a combi asks for its full output in one burst rather than trickling into a cylinder all evening. It means a condensate route to a drain that will not freeze, which in a top floor flat is rarely where the old boiler sat. It often means the old gravity hot water pipework is the wrong size for mains pressure. Those are the line items that turn a simple swap into a real job, and a quote that does not mention them has probably not looked.

Going the other way, putting a cylinder into a property that lost its one, the constraint is the discharge arrangement. An unvented cylinder needs a pipe from the tundish running in the right material, at a continuous fall, terminating somewhere outside where near boiling water can land without scalding a person. In a basement flat or a mid floor conversion, finding that route is frequently the hardest part of the whole installation, and it decides where the cylinder goes long before anyone thinks about the cupboard door.

London hard water changes the answer

Hard water punishes a combi harder than it punishes a cylinder, and in most of the London supply area that should weigh on the decision.

A plate heat exchanger has thin channels and it meets fresh, untreated mains water every single time somebody opens a hot tap. Scale forms on the hottest surface it touches, the passages narrow, and performance drops away years before anything actually stops working. You notice it as a shower that used to be fine and now runs hot and cold, and the fix is either a clean or a new exchanger.

A cylinder meets the same water but stores it, so scale settles as sludge at the bottom and as a crust on the immersion element and the coil. That costs reheat time and efficiency, but the element is a replaceable part and the cylinder keeps working while it scales. Running the store at a sensible temperature rather than as hot as the stat allows slows the whole process and costs nothing. What hard water does over the life of a cylinder is worth reading properly, because it is why London systems age faster than the same kit in a soft water area.

Flats, new builds and when the choice is not yours

In plenty of London properties there is no decision to make, and finding that out early saves a wasted survey.

Purpose built blocks on a communal heating circuit often have a thermal store rather than a boiler at all. A Pulsacoil and units like it take heat from the block and give mains pressure hot water through an internal heat exchanger, with no gas in the flat and often no flue route to create one. Replacing one with a combi is rarely a choice the lease allows, and the right move is to repair or replace like for like.

New builds bring their own constraint. Liquid screed over underfloor heating means the heat source runs at low flow temperatures for long periods, which suits a system boiler and a cylinder, or a heat pump, far better than a combi cycling for short tap draws. Some flats also have no external wall where a flue can terminate.

So which would we fit?

For a one or two bedroom flat with one bathroom and a main that passed the bucket test, a combi, without hesitation. It is the right appliance for that property, the cupboard is worth more than the redundancy, and pretending otherwise to sell a cylinder is the sort of thing that gives the trade a bad name.

For a house with two bathrooms, or one bathroom and a household that all leaves at the same time, a system boiler and an unvented cylinder, sized to the household rather than to the cupboard. The running experience is better and the failure behaviour far more forgiving. We take plenty of calls from people who moved from a cylinder to a combi on a two bathroom house and want to go back, and very few the other way.

For anything with a weak or shared main, fix the main first and then choose. It is the least exciting advice in this piece and the one that saves the most money, because an expensive appliance fed by a poor supply produces exactly the same disappointing shower as a cheap one.

Notice that nothing above has answered which boiler is best. That question only becomes worth asking once the appliance type is settled, because the type is decided by the property, the main and the household, and the badge on the case is decided afterwards. Getting those two in the wrong order is how people end up with a well reviewed boiler that is the wrong shape of appliance for the house it sits in.

Whichever way you go, the question to put to whoever quotes is not which brand they prefer. It is what flow rate your main delivers, what they measured it with, and what happens to your hot water on the day the appliance stops. A quote that answers all three is one worth comparing.