Almost every boiler quote arrives with a brand name and a price on it. Very few arrive with the two figures that actually determine whether you will be happy with the result: how many hot water outlets you use at the same time, and how much water per minute your incoming main can deliver. Get those right and a modest boiler will serve you well for fifteen years. Get them wrong and the most expensive appliance on the market will still give you a shower that drops to a trickle when somebody runs the kitchen tap.
This guide explains what the three types actually do, which sort of London property each one suits, and how to check the numbers yourself before anyone quotes you.
What the three types actually do
A combi boiler heats water as it flows through, on demand. There is no cylinder and no tank, so it takes up the least space and wastes nothing keeping stored water warm. Its limit is that everything it produces comes through one heat exchanger in real time: run two outlets and each gets roughly half. That is a law of physics, not a shortcoming of a particular model, and it does not change if you buy a bigger one.
A system boiler heats a cylinder of stored water. The cylinder sits at mains pressure if it is unvented — a Megaflo or similar — so every tap and shower in the house draws from a reservoir rather than competing for a flow. Two showers at once is a normal expectation with a properly sized cylinder. The cost is space: the cylinder has to live somewhere, and it needs a safe discharge route for its pressure relief.
A regular boiler, sometimes called a heat-only or conventional boiler, is the traditional arrangement: a boiler, a hot water cylinder and one or two tanks in the loft feeding it. It is what most London houses had before the 1990s. It still has its place in large properties with poor mains pressure, where the loft tank does the work the mains cannot, but for most homes a system boiler does the same job with two fewer tanks and a great deal more pressure at the tap.
The two numbers that decide it
The first is your simultaneous outlet count. Not how many bathrooms exist — how many get used at once on a weekday morning. A three-bedroom house where one person showers at a time is a one-outlet house whatever the floor plan says. A house where two people shower while the washing machine fills is a genuine multi-outlet house, and a combi will disappoint in it every single morning.
The second is the flow rate of your incoming water main, measured in litres per minute. This one sets the ceiling on everything downstream. An engineer measures it with a weir gauge or a bucket and a stopwatch at the kitchen tap with everything else closed. In much of London the figure comes back lower than people expect, particularly in older properties where a section of the service pipe is still the original small-bore or lead run. If your main delivers twelve litres a minute, no boiler and no cylinder will give you twenty at the shower.
Those two numbers, taken together, answer the question. One outlet at a time and a decent main: a combi, and you will save space and money. Two or more outlets, or a weak main, or a house on four floors: a system boiler and a properly sized unvented cylinder. A weak main and high demand: the cylinder plus, sometimes, an accumulator or a pump to make up what the street cannot supply.
- Count outlets used at the same time, not bathrooms that exist
- Have the incoming main measured in litres per minute before you accept a quote
- Ask what the flow rate was, and be wary of any quote that never mentioned it
- Height matters: a four-storey converted terrace loses pressure the whole way up
How this plays out in London housing
In the interwar semis that fill Harrow, Barnet, Hillingdon and Enfield, the typical picture is one bathroom, sometimes a second added in an extension, and a loft tank and cylinder that have been in place for decades. Where the bathroom count has stayed at one and the mains flow is reasonable, converting to a combi is usually the right call: it frees the airing cupboard and the loft and simplifies the system. Where a second bathroom has appeared, the calculation flips.
In the Victorian and Edwardian terraces of Wandsworth, Hammersmith, Haringey and Southwark, the pattern is different. Those houses have been extended at the back, into the side return and up into the loft, so a two-storey house with one bathroom has become a four-storey house with two bathrooms and an en-suite. That is a stored hot water house, and the interesting design question is where the cylinder goes once the airing cupboard has become a shower room.
In flats, particularly conversions in Hackney, Camden, Islington and Lambeth, the constraint is usually not the choice of boiler but where a flue may terminate and whether the gas supply serving the building can carry another appliance. Those questions are answered by surveying the building, not the flat, and they should be answered before any appliance is ordered.
What a good quote contains
A quote worth comparing tells you the measured flow rate of your main, the heat loss calculation or at least the basis for the boiler output chosen, what happens to the existing pipework, whether the system will be flushed and what kind of flush, the position of the flue and the condensate route, and what the warranty depends on. Most manufacturer warranties require an annual service and a clean system, and a warranty that is void because the pipework was never flushed is worth exactly nothing.
A quote that contains a brand, a model and a price and nothing else is not cheaper than one that contains all of the above. It is the same job with the thinking left out, and the thinking is where the difference between a good installation and a bad one lives.