In the interwar housing that fills Harrow, Hillingdon, Barnet and Enfield, the standard arrangement is still a boiler, a hot water cylinder in an airing cupboard and one or two tanks in the loft. Converting all of that to a single combi boiler on a kitchen wall is one of the most requested jobs we are asked about, and for a large share of those houses it is the right answer.
For a meaningful minority it is not, and the people it disappoints are almost always the ones who were sold it on the space saving alone. This guide sets out both sides honestly, and what the work actually involves.
What you gain
Space, and it is real space. The airing cupboard becomes a cupboard again, or a shower room, or the top of a staircase that no longer has a box in it. The loft loses two tanks and the boarding and pipework around them, which in a house where the loft is storage or a future conversion is worth a great deal more than the plumbing costs.
You also gain mains-pressure hot water at every outlet. A gravity-fed system delivers hot water at whatever pressure the height of the loft tank provides, which is why an upstairs shower in an old system dribbles. After a conversion every tap and shower runs at the pressure of the street main.
And you gain simplicity: no tank to freeze in the loft, no cylinder losing heat to a cupboard twenty-four hours a day, no immersion element to fail, no ball valve to jam. A combi heats water only when somebody asks for it.
What you lose, and this is the part people are not told
You lose simultaneous hot water. A combi heats water as it flows, through one heat exchanger, in real time — so two outlets share what one would get. Two showers at once is not something a combi does well, and no amount of output on the badge changes that. In a house where two people leave at the same time every morning, this is the whole argument.
You lose the backup. A cylinder with an immersion heater gives you hot water when the boiler fails; a combi gives you none. On the evening your boiler stops, that difference is felt immediately.
And you may lose your shower. This surprises people more than anything else on the list. A pumped shower fed from a gravity system needs low-pressure water to work; put it on mains pressure and it may be unusable or unsafe. The same applies to some older mixer valves. Any pumped shower in the house has to be identified before the conversion is quoted, not discovered afterwards.
- Gain: airing cupboard, loft space, mains-pressure hot water, fewer things to fail
- Lose: two outlets at once, the immersion heater backup, possibly an existing pumped shower
- The decision is made by outlet count and mains flow rate, not by house size
When a combi is the wrong answer
Three tests, and failing any one of them should give you pause. If two or more bathrooms are genuinely used at the same time, a combi will disappoint every morning of its life. If the flow rate of your incoming main is low — and in older London properties with original service pipework it often is — a combi has nothing to work with, because it can only heat the water the street delivers. And if the house is large enough that the furthest bathroom is a long run from the boiler, the wait for hot water gets noticeable.
In those houses the better answer is usually a system boiler with an unvented cylinder. You still lose the loft tanks and still gain mains-pressure hot water, and you keep stored water so several outlets can run at once. You keep the cylinder, but a modern unvented one is a different proposition from the lagged copper thing currently in the cupboard.
The measurement that settles it is the flow rate on your incoming main, taken at the kitchen tap with everything else closed. Any quote for a conversion that did not include that measurement was not an assessment, it was a guess.
What the work actually involves
More than swapping an appliance. The cylinder comes out and the loft tanks come out. The gravity pipework that fed them is capped back or removed, and in a house of this age some of that is large-bore steel or copper running under floors. The cold water supply to the whole house changes over from tank-fed to mains-fed, which means every cold tap in the house is now at mains pressure and every old fitting has to cope with it.
The gas supply usually needs checking and sometimes upgrading, because a combi draws more on hot water than the old boiler did. The system needs a proper clean before a new appliance runs on it — decades of gravity circulation leave a great deal behind, and manufacturers' warranties generally depend on it. And then there is making good: a loft with two tanks removed and an airing cupboard with a cylinder gone both need boarding, plastering and decorating.
It is typically two to three days rather than one, and a quote that prices it as a straight boiler swap has not understood the job. Ours is fixed and in writing after a survey, which on this particular piece of work is not a formality.