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Published by Mo's Plumbing and Heating
How Much Is a Combi Boiler?
Combis are the default choice for most Glasgow properties and they are the right answer more often than not. But the price question has a question underneath it that matters more, which is whether your incoming water supply can actually feed one properly. Get that wrong and you have bought a boiler that disappoints you every morning.
The Short Answer
Fitting a combi where one already exists is the cheapest version of this job, and we supply and fit new boilers from £699 including VAT with the final price confirmed on the call or the visit. Replacing a tank system with a combi costs substantially more, because a cylinder and usually a loft tank have to come out and the pipework has to be reworked. Before either, check the mains flow rate at the property, since a combi is only as good as the water arriving at it.
The Question Underneath the Price Question
A combi has no stored hot water. It heats what you draw, as you draw it, straight off the incoming main. That is why it frees up a cupboard and why it never runs out, and it is also why its performance is dictated entirely by how much water your mains delivers and at what pressure.
The number that matters is flow rate, measured in litres per minute, and it is easy to test: run the cold tap in the kitchen full bore into a measuring jug for exactly ten seconds, then multiply what you collect by six. That gives litres per minute, and doing it in the evening when the street is busy gives a more honest figure than doing it at ten in the morning.
If that number is healthy, a combi will do everything you want. If it is marginal, the boiler will still work, but it will produce a thinner hot shower than you were expecting and it will struggle if anyone runs a tap at the same time. No amount of paying more for a bigger appliance fixes a supply problem, because the boiler cannot deliver water it is not being given.
So the honest sequence is to establish flow first and choose the boiler second. An installer who measures this before recommending anything is doing the job properly, and it takes about a minute.
What Changes the Price
There are really only two versions of this job and they sit a long way apart.
Replacing an existing combi with a new one is the straightforward case. The gas, water and condensate connections are broadly where they need to be, the flue route already exists and is usually reusable or replaceable in the same position, and the work is contained. This is the version our starting price is built around.
Converting from a tank system, meaning a regular boiler with a hot water cylinder and often a header tank in the loft, is a much larger piece of work. The cylinder comes out, the tank comes out, a considerable amount of pipework becomes redundant and has to be stripped and capped rather than left in a wall, the hot supply is rerouted, and the cupboard the cylinder occupied needs making good. It is two or three days rather than one.
Beyond those two, the same things that move any boiler price apply: whether the boiler moves position, whether the flue route is straightforward, whether the gas run can supply the output, and what condition the existing system is in. Those are covered in more detail on the main cost guide.
Are Combis Cheaper to Run?
Generally yes, and for a specific reason rather than because the appliance is inherently more efficient. A stored cylinder loses heat continuously, however well insulated, so a system that keeps a tank hot is paying to reheat water it has already heated and then let cool. A combi heats only what you draw.
That advantage is largest in a household that uses hot water in short bursts and smallest in one that draws a lot at once. A family running two showers simultaneously and a bath in the evening is exactly the pattern a stored cylinder handles better, and where a good unvented cylinder can genuinely be the better answer despite the standing loss.
The other half of running cost is the condition of the system rather than the type of boiler. A sludged circuit with a struggling pump and radiators that are cold at the bottom costs more to run than a clean one whatever is hanging on the wall, and that is usually the cheaper thing to fix.
What You Give Up
Worth being straight about, because the combi is usually presented as an unqualified upgrade and there are two real trade-offs.
You lose simultaneous demand. A combi produces one useful hot outlet at a time, so running a shower while someone fills the kitchen sink means both are worse. In a one or two person household this is close to irrelevant. In a busy family bathroom it is a daily annoyance.
And you lose the immersion heater as a backup. On a cylinder system, if the boiler fails you can usually still get hot water electrically while you wait for a repair. With a combi, a boiler fault means no hot water at all until it is fixed. That is not a reason to avoid a combi, but it is handy to know before it happens in January rather than after.
The thing you gain, besides the cupboard, is that there is nothing stored to run out of, and no loft tank to freeze, overflow or go stale. In a top floor flat where the tank sits in an uninsulated roof space, removing it is a genuine benefit rather than just tidiness.
Sizing It Sensibly
Combi output is quoted in kilowatts and, for a combi specifically, the headline figure is about hot water performance rather than heating. Most flats need far less heating output than people assume, and the boiler is sized up to satisfy the taps rather than the radiators.
Bigger is not automatically better. An oversized boiler in a small well insulated flat will short cycle on heating, firing and stopping repeatedly, which is less efficient and harder on the components. It also costs more to buy for performance you cannot use, since output beyond what the mains can supply is output you will never see.
The right approach is a heat loss assessment for the heating side and the measured flow rate for the hot water side. Both are quick, both should happen at the survey, and between them they narrow the sensible choice to a small range rather than a guess.
Mains Flow, Upper Floors and the Cylinder Cupboard
The flow rate question is not academic in this city, because a good deal of the housing stock is flats above ground level fed through supply pipework that is a great deal older than the boiler being fitted to it. The higher the flat, the more of the incoming pressure is spent simply getting the water up there, and a top floor flat in a four storey tenement can measure noticeably lower than the ground floor flat in the same close on the same street.
The other local factor is the shared supply. Where several flats come off one riser, demand elsewhere in the building affects what arrives at your tap, which is why the evening test is the useful one. A flow rate measured on a quiet weekday morning can flatter a supply that struggles at seven on a weeknight when the whole close is home.
Some older properties are still fed by narrow supply pipework, and where that is the constraint, upgrading the incoming supply is a separate piece of work involving the water authority rather than something an installer resolves on the day. It is not common, but know it exists as a possibility before committing to a combi in a property that measures poorly.
On the other side, the cylinder cupboard is worth more here than the square footage suggests. In a tenement flat with no loft, no garage and no utility room, the cupboard the cylinder occupies is often the only real storage in the property, and reclaiming it is a genuine part of the value of converting rather than a footnote. It is also, in practice, the thing people are happiest about a year later.
When to Call Someone Out
Worth booking a survey once you are weighing a combi against staying with a cylinder, because the flow rate measurement and the heat loss assessment are what settle it and neither can be done remotely. The survey is free on installations and replacements and the fixed price comes out of it. If the flow rate turns out to be marginal we will say so rather than sell you a boiler that will disappoint you, and there are usually better options in that situation than the biggest combi that fits. Mo and Muhammad both hold current Gas Safe registration for domestic work and do these conversions regularly.
Boiler Replacement in GlasgowCommon Questions
Replacing an existing combi is the cheapest version and we supply and fit new boilers from £699 including VAT, with the final price confirmed on the call or the visit. Converting from a tank system costs considerably more because a cylinder and usually a loft tank come out and the pipework is reworked. The survey is what produces your actual number.
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