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Published by Mo's Plumbing and Heating
What Is a Condensing Boiler?
Condensing is not a type of boiler. It is something modern boilers do, and combis, system boilers and heat only boilers can all be condensing. That confusion is worth clearing up first, because it is behind most of the questions people ask about this.
The Short Answer
A condensing boiler extracts extra heat from its own exhaust gases by cooling them until the water vapour in them turns back into liquid. That liquid is the condensate, and it drains away through a small plastic pipe. Recovering that heat is why condensing boilers are substantially more efficient than the ones they replaced, and every gas boiler installed in Britain since 2005 has had to be one. A combi can be condensing, a system boiler can be condensing, and the two questions are unrelated.
Where the Extra Heat Comes From
Burning gas produces heat, carbon dioxide and water vapour. In an older boiler, all that water vapour went straight up the flue while still hot, and the energy it carried went with it. That is what the visible plume from an old boiler was: usable heat leaving the building.
A condensing boiler puts a larger heat exchanger, or a second one, in the path of those exhaust gases and cools them much further before they leave. Cool them enough and the water vapour condenses back into liquid, and the act of condensing releases a substantial amount of energy, which goes into the heating water instead of out of the flue.
That recovered energy is the whole point, and it is why efficiency figures jumped so sharply. An old non-condensing boiler might turn something like seven tenths of the gas it burned into useful heat. A modern condensing boiler operating properly does considerably better.
The liquid produced has to go somewhere, and that is the condensate. It is mildly acidic, which is why it drains through plastic rather than metal, and why it must not simply be allowed to discharge onto brickwork or into a metal gutter.
Why Return Temperature Matters
This is the part almost nobody is told, and it is the difference between a condensing boiler that is efficient and one that is merely modern.
A boiler only condenses when the water returning from the radiators is cool enough to chill the exhaust gases past the point where vapour turns to liquid. If the return water comes back hot, the gases never cool enough, no condensing happens, and the boiler runs at roughly the efficiency of the old one it replaced.
In practice that means flow temperature settings matter. Many boilers are left set high because that is the factory default or because somebody wanted the radiators hot quickly, and a system run at a high flow temperature spends much of its time not condensing at all.
Running a lower flow temperature means radiators are less scorching and take longer to bring a room up, and in exchange the boiler condenses far more of the time. On most systems it is a genuine saving for no comfort cost once the heating is up to temperature, and it is free to try.
It is also why a properly sized system matters. Oversized radiators, correctly sized, let a system deliver the same heat at a lower flow temperature, which is exactly the condition a condensing boiler wants.
The Condensate Pipe
The one new thing a condensing boiler introduced, and the source of a recognisable seasonal fault.
It is a small plastic pipe, usually white or grey and around twenty millimetres, carrying the condensate away to a drain. Ideally it runs internally to a waste pipe, a soil stack or a sink trap, because an internal run cannot freeze.
Where it runs outside, it can. Condensate trickles rather than flows, so in a hard frost it freezes inside the pipe, the blockage backs up, the boiler detects it and shuts down to protect itself. That is the classic cold snap boiler failure and it is a blockage rather than a breakdown.
An external run should be as short as possible, in a larger diameter than the internal pipe, insulated with weatherproof lagging, and terminating somewhere sensible rather than over a path where it will ice. Where an internal route exists, that is always the better answer.
If a boiler stops during a cold snap and shows a fault, this is worth checking before calling anyone. A frozen condensate can often be thawed with a hot water bottle or a cloth soaked in warm water held against the outside pipe, followed by a reset. Not boiling water, which can split the plastic.
Do You Have One?
Three checks, and the first is conclusive.
Look for the condensate pipe. A small plastic pipe leaving the bottom of the boiler and heading for a drain or through the wall means condensing. No such pipe means non-condensing, because there is nothing to drain.
Look at the flue. A condensing boiler produces a visible white plume in cold weather, which is the cooled vapour rather than smoke, and the flue itself is plastic or plastic-lined rather than bare metal.
Check the age. Every gas boiler installed in Britain since 2005 has had to be condensing, with very limited exceptions. A boiler older than that may be either, and one considerably older is almost certainly not.
If yours is non-condensing it is at least twenty years old, which is the more useful thing that check tells you. It is not unsafe by virtue of being non-condensing, but it is old, it is inefficient, and parts availability is likely to be the thing that eventually decides matters.
Replacing a Non-Condensing Boiler
Worth knowing that this is not quite a like for like swap, because the new appliance needs something the old one never had.
The condensate needs a route to a drain. In a property that has never had a condensing boiler there is no such pipe, so one has to be created, and where that route goes is a genuine design decision rather than a detail. An internal route to an existing waste is much preferred and is not always available.
The flue is usually different too. Condensing boilers run cooler exhaust gases and use plastic flue systems, and the existing hole may or may not suit the new appliance's flue position.
And the system itself is likely to be old. A boiler of that age has usually been sitting on a circuit that has never been properly cleaned, and putting a modern appliance with narrow heat exchanger passages onto a sludged system is how a new boiler develops a fault in its second year.
None of that is a reason not to do it. It is a reason the job is properly surveyed rather than quoted from a photograph, and it is why replacing a old boiler often costs more than replacing a ten year old one.
The Boilers That Predate the Rule
Condensing became effectively mandatory for new gas boiler installations in Britain in 2005, which means anything non-condensing still running has been in place for two decades or more. There are more of those in Glasgow than the national picture suggests, and it is worth knowing why, because it affects what a replacement involves.
Two things kept them going here. A large private rented sector where appliances are replaced when they fail rather than when they age, and a lot of property where the practical difficulty of creating a condensate route made replacement more involved than a straight swap, so the old unit stayed while it still worked. The result is a tail of very old appliances in otherwise updated flats.
The condensate route is the specific difficulty in this housing. A tenement flat may have no convenient internal waste near the boiler position, particularly where the boiler sits in a hall cupboard or on an external wall away from the kitchen. Running condensate externally down a gable is possible and is exactly the arrangement that freezes in a Scottish January, so it is a last resort rather than a default. Where an internal route to a kitchen waste or a soil stack can be found, it is worth the extra pipework at installation to avoid an annual cold snap callout.
That is also why the freezing fault is so seasonal and so clustered here. A run of nights below freezing produces a wave of boilers shutting down across the city that are not broken at all, and a good number of those are external condensate runs that were the only option available at the time.
When to Call Someone Out
Worth calling if a boiler has shut down during a cold snap and thawing the external condensate pipe does not bring it back, since that means the fault is something else and the frozen pipe was a coincidence. Worth calling too if you have a non-condensing boiler and want to know realistically what replacing it involves in your property, because the condensate route and the flue are what decide that and both want looking at. Mo and Muhammad are Gas Safe registered for domestic work. If your external condensate run has frozen more than once, ask about rerouting it internally rather than treating it as weather.
Boiler Installation in GlasgowCommon Questions
No, and they are not even the same kind of category. Condensing describes how a boiler extracts heat from its own exhaust gases. Combi describes whether it stores hot water. A combi can be condensing, a system boiler can be condensing, and virtually all modern boilers of every type are.
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Read Next
Full guide
What Is a System Boiler?
A system boiler heats your radiators and heats a separate hot water cylinder, which stores hot water ready to use.
How Does a Combi Boiler Work?
A combi heats your radiators, and when you open a hot tap it stops heating the radiators and diverts all its output into heating the water flowing through to that tap instead.
What Is a Heat Only Boiler?
A heat only boiler heats water and nothing else.
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