Internal boilers last longer and avoid frozen condensate but need a flue route, ventilation and space. External boilers free up room and are simpler to install but suffer from weather and are the ones that fail in January. Boiler houses combine the advantages and cost more.
The three options
Internal
Inside the heated envelope of the building – a utility room, kitchen, garage or purpose-made cupboard. The appliance lives in a dry, temperature-stable environment, which is the single biggest determinant of how long it lasts.
External
Outdoors in a weatherproof casing, mounted against a wall. Frees internal space entirely, removes the flue routing problem, and removes noise from inside the house. Exposed to everything the weather does.
Boiler house
A small outbuilding or dedicated structure housing an internal-pattern boiler. Common on farms, larger properties and where a boiler serves multiple buildings. Combines most of the advantages of both and costs more to build.
What each one costs you
| Internal | External | Boiler house | |
|---|---|---|---|
| Expected lifespan | Longest – dry and temperature stable | Shortest – weather and temperature cycling | Long |
| Frozen condensate risk | Lowest – internal termination is usually possible | Highest – the condensate is outside by definition | Low to moderate |
| Space taken from the house | Significant | None | None |
| Flue routing | Must be solved – and it constrains position | Simple | Simple |
| Ventilation requirement | Must be provided and kept clear | Inherent | Must be provided |
| Noise indoors | Audible – oil burners are not quiet | None | None |
| Ease of servicing | Good – dry and lit | Poorer in bad weather | Good |
| Corrosion of casing and controls | Minimal | Real – casings rust, electrics suffer | Minimal |
| Heat loss from the appliance | Recovered into the house | Lost outside | Lost, unless the space is useful |
| Installation cost | Higher – flue, ventilation, making good | Lower | Highest – the structure |
Lifespan and frozen condensate. An external oil boiler lives a harder life than an internal one and it is the appliance that shuts down in a cold snap, because its condensate pipe is outside by definition. If those two matter to you, they should weigh heavily.
External boilers in practice
They are extremely common in Norfolk, for good reasons – the housing stock frequently has nowhere sensible to put a boiler internally, and an external unit is simple and cheap to install. They also generate a disproportionate share of our winter call-outs.
What goes wrong with them:
- Frozen condensate. The single most common winter fault. The condensate is outdoors, in small-bore plastic, in a cold wind – see frozen condensate pipes.
- Water ingress into the casing. Perished door seals, corroded casings and failed weather seals let water into the burner housing and onto electrical components.
- Corroded casings. The steel casing is a consumable on a coastal or exposed site, and once it is holed the appliance inside deteriorates quickly.
- Nobody looks at it. An external boiler is out of sight, and small problems – a rusting casing, a weeping oil line, a dangling fire valve sensor – go unnoticed for years.
- Servicing in bad weather. A service done properly in driving rain in February is harder than the same service in a dry utility room, and the quality can suffer.
None of these make external boilers a bad choice. They make it a choice that needs the condensate designed properly and the annual service taken seriously.
Internal boilers in practice
The constraints are flue, ventilation, space and noise.
- Flue terminal position is the big one. Minimum clearances to openings, boundaries and surfaces apply, and they frequently dictate where the boiler can go rather than the other way round. Plume from a condensing appliance is visible and it goes somewhere.
- Ventilation must be provided to the manufacturer’s requirements and must stay clear. An air vent blocked up because it was draughty is a genuine safety issue.
- Noise. An oil burner is not quiet – there is a motor, a fan and a pump. A boiler in a utility off a kitchen is fine; one in a cupboard next to a bedroom is not.
- Space and access. Not just for the appliance but for servicing it. A boiler boxed into a cupboard with shelving over it will be serviced badly.
- The condensate can usually go internally, which removes the freezing problem entirely. This is the main advantage and it is worth designing for deliberately.
Garages deserve a note. A boiler in an unheated garage gets some of the disadvantages of both – it is indoors for flue purposes but it is not in the heated envelope, so the condensate can still freeze and the appliance still experiences cold. It is workable and it needs the condensate treated as though it were external.
Boiler houses
A dedicated boiler house is the right answer in a few specific situations: where a boiler serves several buildings, on a farm or estate, where bulk fuel storage and plant are sensibly kept together, or where the property is large enough that the noise and space arguments both point outdoors.
Requirements worth knowing: it needs proper ventilation for combustion and heat rejection, adequate separation and fire protection, a solid floor, lighting and power, and enough room to work all the way round the appliance. A boiler house that is really a shed with a boiler in it will cause problems.
The advantage worth emphasising is servicing. A boiler in a dry, lit, spacious boiler house gets a better service than one in a cramped cupboard or in the rain, every time.
Replacing a boiler? The position is the decision that most affects what happens over the next fifteen years, and it is worth deciding deliberately.
Questions we get asked about this
Do external oil boilers last as long as internal ones?
Generally not. Weather, temperature cycling and water ingress all shorten life, and casings corrode. A well-maintained external boiler can still do very well – it just has a harder life.
Can I move my boiler from outside to inside?
Often, and it needs a flue route that satisfies terminal clearances, ventilation provision, space, and an oil line and electrical supply brought to the new position. It is a proper job rather than a relocation, but it usually pays back in reliability.
Is an external boiler cheaper to install?
Usually, because there is no flue routing through the building, no internal ventilation to provide and no making good. The saving is at installation; the cost tends to appear later.
Will an external boiler be noisy for my neighbours?
Oil burners produce a mechanical noise while running and it is outdoors. On a tight site it is worth thinking about where the boiler faces, in the same way as an air conditioning condenser.
Can an oil boiler go in a garage?
Yes, commonly, with appropriate ventilation and separation. Treat the condensate as though it were an external installation, because an unheated garage gets cold enough to freeze it.
Does the boiler position affect efficiency?
Marginally. An internal boiler’s standing heat losses go into the house rather than outside, which is a small real benefit. The bigger efficiency factors are sizing, commissioning and controls.
You might also need
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- Buying heating oil: order size, timing and tank management
- Eight signs an oil tank is near the end of its life
- Frozen condensate pipes: why it happens and how to stop it recurring
- Grant, Firebird, Warmflow, Worcester Danesmoor and Mistral, compared honestly
- Oil boiler lockouts: what the reset button does, and when to stop pressing it
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