Open Flue vs Room Sealed Appliances: Installation Rules

Why the Distinction Matters

Getting the classification wrong between an open flue and a room sealed appliance is one of the most common — and most dangerous — mistakes on a gas installation. The two types behave fundamentally differently in how they source combustion air and expel combustion products, and the installation rules reflect that. Mixing them up can result in carbon monoxide ingress, failed inspections, and invalidated certificates.

This guide breaks down the practical differences so you know exactly what to check before you start work and what to document on your certificate.

The Fundamental Difference

Open Flue Appliances

An open flue appliance draws its combustion air from the room in which it is installed. The burner is open to the room atmosphere. Combustion products are then expelled up a conventional flue — typically a lined chimney, a pre-fabricated metal flue system, or a purpose-built flue pipe. Because the appliance is breathing the same air as the occupants, any spillage of combustion products is a direct risk of carbon monoxide poisoning.

Common open flue appliances include older back boilers, some decorative gas fires, and many commercial catering appliances.

Room Sealed (Balanced Flue) Appliances

A room sealed appliance has a sealed combustion chamber. Both the air supply and the flue exhaust are connected directly to the outside via a concentric or twin-pipe system — the appliance takes no air from the room and expels no products into it. This makes them inherently safer in terms of CO risk from the appliance itself and gives you much more flexibility on installation location.

Modern combination boilers, system boilers, and most current wall-hung boilers are room sealed. Fan-assisted (fan flue) variants are the most common type you will encounter.

Ventilation Requirements

Open Flue: Permanent Air Supply is Non-Negotiable

Because an open flue appliance consumes the room air for combustion, you must provide a permanent, free air supply sized to the appliance input. The relevant standard is BS 5440-2, which gives the calculation method for ventilation openings.

The basic rule for a natural draught open flue appliance installed in a room: you need 5cm² (500mm²) of free area per kilowatt of rated heat input above 7kW. The first 7kW is covered by adventitious ventilation — the natural infiltration through gaps in the building fabric — so an appliance at or below 7kW input normally needs no purpose-provided vent in a room. Compartments are a different calculation entirely: check BS 5440-2.

For installations in modern, well-sealed buildings — particularly post-Part L 2021 new builds — do not assume infiltration is sufficient. Measure the room volume, calculate the input rating, and fit a permanent vent if the figures demand it. A hit-and-miss vent or a closeable vent is not acceptable: the vent must be permanently open and positioned to avoid obstruction.

Room Sealed: Ventilation for the Room vs. Ventilation for the Appliance

A room sealed appliance does not need a permanent air vent for combustion purposes. However, if it is installed in a compartment (e.g., a purpose-built boiler cupboard), you still need to provide ventilation for cooling the compartment — refer to the manufacturer's installation instructions and BS 6798 for the specific free area required, which is typically based on the boiler's casing heat loss.

A common oversight: installers fit a room sealed boiler in a tight under-stair cupboard with no ventilation grilles and then wonder why the boiler is overheating on the PCB. The appliance breathes outside air for combustion, but the cupboard still heats up from casing losses.

Flue Routes and Termination

Open Flue: Draught, Height, and Spillage

Open flue systems depend on natural draught to pull combustion products away from the appliance. This means:

• Flue height matters. A minimum effective flue height is required to generate sufficient draught — typically at least 3 metres from appliance draught diverter to terminal, though always check the appliance installation manual for the specific minimum.
• No excessive bends. Each 90° bend reduces effective draught. BS 5440-1 limits the number of bends and requires equivalent length calculations to ensure the system still draws adequately.
• Spillage testing is mandatory. After installation (and after any work on an existing system), you must carry out a spillage test using smoke matches or a CO analyser at the draught diverter. If spillage persists once the appliance has warmed up and you have followed the full BS 5440-1 test procedure (including worst-case conditions with fans running), the installation has failed and must not be left in use.
• Terminal position. The terminal must be sited to avoid recirculation. Minimum clearances from openings, corners, and roof lines are specified in BS 5440-1.

Room Sealed: Positioning and Clearances

Room sealed flues are far less sensitive to draught, but terminal positioning rules are strict and frequently misapplied:

• 300mm from any openable window, air brick, or other opening into a building — the typical minimum for fanned-draught room sealed appliances. Manufacturer instructions vary and always take precedence; natural draught balanced flues need larger clearances.
• 300mm from an internal or external corner — products can recirculate around corners and re-enter the building.
• Not below a window or under a roof overhang where condensate or products can stain or re-enter.
• Pitched roof terminals — if a room sealed flue exits through a pitched roof, follow Document J of the Building Regulations and the appliance manual for above-ridge height requirements.

For fan-flued room sealed appliances (which covers virtually all modern condensing boilers), the fan provides the driving force, so long horizontal runs are possible — but there are still maximum equivalent length limits. Always work these out from the manufacturer's data, as they vary significantly between models.

Location Restrictions

Open Flue in Bathrooms and Shower Rooms

Open flue appliances are prohibited in bathrooms, shower rooms, and rooms containing a shower under Gas Safety (Installation and Use) Regulations 1998 Regulation 30, and reinforced by BS 6798. The risk of CO accumulation in a sealed, humid space with occupants is unacceptable. There are no exceptions for small appliances or historic installations — if an open flue appliance is found in a bathroom, it must be disconnected.

Open Flue in Sleeping Rooms

Under GSIUR Regulation 30, a new gas appliance over 14kW (gross) input installed in a bedroom must be room sealed. At or below 14kW, an open flue appliance is only permitted if it has an atmosphere-sensing (oxygen depletion) device — in practice, fit room sealed. Room sealed appliances may be installed in bedrooms subject to satisfying ventilation (compartment cooling) and flue termination rules.

Garages and Semi-Exposed Locations

Both types can be installed in integral garages, but consider frost protection, the appliance's IP rating for the environment, and — for open flue — the risk of contaminated air (vehicle exhaust, solvents) being drawn into the combustion chamber and flue.

Documentation and Certification

When completing your Benchmark commissioning checklist or issuing a Gas Safe certificate, you must correctly identify the flue type. For open flue appliances, the certificate should record:

• Ventilation provision (permanent vent size and location, or confirmation of adequate room volume)
• Spillage test result and test method
• Flue flow test result (using smoke pellet or equivalent)
• CO alarm fitted (a legal requirement in rented homes in England, Scotland and Wales where there is a fixed combustion appliance; strongly recommended on every gas installation)

For room sealed appliances, record the flue system type (e.g., concentric horizontal, twin pipe vertical), equivalent flue length calculation reference, and terminal clearance compliance.

Common Mistakes to Avoid

1. Using output kW instead of input kW for ventilation calculations. The appliance consumes input energy; the ventilation must match it.
2. Skipping the spillage test on open flue because the flue "looks fine." Partial blockages, bird nests, or down-draught conditions from nearby structures can cause spillage that is invisible to the eye.
3. Fitting a room sealed boiler in a sealed cupboard and assuming no ventilation is needed. Compartment ventilation for cooling is separate from combustion air.
4. Terminating a room sealed flue too close to an opening (within 300mm for a typical fanned-draught appliance). This is one of the most commonly cited flue defects on inspection.
5. Not checking the equivalent flue length on long runs. Fan flue systems have limits — exceeding them causes nuisance lockouts and, in edge cases, incomplete combustion.
6. Assuming an existing open flue liner is adequate without inspection. A lined chimney must be inspected and cleaned before connection. A defective liner can cause CO to leak through the masonry into the building.

Quick Reference Summary

Use this as a sanity check before signing off any appliance:

• Open flue: Permanent ventilation sized to input kW — spillage test — no bathrooms or bedrooms — flue height and bend limits per BS 5440-1
• Room sealed: No combustion air vent needed — compartment cooling vents if in enclosure — terminal clearances per manufacturer and Gas Safe standards — equivalent flue length within limits

When in doubt, the appliance manufacturer's installation manual takes precedence over general guidance — and if the manual conflicts with a BS standard, the safer requirement applies. Keep a copy of the manual on file with your certificate; it forms part of the installation record.

Published 2026-07-06. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.