[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

I am reviewing layouts where gas boilers, water heaters and cookers are placed in kitchens, utility rooms and a small plant room. Architects often ask whether a vent is “really needed” if the boiler is room-sealed, and how the requirements differ for open-flued appliances and for a central boiler room.

• What are the principles behind combustion and ventilation air for gas appliance rooms?
• How do appliance types A, B and C differ?
• What should the designer show on drawings?

I will take the actual opening sizes from our local gas code.

[MEP Center Engineering]

The requirement depends on where the appliance takes its combustion air: open-flued (type B) and flueless (type A) appliances need permanent, non-closable air supply to the room, sized by the local gas code from the heat input; room-sealed (type C) appliances take air from outside through their own flue system and normally need no combustion air opening, although the room may still need ventilation for other reasons.

Appliance types (EN classification)

• Type A – flueless: products of combustion go into the room (e.g. cookers). The room needs ventilation to dilute them and an openable window or extract as the code requires.
• Type B – open-flued: combustion air from the room, products through a flue. Needs permanent air openings; very sensitive to extract fans and to rooms being too airtight.
• Type C – room-sealed: air and flue gas pass through a sealed concentric or twin-pipe system to outside. Most modern combi and condensing boilers are type C.

Principles behind the openings

1. Combustion air: natural gas needs roughly 9.5-10 m³ of air per m³ of gas for complete combustion, plus excess air.
2. Dilution and cooling: open-flued appliances with a draught diverter draw extra room air into the flue, and plant rooms need air to remove heat from boilers, pipes and pumps. This is why plant rooms usually have both low-level and high-level openings or mechanical ventilation.
3. Pressure balance: a kitchen extract hood or bathroom fan in the same space can reverse the flue of an open-flued appliance and cause spillage of combustion products. Codes either prohibit the combination or require a spillage test.
4. Location bans: most codes prohibit open-flued appliances in bedrooms, bathrooms and small rooms below a minimum volume.

Small example: the order of magnitude

A 24 kW open-flued water heater at 9.5 kWh/m³ uses 24 / 9.5 = 2.53 m³/h of gas, so it needs about 2.53 x 9.6 = 24 m³/h of air just for stoichiometric combustion. With excess air and draught-diverter dilution the actual air drawn from the room is several times higher. This is why a closed door and sealed windows can starve an open-flued appliance within minutes, and why the openings must be permanent.

Central boiler rooms

For plant rooms, the local code (for example EN 1775-based national rules, BS 6644 in the UK, NFPA 54 in the US, or the utility’s technical specification) typically requires: dedicated combustion air openings or fans interlocked with the burners, high-level openings for the lighter-than-air natural gas, gas detection with an automatic shut-off valve at the room entry, emergency stop outside the door, and in some codes a pressure relief area. Openings must not face escape routes and must not be closable.

What to show on drawings

• Appliance type (A/B/C) and heat input for each appliance.
• Position, free area and level of every air opening, with the code clause used.
• Extract fans in the same room and how the conflict is resolved.
• Gas detection, shut-off valve and CO alarm locations where required.

Common mistakes

• Counting a grille’s overall size instead of its free area.
• Vents later closed by tenants because they cause drafts.
• Specifying open-flued appliances in new airtight dwellings.

Related: Natural gas pipe sizing basics. Final opening areas, locations and permits come from the local gas code and the gas utility.

Check the edition of the code that applies to your project.

[Author]

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