Sizing Ventilation for an Open-Flued Heater in a Sealed Flat
A gasfitter is asked to service and certify an open-flued space heater in a ground-floor flat that has been extensively renovated. New double glazing, draught seals on all doors and a range hood exhausting 200 L/s have been installed. The heater is rated at 25 MJ/h and sits in a living room of 45 m³ connected by a permanent 1.8 m × 0.8 m opening to a kitchen of 20 m³ and a hallway of 15 m³. The occupant reports the flame occasionally lifting and a faint smell of exhaust when the range hood runs. There is one high-level wall vent of 8000 mm² and one low-level vent of 6000 mm², both opening to the outside.
1. Explain what is happening to combustion and ventilation in this flat and why the symptoms appear when the range hood runs.
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The range hood removes 200 L/s from the flat. With new double glazing and draught seals, the flat has very little uncontrolled infiltration, so the hood depressurises the space. An open-flued appliance relies on the room's air supply and on a stable pressure at the draught diverter to establish a proper flue draught.
When the flat goes negative, two things happen:
- Spillage or reversed draught. Combustion products that should rise up the flue are drawn back into the room, producing the exhaust smell. This is a carbon monoxide risk, not a nuisance.
- Flame disturbance. Reduced oxygen and pressure changes at the burner cause the flame to lift or burn poorly, which can generate higher CO levels.
The existing vents are inadequate for the combined demand: the heater needs combustion air plus dilution air, and the exhaust fan competes for the same room air. The permanent opening between rooms helps distribute air but does not create a supply. The correct finding is that the installation is unsafe as it stands and requires additional permanent ventilation sized to the appliance and the exhaust fan, or replacement with a room-sealed appliance.
2. What steps should the gasfitter take before leaving the site, and what should the certificate say?
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The gasfitter should not leave the appliance operating in its current state.
Immediate actions:
- Isolate the gas supply to the heater and attach a warning tag.
- Test the appliance under worst-case conditions: all exhaust fans running, doors and windows closed. Confirm spillage at the draught diverter and record it.
- Perform a combustion test (CO/CO₂ and oxygen) before and during fan operation to document the deterioration.
- Check that the flue is sound, unobstructed and correctly terminated.
Rectification options:
- Increase permanent ventilation to the required free area for the appliance input and the exhaust fan, positioned to avoid dead spots.
- Fit a spillage-sensing device only where permitted, and never as a substitute for adequate ventilation.
- Recommend replacing the open-flued heater with a room-sealed unit, which removes the room-air dependency.
Certification: the compliance certificate must reflect the actual condition. If the appliance is left isolated, the certificate should record the defect and the isolation, not certify the installation as compliant. A certificate stating the appliance is safe when spillage has been observed would be a false record and exposes the gasfitter to disciplinary action.