Ventilation for a Boiler in a Sealed Cupboard
A wall-mounted 30 kW oil boiler is to be installed in a purpose-built cupboard off a kitchen in a flat. The cupboard has a close-fitting door, is 1.2 m wide by 0.8 m deep by 2.4 m high, and contains the boiler, a consumer unit, and a stopcock. The flat has modern draught-proofing and a mechanical extract fan in the bathroom. The installer proposes a single 100 mm core hole in the external wall at high level and says the cupboard door can be left with a gap at the bottom. The customer is worried about draughts and asks whether the hole is really needed.
1. Assess the ventilation requirement and whether the proposed single high-level hole is adequate.
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Ventilation for an oil appliance serves two purposes: combustion air for the burner and cooling/clearance air for the compartment. The requirement depends on whether the appliance draws combustion air from the compartment or is room-sealed, and on the manufacturer's instructions, which must be followed.
For a conventional (open-flue) appliance in a compartment:
- High- and low-level ventilation is normally required to the outside (or to a well-ventilated space), sized to the appliance rating and the compartment. A single high-level hole does not provide the through-flow needed for cooling and combustion air; low-level air is also needed so that hot air can escape at high level and cool air enter at low level.
- Free area: the hole must give the required free area (not just core diameter); a 100 mm core hole gives roughly 0.0079 m², which is likely to be far short of the requirement for a 30 kW appliance, which is typically expressed in cm² per kW and can be several hundred cm² in total.
- Door gap: a gap under the door is not a substitute for correctly sized permanent vents, and it compromises fire separation and draughts.
- Extract fan: a mechanical extract fan in the same dwelling can depressurise the compartment and cause spillage of products of combustion; the interaction must be assessed, and the appliance must not be starved of air when the fan runs.
- Other equipment: a consumer unit in the cupboard is an electrical enclosure; ventilation and clearance must not compromise it, and the boiler must not overheat the space.
On the information given the single 100 mm high-level hole is inadequate and the arrangement needs redesign.
2. State what you would specify and how you would verify it at commissioning.
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Specification:
- Determine from the boiler manufacturer's instructions whether the appliance is open-flue or room-sealed and the exact compartment ventilation free areas required at high and low level.
- Provide both high-level and low-level permanent vents to outside (or to a permanently ventilated space) with the correct free area after allowing for grilles/louvres (use the grille's free-area figure, not the hole size).
- Ensure the vents cannot be blocked and are positioned to give through-flow across the appliance.
- Address the extract fan: confirm the compartment and appliance are not adversely affected when the fan operates; if necessary, interlock or provide additional air.
- Maintain fire separation and do not rely on door gaps.
Verification at commissioning:
- Measure and record the actual free area of each vent and compare with the manufacturer's requirement.
- With the appliance running and all doors closed, carry out a spillage/combustion test (e.g. smoke match or flue gas analysis) at the draught diverter/flue to confirm products are clearing, and repeat with the extract fan running and any internal doors in the positions likely in use.
- Check the flame is stable and the burner settings are correct; record flue gas readings.
- Confirm the consumer unit and stopcock remain accessible and the compartment temperature is acceptable.