Converting a Domestic Shed to a Commercial Spray Booth
A automotive refinisher wants to use a detached 7 m × 5 m × 3 m shed as a spray booth. They propose installing a second-hand gas-fired space heater rated 60 MJ/h, flued horizontally through the wall, with a 450 mm ceiling fan for air movement and a 900 mm × 900 mm roller door plus one 300 mm low-level vent. The heater's data plate states it requires 3.5 m³/min of combustion air and a 150 mm flue with 1 m minimum vertical rise before any offset. The shed sits 1.2 m from a neighbouring boundary and the flue would discharge 2.2 m above the roof line, 1.5 m from an openable window of the neighbouring property. The owner has a current Type B licence and asks whether they can do the work themselves under their own licence.
1. Assess the ventilation and flue termination proposal against the heater's requirements and general clearance principles, and identify the deficiencies.
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Work through the heater's stated requirements first, then the flue termination, then the room.
Combustion air: the heater needs 3.5 m³/min. The proposed openings are a 900 × 900 roller door (often closed in use) and a single 300 mm low-level vent. A 300 mm opening gives roughly 0.07 m² of free area; even allowing for the door when open, the permanent ventilation is far short of what 3.5 m³/min demands once you account for the spray booth's exhaust and the door being shut during operation. This is a deficiency.
Flue: the data plate requires 150 mm diameter and at least 1 m vertical rise before any offset. A horizontal through-wall termination with no stated vertical rise does not meet that, and horizontal runs collect condensate and reduce draught. Deficiency.
Termination clearances: discharging 2.2 m above the roof but only 1.5 m from a neighbour's openable window is a clear problem. Flue products must not enter openable windows, and the proximity to the boundary plus the window means the termination is unacceptable regardless of height above the roof. Products of combustion in a spray booth environment also carry solvent vapour risk.
Conclusion: the proposal as described cannot be approved. The heater needs a compliant vertical flue, adequate permanent combustion and dilution air, and a termination relocated away from the neighbour's window and boundary.
2. Can the owner perform this work under their own Type B licence, and what regulatory steps and records are required?
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Licensing is separate from technical adequacy. Holding a Type B licence authorises the class of work it covers, but it does not authorise work outside its scope, work on another person's behalf without the required contractual and insurance arrangements, or work that breaches the installation standards.
Steps:
- Confirm the licence class actually covers the appliance type and the commercial application. A Type B licence is not a blanket authorisation for every gas appliance; check the endorsement and any restrictions.
- The work is a new commercial installation in a changed-use building. That triggers planning or building approval for the change of use and for the flue penetration and boundary clearance. The gasfitter cannot substitute for that approval.
- A compliance certificate must be issued for the gasfitting work, and the installation must be certified before the booth is used.
- The spray booth itself raises fire and hazardous-area requirements that sit outside gasfitting; the owner should confirm with the relevant authority whether the heater location is permissible at all in a solvent atmosphere.
Records: retain the compliance certificate, appliance data plate details, ventilation calculations, flue termination details and test results. If the owner proceeds without the building approval or outside licence scope, the certificate would be invalid and the installation non-compliant.