Replacement Combi With a Flue Terminal Close to an Openable Window
You are replacing an 18 kW wall-mounted combi boiler in a first-floor flat. The new boiler is in the same cupboard, but the flue must pass through the existing 100 mm hole in the external wall. You measure the new terminal 250 mm horizontally from an openable kitchen window, 600 mm from the boundary, and 1.8 m above the ground. The manufacturer's instructions state a minimum of 300 mm from any openable window and 300 mm from an internal or external corner; the flue is 400 mm from an internal corner. The condensate is connected to a rainwater downpipe. The gas supply is 15 mm copper, 12 m long, serving only the boiler. The customer wants the boiler working before the weekend and says the old flue was 'in the same place for 20 years'.
1. Assess the flue terminal position and state the options.
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The terminal is non-compliant: 250 mm is less than the manufacturer's 300 mm minimum from an openable window. The 400 mm corner distance is acceptable against the 300 mm minimum. An existing flue position is not a defence-the manufacturer's instructions apply to the new appliance. Options are to relocate the terminal, route the flue to another wall, or use a manufacturer-approved plume-management kit if it achieves the required clearance. Do not commission until the terminal complies. Record the measurement and the manufacturer's instruction.
2. Assess the condensate discharge and the gas supply. What checks and changes are needed?
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Condensate to a rainwater downpipe is normally unacceptable because rainwater systems are not designed for acidic condensate and may discharge to surface water. Use an internal soil stack via a suitable trap, or a condensate pump to an internal discharge point, with neutralisation if required. The 15 mm gas pipe over 12 m for 18 kW is likely marginal: calculate equivalent length and pressure drop, and upsize to 22 mm if the permissible drop is exceeded. Also confirm the boiler minimum inlet pressure under full load.
3. How should you handle the customer's '20 years' argument and document the outcome?
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Explain in writing that the flue and condensate do not comply, so the boiler cannot be commissioned. Give the customer the options and a revised date. Record the old flue position and the new measurements; do not rely on the '20 years' argument. If the customer refuses remedial work, leave the boiler isolated and issue a warning notice. Photograph the terminal, condensate connection and pipe route, and keep copies of the manufacturer's clearance instructions with the job record.