Guidance, not a calculator
Gas pipes, boilers and flues
Sizing gas pipes, a gas boiler or its flue is gas work. By law it's for a registered gas engineer.
What the work involves
Gas work covers the pipework from the meter, the appliances, and the flue and air supply that keep their fumes out of the house. Sizing any of it depends on every appliance on the supply, the route of the pipe and the ventilation of the rooms.
Why it's regulated
Only registered engineers may work on gas: Gas Safe registered in the UK, and qualified, certified installers in France and Spain. Mistakes cause carbon monoxide poisoning, fires and explosions, and work by anyone else can void your insurance.
Who to ask
A Gas Safe registered engineer in the UK, or a qualified gas installer in France or Spain. Ask to see their card or certification, and fit a carbon monoxide alarm near every gas appliance.
No figures here, on purpose. We don't give sizes, rules of thumb or typical values for this work. Only a qualified professional who has seen your building can.