Chimneys and parapets
What is an open flue, and why is that not a gas-safety certificate?
An open flue takes combustion gases up a chimney from an appliance that also draws air from the room. Naming that arrangement is not a test of the flue, the ventilation or the appliance. Gas safety checks are for an engineer on the Gas Safe Register.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
A home surveyor may see a gas fire, a back boiler or an open fireplace and say the flue is open rather than a balanced flue through the wall. They do not measure carbon monoxide, do not smoke-test the flue as a gas engineer would, and do not certify it. The Health and Safety Executive’s domestic gas guidance is clear that gas work belongs with a registered engineer.
If the fire is in use, ask for service records and consider your own Gas Safe inspection before you rely on it. The fee is the engineer’s quote. If the fire is dead, the chimney questions about rain, pots and ventilation still apply. Do not let “flue noted” stand in for either job.
What should you ask your surveyor?
- Did you identify an open flue, and did you test it? The second answer should be no, unless a specialist test was agreed.
- Which appliance uses it, and was that appliance lit?
- What ventilation into the room did you see?
What are the typical next steps?
- Book a Gas Safe engineer for any appliance you will use. Do not ask the surveyor to sign it off afterwards.
- Keep the chimney weatherproofing as a separate list: pot, flaunching, flashing.
- Fit no stove and light no fire until the flue has been checked by the person who is allowed to do that work.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Is a carbon-monoxide alarm a substitute for a test?
No. An alarm is a warning device. It does not prove the flue works, and it is not the inspection the survey skipped.
What is a balanced flue in one sentence?
It takes air from outside and sends fumes outside, usually through a wall terminal, rather than using the room and a chimney. The survey still does not test it.