Electrics and gas
What is a balanced flue, and why does a home survey not test it?
A balanced flue draws air from outside and sends combustion gases outside, usually through a terminal on the wall. It does not use a traditional chimney. A home survey can see the terminal and the boiler. It does not test the combustion or the seal.
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?
The surveyor may note a terminal that is too close to a window or a terminal that is stained. That is a visual flag. Distances and clearances are for the person who installs and inspects under the gas rules and the building regulations. Do not measure a terminal with a guess and call it illegal. Ask a Gas Safe engineer.
Balanced-flue boilers can still leak water, fail and be badly installed. The water leak is visible sometimes. The gas fault often is not. Book the engineer if the survey has flagged the terminal, the boiler is old, or you simply want it checked. The fee is the engineer’s. A new terminal position, if required, is a further quote.
What should you ask your surveyor?
- Where is the terminal, and did anything about its position concern you?
- Did you see staining, a guard, or a plume mark?
- Are you excluding a combustion test?
What are the typical next steps?
- If the terminal looks wrong, ask a Gas Safe engineer, not a bricklayer, to say so.
- Do not box in a terminal to tidy the outside wall.
- Read the boiler service history alongside the terminal note.
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 combi boiler always balanced-flue?
Many modern combis are. Do not assume. The engineer identifies the appliance. The surveyor may only have seen a white case and a terminal.
Does a plume of steam mean a fault?
Pluming can be normal in cold air. A surveyor who calls it a defect should say what else they saw. If they only saw steam, ask them before you treat it as a failure.