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Electrics and gas

What should you ask if a boiler flue runs through a ceiling void the surveyor could not see?

Ask a Gas Safe engineer to consider the concealed flue, and do not treat “boiler present” as a test of that run. A flue hidden in a void can be the part that leaks products of combustion. The survey sees the boiler and maybe a terminal on the wall. It does not open ceilings, and HSE’s gas guidance does not make the surveyor the engineer.

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?

Modern boilers often have fan flues that travel further than an old balanced flue straight through a wall. Where that path is boxed in, a visual survey stops. The engineer’s inspection is the one that can include the flue route. A carbon monoxide alarm on the landing is not that inspection.

If the report says the flue route was not visible, keep that limit in any question to the seller. Ask for installation papers. Papers are not a substitute for an engineer if you need the appliance declared safe to use. There is no RICS fee for opening a ceiling. Agree any opening with the seller. Do not cut plaster on a viewing.

What should you ask your surveyor?

  • Where does the flue appear to run, and which part was hidden?
  • Did you see the terminal outside?
  • Is your comment a limit, or a defect you could see?

What are the typical next steps?

  1. Do not take the ceiling down yourself.
  2. Give the engineer the survey sentence so they know the void was not seen.
  3. Treat an alarm as an alarm. It has not inspected the flue.

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 boxed-in flue always wrong?

Not always. The point is that it was not inspected. An engineer can say whether the installation is acceptable. The survey cannot.

Does a short flue through the wall avoid this?

A terminal you can see is easier to look at. The survey still has not tested combustion.

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