Electrics and gas
Does a white plume at the boiler terminal prove a concealed flue is jointed properly?
No. A white plume shows a condensing boiler is running and sending vapour out of the terminal you can see. It does not show whether joints inside a ceiling or a boxed void are supported, sealed or leaking combustion products. A home survey that could not open that void has not cleared those joints. Only a Gas Safe registered engineer should inspect or alter a gas flue. HSE’s domestic gas guidance is that gas work belongs with those engineers, not with a visual survey.
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?
Fan flues often travel further than a short balanced flue straight through an outside wall. The terminal can look tidy while the run above a ceiling stays inaccessible. The honest survey sentence is that the route was not inspected. A warm boiler, a neat cupboard or a plume should not be upgraded into a pass. A carbon monoxide alarm on the landing is a warning device. It is not an inspection of the joints, and seeing one fitted does not remove the limit.
Opening the ceiling is outside the survey. If an engineer needs access before exchange, agree it with the seller in writing and do not cut plaster on a viewing. Installation papers may sketch the route. They are not a substitute for looking at a void that is now sealed. There is no RICS price for that opening or for the engineer’s visit. If the engineer later says a joint is unsafe, negotiate from that finding, not from the colour of the plume.
What should you ask your surveyor?
- Did you see a plume, a terminal, or the flue run itself?
- Which lengths were boxed in or above a ceiling?
- Are you leaving the joints as not inspected?
What are the typical next steps?
- Do not treat the plume as a commissioning test.
- Instruct a Gas Safe engineer and tell them the internal run was not seen.
- Agree any opening of the ceiling with the seller before exchange.
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 the plume the same thing as a condensate leak?
No. The plume is vapour at the terminal. A condensate leak is water from the small drain pipe. Both can be present. Neither proves the flue joints.
Who may open the ceiling to look at the flue?
Not a buyer on a viewing. Before exchange the seller has to agree. The person who inspects the gas flue should be Gas Safe registered.