Electrics and gas
The air vent on a buffer vessel is dripping. Is that drip from the domestic water or from the heating circuit?
The survey has not shown which. The air vent on the buffer vessel is dripping, and that drip was seen. The visit did not trace the pipes or sample the water, so it has not proved domestic water or heating water. A buffer normally sits on the heating side, but this inspection did not confirm the circuit by test and did not rule out a cross-connection. The drip is established. Which circuit it comes from is not.
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?
An air vent should let air out and then stay shut. If it drips, it is passing water from the circuit the vessel is on. Buffers store heating water. Domestic hot water is usually a separate cylinder. A label and one weeping vent do not prove the two stayed apart. No temperature was taken and no sample was taken. The survey will not guess which water is escaping. The drip is real. The circuit is untraced.
Ask the solicitor for a commissioning diagram that separates the buffer from the domestic cylinder. An engineer can trace the vent. The survey has not done that. It does not decide Boiler Upgrade Scheme eligibility and quotes no grant. The scheme is a route in England and Wales, not in Scotland or Northern Ireland. Approved Document L is guidance for building work. This visit did not issue a building-regulations certificate. Do not say the domestic water was tested.
What should you ask your surveyor?
- Is the buffer labelled apart from any domestic cylinder?
- Is the drip only at the air vent?
- What trace would name the circuit?
What are the typical next steps?
- Report the drip without naming a proved circuit.
- Ask for the commissioning diagram.
- Have an engineer trace the vent to the heating side or the domestic water.
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
Was the drip tested by temperature?
No. No temperature was taken and no sample was taken. The survey will not invent a figure for the drip. The air vent was seen dripping. Whether that is domestic water or heating water is unresolved. An engineer can trace the pipe. This visit did not separate the circuits by any test.
Does the drip settle a Boiler Upgrade Scheme application?
No. The survey does not decide eligibility and does not quote a grant figure. The Boiler Upgrade Scheme is a grant route in England and Wales toward a heat pump or biomass boiler, not in Scotland or Northern Ireland. A dripping vent is an untraced leak at the buffer. It is not a grant decision. The circuit is still unproved.