Survey finding
Unvented cylinder on your survey: what's flagged
Unvented cylinders need annual servicing. This page covers the regulatory framework and costs.
Last updated: 6 May 2026. Editorially reviewed: 20 May 2026.
Finding
Unvented hot water cylinder issues
What this usually means
Unvented hot water cylinders are pressurised and require annual servicing under G3 of the Building Regulations. Surveyors flag missing service records, expansion vessel pressure issues, or visible corrosion.
Why it matters
Failure to maintain can void warranties. Severe failure has caused explosions, hence G3 regulation.
Ask your surveyor
- Check:Is the cylinder G3 certified?
- Check:When was it last serviced?
Ask the seller
- Check:Do you have G3 service records?
- Check:Is there a pressure relief valve discharge log?
Next steps
- •Get two written quotes from local trades before negotiating with the seller.
- •Speak to your mortgage broker before exchanging if the finding affects mortgageability.
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What you need to know
Severity
Maintenance item. Worth quoting and including in negotiation.
Typical cost to fix
G3 service £100-£250/year. Expansion vessel replacement £150-£400. Full unvented cylinder replacement £1,500-£3,500.
Mortgage impact
Standard unless cylinder is condemned.
Insurance impact
Standard if maintained.
When to pull out
Not relevant.
When to renegotiate, and by how much
Service plus likely component replacement; £200-£800.
Thinking of pulling out or renegotiating? What to do after a bad survey
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Run a free previewHow MyPropertyScan produces this guide, and what it is not
Published by MyPropertyScan and written by Ziad Nasr, who is accountable for its accuracy. No page here is written or reviewed by a chartered surveyor, conveyancer or regulated adviser, and we do not claim otherwise.
This page is not:
- Chartered surveying — we do not inspect properties or produce RICS Home Survey reports
- Legal advice — we do not act as conveyancers or advise on title, tenure or contracts
- Mortgage or insurance advice — we are not regulated by the FCA and give no recommendations
Figures and process steps come from the published UK datasets and standards listed on this page, not from professional practice. Pages are updated when source coverage, survey cost assumptions, or lender and legal requirements materially change. Dataset limitations are set out on the methodology page.
Spotted something out of date or wrong? Email hello@mypropertyscan.com and we will correct it and change the update date.
Sources used
We use UK public and specialist sources where they are available. Public datasets can be incomplete, delayed, or missing for some addresses. Treat them as a starting point, not as a replacement for professional advice.
Source standard: preference goes to official government datasets, statutory bodies, professional standards, and primary dataset publishers. We cite the source family on the page and explain coverage limits rather than filling gaps with unsupported estimates.
- Professional standard: RICS Home Survey StandardRICS standard for condition-based residential surveys in the UK.
- Learn more from: RICS consumer guide to surveysConsumer-facing guide to survey types and when each level is appropriate.
- Official guidance: GOV.UK buying or selling your homeGovernment overview of the home buying and selling process.