Appointing the wrong ventilation subcontractor is one of the more costly mistakes on a commercial or residential project. Ductwork and MVHR installation failures — whether from poor workmanship, incorrect materials or absent commissioning documentation — don't typically show up until the system is running or until Building Control inspects at completion. By then, the ceilings are closed and remediation is expensive.
These eight questions, asked before you send out a tender or accept a quotation, will reveal a great deal about whether a ventilation subcontractor can genuinely deliver what your project requires.
Can you provide DW/143 air leakage test certificates from recent projects?
Any subcontractor that genuinely works to DW/144 will have test certificates available. Ask for examples from projects in the last 12 months, ideally of similar scale and pressure class to yours. A contractor who can't produce certificates, or who needs to explain why testing isn't necessary, is a risk. For medium and high-pressure ductwork on commercial or healthcare projects, DW/143 testing isn't optional — it's in almost every specification.
What insurances do you carry, and can I see the certificates?
You need to see actual insurance certificates — not just a confirmation email. Check that public liability is at least £2M (£5M or more for commercial and healthcare work), that employers' liability is in place (legally mandatory), and that the renewal dates haven't passed. On projects with design responsibility, ask about professional indemnity. An uninsured subcontractor that causes damage or injury leaves the main contractor or M&E contractor exposed.
Are all your operatives CSCS-carded for the correct trade?
Commercial sites in Scotland require CSCS cards (Construction Skills Certification Scheme) as a minimum. For specialist trades, the relevant card is the Mechanical Engineering Services card. Ask whether the operatives who will actually be on site hold current cards — and be aware that a company with appropriate cards on paper can send uncarded operatives if site supervision is lax. For sensitive sites (healthcare, occupied buildings), ask specifically about supervision arrangements.
What is your proposed installation programme, and what are your sequencing requirements?
Ventilation installation has a direct dependency on other trades — structure must be in place, ceiling grids may need to be coordinated, and access for ductwork often conflicts with other services. Ask the subcontractor to give you their proposed sequence and identify where they need other trades to be ahead of them. A subcontractor who can articulate their programme requirements clearly has done this before. One who can't — or who says they'll "sort it out on site" — is likely to create programme conflicts.
What commissioning documentation will you provide at handover?
This question separates contractors who understand building compliance from those who don't. For MVHR: you need a commissioning certificate with individual airflow measurements at every terminal. For commercial ventilation: TAB (Testing, Adjusting and Balancing) reports. For ductwork: DW/143 test certificates. Ask to see an example of the handover documentation from a previous project. If the subcontractor is unclear what documentation the project requires, that's a red flag for compliance risk.
Have you worked on similar projects — same building type, same system type?
Healthcare ventilation is not the same as commercial office ventilation. MVHR in high-airtightness Passivhaus housing is not the same as MVHR in a typical new build flat. The standards, the sequencing, the documentation requirements and the construction process are different. Ask for references from comparable projects and follow them up. A subcontractor who has never installed MVHR in a housing development will have a learning curve on your project — make sure you're pricing that risk appropriately.
Who will be the named supervisor on site, and what is their experience?
The quality of the ductwork installation depends on the person leading the installation team on site — not the person who answered the tender enquiry. Ask specifically who the site supervisor will be and what projects they have supervised. On anything other than a very small project, you want a named individual with relevant experience, not "one of our team." For healthcare projects, the supervisor should have healthcare-sector installation experience.
What is included in your price, and what is explicitly excluded?
Ductwork tenders are notorious for unclear inclusions and exclusions. Fire dampers — who supplies and installs? DW/143 testing — included or extra? MVHR commissioning — part of the package or a separate appointment? Insulation — full specification or first fix only? Grilles and diffusers — included? Ask the subcontractor to confirm their inclusions in writing before you accept a price. A price that looks low because half the scope is excluded is not a competitive price — it's a contractual problem waiting to happen.
A Note on Price vs Value
The cheapest ventilation quote is almost never the best value. Ductwork installation is a trade where the materials are a relatively small percentage of the total cost — labour and programme are the dominant variables. A subcontractor pricing significantly below market is either under-resourced, intending to cut corners, or pricing for disputes later.
The consequential costs of a failed DW/143 test (ceiling removal, remedial sealing, retesting), a missed commissioning certificate (delayed Completion Certificate, delayed handover) or a poorly balanced MVHR system (resident complaints, warranty calls, potential Building Standards non-compliance) are typically far higher than any saving on the initial tender price.
Practical suggestion: Ask all tenderers to confirm in their submission that they have read the full specification and that their price includes DW/143 testing (if specified), commissioning documentation, fire damper installation and all handover documentation. This makes post-contract disputes about inclusions much harder to sustain.
A Ventilation Subcontractor That Answers All Eight
Cliventa Engineering works with M&E contractors and main contractors across Scotland on ductwork, MVHR and ventilation installation. We provide DW/143 test certificates, commissioning reports and full handover documentation as standard.
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