CO inspection: why there is no fixed retesting interval laid down

Installers often look for a fixed interval for CO inspections — "once every so many years" — the way that is explicitly set out in a BRL for leak checks on refrigeration installations. For the CO inspection of gas-fired combustion appliances, that fixed retesting interval does not exist in the certification schemes themselves. This article explains what regulation does lay down for the CO inspection, why a fixed cadence is deliberately left out of it, and what that means in practice for your work order and inspection file.

Which BRL regulates the CO inspection — and which doesn't

A common confusion: the CO inspection is sometimes loosely linked to "BRL 100", the assessment guideline for refrigeration and climate-installation companies (F-gases, refrigerants, leak checking — see BRL 100 in 2026: what changes?). That is not the correct source. Work on building-related gas-fired combustion appliances (central heating boilers, gas water heaters, gas heaters) has fallen under a mandatory process certificate since 1 April 2023 — at the time via article 1.35 of the then Bouwbesluit 2012, and since the introduction of the Omgevingswet on 1 January 2024 continued via the Besluit bouwwerken leefomgeving, article 6.45(1) (source: Informatiepunt Leefomgeving — gas combustion installations (opent in nieuw tabblad)). The certification schemes designated for that purpose: BRL 6000-25 from InstallQ and K25000 from Kiwa for central heating boilers and gas water heaters (see also CO-person certificate), and the Conformiteitsbeoordelingsschema NHK CBS 2021 specifically for gas fires and gas heaters. BRL 100 plays no role here — it is a different guideline, for a different field.

Why is there no fixed cadence in the BRL?

We checked the official text of BRL 6000-25 (version 10-07-2024, declared binding by InstallQ, source: installq.nl/files/brl/brl6000-25...pdf (opent in nieuw tabblad)). What it does say:

• Chapter 5 describes requirements for the work itself — including the CO measurement in the installation space (§5.2.6) — as part of installing, repairing, maintaining or replacing an appliance.
• §9.2 Period of validity determines that the company certificate is valid for a maximum of three years, after which the certifying body renews it in good time on the basis of regular follow-up audits (audits of the company, not of individual installations).

What it does not say: an obligation to have an already inspected installation reinspected after X months or years. The BRL regulates who may carry out the work and how that work must be carried out at the moment it takes place (new construction, maintenance, repair, replacement) — not a recurring inspection rhythm separate from that work. That is a deliberate choice in the design of the scheme, not an omission that we should fill in here with an educated guess.

So what determines how often inspections take place?

In practice, the CO check runs alongside the maintenance moment of the appliance, and that moment is not determined by the Gasketelwet certification schemes but by:

• The manufacturer's instructions for the appliance — these range from annually to once every two or three years, and are often a condition for the warranty (source: Kemkens — how often does a central heating boiler need maintenance? (opent in nieuw tabblad)).
• The maintenance contract that the installer concludes with the owner — see also our checklist and work order requirements for what needs to go into it.

So there is no nationally uniform "once every X years" figure to program into software. Any tool that does present that as a hard given is making up a cadence that isn't based on anything.

What does this mean for your records?

Because there is no fixed retesting interval, you cannot automatically calculate a "next CO inspection expected on" date without a source that can substantiate it — the manufacturer's instructions or the maintenance contract, per installation. Koldwerk therefore deliberately does not calculate an automatic expiry date for CO inspections; our CO inspections module keeps the inspection history chronologically and source-traceably per installation (who, when, which values, which result), so that you — with the maintenance contract or the manufacturer's instructions at hand — decide for yourself when the next inspection needs to take place. Two things you can record yourself:

• The frequency from the maintenance contract as a fixed field per installation, so that you can still set an internal reminder — based on what you have agreed, not on a made-up legal standard.
• The BRL 6000-25 / K25000 company certificate of every technician who carries out CO inspections, with the actual validity date (max. 3 years) from that certificate — see CO-person certificate.

Further reading

• CO inspection: checklist and what needs to be on your work order
• CO-person certificate: what you need to know
• BRL 100 in 2026: what changes for installation companies?

Disclaimer: this article is based on the official BRL 6000-25 text (InstallQ, version 10-07-2024), the Besluit bouwwerken leefomgeving and public industry information, consulted on the dates mentioned in this article. Certification schemes and regulation can change. Ask about the current retesting and maintenance requirements for a specific installation at InstallQ (opent in nieuw tabblad) or Kiwa (opent in nieuw tabblad), or consult your manufacturer's instructions. Not legal advice.