Flues in voids: inspection hatch rules
Yes, where the flue is concealed and cannot be inspected. HSE's bulletin OPSTD 10-2010 set 31 December 2012 for fitting hatches, and Gas Safe Register's TB 008 was revised in April 2013 to allow a carbon monoxide void-monitoring shut-off system instead in some cases. Makers print their own hatch rules.
Reviewed by Jordan Valentine-Dunn, Gas Safe registered engineer · Portsmouth Gas Heating · Last reviewed October 2026
A concealed boiler flue must be inspectable, because the engineer has to see that it is continuous, sealed at every joint and supported along its length. HSE's bulletin on flues in voids set the deadline for hatches, and each maker's installation manual then sets the hatch size and spacing for its boilers.
What does HSE say about flues in voids?
HSE's safety bulletin OPSTD 10-2010 was issued on 2 December 2010 and updated on 20 May 2013. It followed a death in 2008 and covers existing gas central heating where the flue runs through a ceiling or wall void, mostly in flats built since about 2000. It rests on one test: the original installer and every later engineer must be able to check three things.
| Point | What the bulletin says |
|---|---|
| Continuity | The flue is continuous throughout its length |
| Joints | All joints are correctly assembled and appropriately sealed |
| Support | The flue is adequately supported throughout its length |
| If the engineer cannot make those checks | They cannot ensure the flue is safe, which is why appropriate inspection hatches are needed in the ceiling and, where relevant, the stud wall |
| Deadline for hatches | 31 December 2012 for the homeowner, landlord or other responsible person to have them installed |
| From 1 January 2013 | An engineer working on an affected system advises that it is At Risk (AR) and, with the owner's permission, turns off the gas supply to the boiler |
| Before the deadline | A risk assessment: look for leakage along the flue route, a satisfactory flue combustion analysis, and suitable audible CO alarms fitted |
Can a carbon monoxide monitor replace the hatch?
In some circumstances, yes. HSE's note on the bulletin says Gas Safe Register's Technical Bulletin 008 was revised in April 2013 to introduce the option of a CO void-monitoring safety shut-off system as an alternative to inspection hatches. HSE does not set out those circumstances. Gas Safe Register's page for TB 008 answered a bot check when this guide was written, so read the current bulletin for the conditions.
How big must the inspection hatch be?
The two manuals checked for this guide agree on the spacing and differ in how they print the size. Worcester's manual says no less than 300mm square. Ideal's manual prints at least 300 mm², which as written is an area, not a side length. This guide does not read the two as the same size: ask the maker, and work to the stricter reading until it answers.
| Point | Worcester Bosch Greenstar 4000 combi | Ideal Logic Combi2 C24, C30, C35 |
|---|---|---|
| Number and size | At least one hatch no less than 300mm square | Inspection hatches at least 300 mm² |
| Distance from flue joints | Flue joints within the void not more than 1.5 metres from the edge of the hatch | Hatches no more than 1.5 metres from joints |
| Position | Hatches should be located at changes of direction | Where possible, located at changes of direction |
| If that is not possible | Bends should be viewable from both directions | Bends must be viewable from both sides |
| Wider rule | Provision must be made for service and inspection of a concealed flue | Concealed flues must have hatches; commissioning notes cite Gas Safe Register TB 008 where flues are in voids |
Who is responsible for fitting a hatch?
HSE puts the cost and the arrangement on the owner. A homeowner has to arrange for hatches to be installed, and a landlord is responsible for hatches being installed and the boiler and flue checked every year. HSE advises against householders trying to fit hatches themselves, as they may damage fire or noise proofing in the ceiling. A builder or building services company fits the hatch; the engineer inspects through it.
- Worked example 1: a flat built in 2004 with a boiler on an internal wall and a flue running 3 metres through the ceiling void, with one 90 degree bend. Under Worcester's rule the joints must be within 1.5 metres of a hatch edge and the bend should be at a hatch or visible from both directions, so the hatch position follows the bend and the joints either side of it. Without a hatch, the engineer cannot inspect the flue and applies the HSE route: At Risk and, with permission, isolation.
- Worked example 2: a flue in a stud wall void behind a cupboard, with a CO void-monitoring shut-off system fitted. HSE's note says the 2013 revision of TB 008 accepts that as an alternative to hatches in some circumstances. Whether this installation qualifies depends on the current TB 008 and the maker's instructions, which the engineer must check.
Only a Gas Safe registered engineer can install, service or certify a boiler and its flue, and whether a hatch or a CO monitoring system is acceptable is their call under the current TB 008 and the maker's instructions. This page reports what HSE and two makers print.
If you suspect carbon monoxide, turn the appliance off and call National Gas on 0800 111 999, free, at any hour. On a Manifold gas safety check each appliance is recorded with its location and flue type, and the record lists every appliance with its flue result.
Frequently asked
Sources
- HSE, Gas boilers - flues in voids, safety bulletin OPSTD 10-2010 (updated 20 May 2013)
- Worcester Bosch, Greenstar 4000 combi installation manual (6720891161, 2021/10), section 4.4
- Ideal Heating, Logic Combi2 C24, C30, C35 installation and servicing manual (UIN 237664 A08), section 2.8
- National Gas, emergency contacts
Related guides
Last reviewed October 2026. This guide is general information, not legal or safety advice, gas safety work must be carried out by an appropriately Gas Safe registered engineer. Rules can change, so check the linked official sources for the current position.