A concealed duct run and 6 mm² bonding on a boiler replacement
You are pricing the connection of a new 24 kW system boiler in the attic of a two-storey semi-detached house, replacing a boiler in the kitchen. Your proposed route runs 22 mm copper from the external meter box on the front elevation, through the front wall into the hall, then up through a boxed-in service duct to the attic. The route needs two compression elbows inside the duct, and the duct will be closed with plasterboard on completion with a small access hatch at the bottom only. The existing gas pipe is bonded with a 6 mm² conductor and a proprietary clamp at a point 3.2 m from the meter outlet, downstream of the tee that feeds the hob. The bonding clamp is painted over and the connection to the main earthing terminal cannot be traced. The householder wants the work done in one day and asks you to "just box it in and I'll get the electrician later".
1. Identify the non-compliances in the proposed route and bonding arrangement, explaining why each matters.
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Pipework route
- Compression elbows inside a duct that will be sealed are joints in an inaccessible location. Gas pipe joints must be accessible for inspection and testing; a joint behind plasterboard cannot be examined, leak-detected or repaired, and a leak there can migrate through the building fabric. Either reroute with bends, or provide full access at every joint.
- The duct must be ventilated at high and low level so that any leakage can escape and be detected rather than accumulating in an enclosed void. A single small hatch at the bottom does not achieve this.
- Where the pipe passes through the front wall it must be sleeved and sealed so the pipe is not in direct contact with mortar (corrosion) and gas cannot track into the cavity; the sleeve should be sealed to the wall.
- Sizing and pressure - 22 mm may be adequate for 24 kW, but that is a calculation, not an assumption, using the actual equivalent length, and the boiler in the attic is at the far end of the run.
Bonding
- 6 mm² is not acceptable where 10 mm² is required; the conductor must be sized to the installation's earthing requirements.
- Position is wrong. Main protective bonding must be connected on the consumer's side of the meter, within 600 mm of the meter outlet, and before any branch tee. A clamp 3.2 m away and downstream of the hob tee does not bond the installation as required.
- The clamp is painted over and the connection to the main earthing terminal cannot be traced - the bonding is unverifiable and must be remade and proven continuous.
2. The householder insists on "boxing it in" today. How do you sequence the work so concealment happens only after the checks that cannot be done later?
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The sequence is driven by one rule: anything that will be hidden must be proven before it is hidden.
- Agree the route and the access - settle positions so joints are accessible or eliminated, and agree the duct ventilation and sleeve details before a single board goes on.
- Install the pipework with bends in preference to elbows, correct clipping and protection at the wall penetration.
- Pressure/tightness test the run before concealment, with the test method and permissible drop appropriate to the volume and test pressure, and with all joints exposed. Note that this is a test of the new section; the final installation test from the meter must still be completed once everything is connected.
- Photograph and record the route, joint positions, sleeve and duct vents while they are visible, so the installation is traceable later.
- Bonding sorted by a competent person - or your own work if you are competent and authorised to do it - 10 mm², within 600 mm of the meter outlet on the consumer's side, before any tee, with a sound connection to the main earthing terminal and a durable "Safety Electrical Connection - Do Not Remove" label.
- Only then close the duct with ventilation high and low and the access hatch positioned so joints can be reached.
- Commission and document - purge, light, gas rate and working pressure, combustion readings, and a final tightness test from the meter.
If any of steps 2–5 cannot be completed today, the boarding waits. Boxing in is reversible; an inaccessible leaking joint is not.
3. What would you do about the untraceable bonding before leaving, given the electrical work is outside your scope?
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Untraceable main protective bonding on the gas installation is a genuine safety issue and must not be left as "someone else's problem later".
- Treat it as unresolved, not acceptable. Bonding that cannot be verified as continuous to the main earthing terminal provides no assured protection, and a painted-over clamp on a possibly corroded pipe cannot be assumed sound.
- Record and report it. Note on the job documentation exactly what exists (6 mm², painted clamp, 3.2 m from the meter, downstream of the hob tee, no traceable route to the main earthing terminal) and that it does not meet the requirement.
- Communicate in writing to the householder - and to the landlord or letting agent if applicable - stating that the bonding must be brought up to standard by a competent person, with the specifics of what is required (10 mm² minimum, consumer's side of the meter, within 600 mm of the meter outlet, before any branch, connected to the main earthing terminal and labelled).
- Do not simply carry on and conceal. Complete the gas work you are competent to do, but make the handover conditional on the bonding being rectified and verified, and record that the customer was told.
- Coordinate rather than duplicate - arrange for the electrician to attend before the duct is closed if practical, so the bonding can be inspected while access exists.
- Keep it proportionate but firm: this is not a reason to walk away from a compliant gas installation, but it is a reason to refuse to sign the job off as complete without the written caveat.