Part G3 of the Building Regulations covers hot water supply and systems, requiring unvented installations to have appropriate safety devices, to be installed by a competent person, and to discharge safely. The work is notifiable and you should receive a certificate.
What G3 covers
Part G of the Building Regulations deals with sanitation, hot water safety and water efficiency. G3 specifically addresses hot water supply and systems, and its requirements include:
- Adequate precautions to prevent stored hot water exceeding a safe temperature under any circumstances, including failure of the normal controls.
- Safe discharge of any hot water released by safety devices, so it cannot cause danger to people in or about the building.
- Installation by a competent person, with the work notified to building control.
- Limits on the temperature of hot water delivered to a bath, addressing scalding risk.
The requirements apply to unvented systems because a vented system’s open vent pipe inherently satisfies the first of them – it has a permanent, unblockable route to atmosphere.
Why it is notifiable, and what that means for you
Installing an unvented hot water storage system is notifiable work under the Building Regulations. It can be self-certified through a competent person scheme, which is the normal route, or notified to the local authority building control directly.
What this means practically:
- The installer must hold the relevant competence for unvented hot water systems, which is a specific qualification distinct from general plumbing.
- You should receive a certificate confirming the installation was notified. Keep it.
- A solicitor will ask for it when you sell the house. Reconstructing it years later is difficult and sometimes impossible – a retrospective regularisation application through building control is the fallback and it is not straightforward.
- Your insurer may ask, particularly after any incident.
- An installation with no certificate is a problem at exactly the moment you least want one.
Not ‘it will follow’. A great many people discover fifteen years later that they never received one, at the point their buyer’s solicitor asks. If you are having an unvented cylinder installed, the certificate is part of the deliverable.
The required safety devices
G3 requires that hot water cannot exceed a safe temperature even if the normal controls fail. In practice that means a layered set:
| Device | Function |
|---|---|
| Cylinder thermostat | The normal operating control |
| High-limit thermostat (energy cut-out) | Non-self-resetting. Cuts the heat source if the control thermostat fails |
| Temperature and pressure relief valve | Discharges if temperature or pressure exceed safe limits despite everything else |
| Expansion vessel or internal air gap | Accommodates expansion as water heats, so pressure does not climb in normal use |
| Expansion relief valve | Discharges if the expansion arrangement fails |
| Pressure reducing valve | Limits incoming mains pressure to what the cylinder is rated for |
The high-limit thermostat must not self-reset. That matters: a device that resets itself would allow a fault to recur indefinitely without anybody knowing. A manually resettable cut-out means somebody has to notice.
Discharge pipework: D1 and D2
This is where installations fail inspection most often, and it is the part that gets skimped.
If a relief valve opens, it discharges water at or above boiling point. That has to be taken somewhere safe. The regulations describe two sections:
D1 – valve to tundish
- The short length from the relief valve to the tundish.
- Maximum 500mm from the valve.
- Same size as the valve outlet.
The tundish
- An open air break with a visible gap, so discharge can be seen.
- Fitted vertically, in the same room or space as the cylinder, where somebody will actually see it.
- Not concealed inside a sealed box – which defeats the entire purpose.
D2 – tundish to termination
- At least one pipe size larger than D1.
- Continuous fall from the tundish, with a minimum vertical drop below it before any horizontal run.
- Maximum equivalent length, which is reduced by every bend. Each elbow counts against the allowance, so a run with several bends may need upsizing again.
- Metal pipe, because it may carry water above 100 degrees.
- Terminating safely – low level to a gully below a grating, or a visible external position where discharge cannot scald anybody, including someone at a window or on a path.
A tundish hidden inside a boxed-in cupboard. A D2 run the same size as D1. A long D2 run with several bends and no upsizing. A near-horizontal section holding water. A termination discharging over a doorway or a path. All of these are found regularly, and all of them mean the system cannot discharge safely.
The scalding requirement
G3 also addresses the temperature of hot water delivered to a bath, requiring that it does not exceed a stated maximum. In practice that means a thermostatic mixing valve on the bath supply.
This resolves the same tension that appears throughout hot water design: water must be stored hot enough to suppress bacterial growth, and delivered cool enough not to scald. Storing cool is not the answer; blending at the outlet is.
A thermostatic mixing valve is a serviceable item, not a fit-and-forget one. It should be checked and its performance verified periodically – a valve that has drifted is providing neither protection nor reliable temperature.
What to check on an existing installation
- Is there a certificate? If not, that is worth knowing before you sell rather than during.
- Can you see the tundish? It should be visible in the same space as the cylinder.
- Is D2 larger than D1? Look at the two pipes either side of the tundish.
- Does D2 fall continuously? Any near-horizontal section is a problem.
- Where does it terminate? Somewhere safe, or over a path?
- Has it been serviced? The relief valves and the expansion vessel need annual attention.
- Is there a thermostatic mixing valve on the bath?
Most of that is a two-minute visual check and it tells you a great deal about the quality of the installation.
Unvented cylinder with no certificate, or a hidden tundish? Both are worth resolving before you need to explain them.
Questions we get asked about this
Is installing an unvented cylinder notifiable?
Yes, under the Building Regulations. It requires a competent person and you should receive a certificate confirming notification. Keep it – a solicitor will ask for it when you sell.
What is a tundish for?
It provides a visible air break in the discharge pipe, so that if a relief valve opens you can see it happening. That is why it must be visible, in the same space as the cylinder, and not boxed in.
Why must D2 be bigger than D1?
Because the discharge flashes to steam as pressure drops, and the larger bore accommodates the increased volume. A D2 the same size as D1 cannot carry the discharge.
What if my installation has no certificate?
It is worth resolving before you sell. A retrospective regularisation application through local authority building control is the route, and it is easier if the installation actually complies – which is worth checking first.
I can see water coming out of the tundish. Is that serious?
It means a relief valve has opened. The usual cause is a failed expansion vessel. Not dangerous in itself but it should not be left, because a valve that keeps discharging will eventually fail to reseat.
Does a vented cylinder need any of this?
No. A vented system’s open vent pipe provides a permanent unblockable route to atmosphere, which satisfies the requirement inherently. That is why vented work is not notifiable.
Regulations and standards referenced on this page
- Approved Document G: sanitation, hot water safety and water efficiency — Ministry of Housing, Communities and Local Government
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.