G3 revision notes
Unvented cylinders — the layered safety devices and safe discharge arrangement.
- 11notes, free to read
- 134questions; 25 free to try
- 4topics
Approved Document G Section G3: Discharge Pipework & Expansion
Approved Document G Section G3: Safety Devices & Overheat Protection
Safe termination options for unvented cylinder discharge pipework
Choosing a safe D2 termination, including the conditions for a gully, guarded external outlet or suitable soil-stack connection.
The layers of protection on an unvented hot water system
Why an unvented cylinder has stacked, independent safety controls and what each layer does.
Tundish and discharge pipe basics (D1 and D2)
Distinguish D1 from D2 and check the air break, fall, material and complete route resistance before choosing a termination.
Unvented hot water: layered safety and discharge
Read the unvented-cylinder diagram: inlet controls, expansion provision, relief valves, tundish and discharge route.
Unvented hot water: layered safety controls (diagram)
How the layered safety controls on an unvented hot-water cylinder protect against overheat, and where a discharge ends up.
Approved Document G3: Expansion Provision & Inlet Controls
Expansion vessels and the air charge
How a pressurised expansion vessel takes up expanding hot water and why its air charge matters.
Sizing the expansion for an unvented cylinder
A worked first calculation for unvented expansion volume, with sizing left to the manufacturer data.
The cold-water control set on an unvented cylinder
The group of inlet controls that condition the cold supply to an unvented cylinder and what each one does.
Unvented Hot Water: Building Regulations & Competence
Trace an unvented cylinder diagram before answering G3 questions
Name the job of each component on the unvented cylinder diagram, then trace the supply, heat source and safety-discharge routes separately. In a fault question, explain which observation supports your conclusion and what remains unknown.
Try the free G3 practice questions after identifying each safety control. Use the G3 revision guide to connect controls and expansion, and the D1 and D2 discharge guide for the tundish arrangement. Check your working with the cylinder heat-up calculation when the question supplies a volume, temperature rise and heat input.
In-depth guides for this module
Longer walkthroughs that sit alongside the revision notes.
G3 unvented hot water revision guide: safety controls and discharge
Revise the unvented control chain: normal thermostat control, high-limit cut-out, expansion provision, temperature and pressure relief, then a visible tundish discharge to a safe termination. Stored hot water expands — the layered controls exist so no single failure is dangerous.
Unvented cylinder explained: diagram, safety controls and G3 revision
An unvented cylinder is mains-fed and sealed, so it needs layered pressure, temperature, expansion and discharge safety controls.
What is a tundish? Unvented cylinder discharge pipe (D1/D2)
The tundish makes safety discharge visible; the D2 pipe then has to carry hot water and steam safely to a visible, safe termination.
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