G3 revision notes
Unvented cylinders — the layered safety devices and safe discharge arrangement.
- 10notes, free to read
- 73questions; 25 free to try
- 4topics
Approved Document G Section G3: Discharge Pipework & Expansion
- Servicing an unvented cylinder: annual safety checksPeriodic checks that keep each protection layer working, and how to respond to a discharge, a tripped cut-out or overheated water.
- Unvented cylinder commissioning and safety checksBefore and at handover: confirm the notification route, the mains supply, each heat source's cut-out and a safe discharge, then leave the records.
Approved Document G Section G3: Safety Devices & Overheat Protection
- Safe termination options for unvented cylinder discharge pipeworkWhere D2 can end: a guarded outlet over hardstanding, a trapped gully, a suitable soil stack or at high level, and the terminations that fail.
- The layers of protection on an unvented hot water systemWhy an unvented cylinder needs independent layers of overheat protection, how each layer behaves, and how outlet temperatures are kept safe.
- Tundish and discharge pipe basics (D1 and D2)D1 runs from the valve to the tundish, D2 from the tundish to a safe end: the air break, positions, fall, material and sizing with bends.
- Unvented hot water: layered safety and dischargeWalk an unvented cylinder from the cold inlet to the discharge: what each device does, and what water at the tundish tells you.
Approved Document G3: Expansion Provision & Inlet Controls
- Expansion vessels and the air chargeHow the gas cushion takes up expansion, what the pre-charge should match, how to check it, and what weeping on every heat-up means.
- Sizing the expansion for an unvented cylinderA worked expansion-volume sum, why the vessel is bigger than the result, and how the pre-charge setting decides how much of it is usable.
- The cold-water control set on an unvented cylinderThe inlet controls in order, what each does, why the order matters, and how the expansion relief valve differs from the temperature relief valve.
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 suitable termination.
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