Unvented cylinders are high-search, high-confusion plumbing topics because they look simple from the outside. The important revision idea is that a mains-fed sealed hot-water store needs a chain of controls: normal temperature control, overheat protection, expansion control, pressure relief and a safe visible discharge route.
An unvented cylinder is mains-fed and sealed, so it needs layered pressure, temperature, expansion and discharge safety controls.
Unvented cylinder diagram
The cold mains feed enters through an inlet-control group. Expansion is handled by an expansion vessel or internal air gap. Temperature and pressure relief protects the stored hot water side. Discharge passes through a tundish and then to a safe termination.
Vented vs unvented
A vented cylinder is fed from a cold-water storage cistern and has an open vent path to atmosphere. An unvented cylinder is sealed and fed from the mains, which improves pressure but raises the safety stakes because expansion and over-temperature faults need engineered controls.
Direct vs indirect
A direct cylinder heats the stored water directly, often with immersion heaters. An indirect cylinder heats the stored water through a coil supplied by a boiler or primary circuit. The heating method changes the controls, but the unvented safety principle remains layered protection.
The G3 safety chain
- Control thermostat: normal temperature control.
- High-limit cut-out: stops overheating if normal control fails.
- Expansion provision: absorbs water expansion as it heats.
- Expansion relief and pressure relief: protect against over-pressure.
- Temperature and pressure relief valve: protects against dangerous stored hot-water faults.
- Tundish and discharge pipework: makes discharge visible and routes it safely.
Common faults and what they tell you
Unvented faults are unusually readable once you know the safety chain, because the discharge route makes problems visible instead of hiding them.
- Intermittent dripping at the tundish while the cylinder heats: think expansion — a flat or failed expansion vessel (or lost internal air bubble) leaves nowhere for the expanding water to go, so the expansion relief valve weeps on every heat-up cycle.
- Steady discharge that does not track heating cycles: think incoming pressure — a failed pressure-reducing valve can hold the system above the relief setting all day.
- Sudden, heavy, hot discharge: treat as the temperature relief valve doing its job against overheat — isolate the heat source and investigate the thermostat and cut-out chain rather than silencing the symptom.
- Poor flow or pressure at hot taps: check the inlet control set — strainer, pressure-reducing valve and check valve — before blaming the cylinder.
- No hot water at all: usually controls, immersion or a tripped thermal cut-out. A cut-out that keeps tripping is a fault clue, not a reset routine.
- Any of the above capped, bunged or “fixed” by blanking the discharge: that converts a visible fault into a concealed hazard.
Changing from vented to unvented
The conversion question comes up constantly, and the honest answer is: often yes, but three checks decide it. First, the mains supply must actually deliver the flow and pressure the cylinder needs — a survey, not an assumption. Second, the work is controlled: it needs a competent installer for unvented hot water and the correct Building Regulations notification route. Third, a compliant discharge route has to exist from the cylinder position — in real houses, finding a safe, visible tundish and D2 run is frequently the thing that decides where (or whether) the cylinder can go.
What you gain is mains-pressure hot water at every outlet and no loft cistern; what you take on is dependence on the incoming main and a safety chain that must be serviced. The learner takeaway: the cylinder swap is the easy part — the system design decisions around it are the exam material.
Quick answers
What is an unvented cylinder?
It is a sealed hot-water storage cylinder fed directly from the mains, using safety controls for pressure, temperature, expansion and discharge.
Is it illegal to fit an unvented cylinder yourself?
Unvented hot-water work is controlled by Building Regulations and needs competent installation, the correct certification/notification route and manufacturer instructions.
Why does an unvented cylinder need a tundish?
The tundish makes safety discharge visible and forms part of the discharge arrangement so faults are noticed rather than hidden.
What are the risks of an unvented cylinder?
Stored hot water under mains pressure needs layered protection: thermostat, overheat cut-out, expansion control and relief valves with a safe discharge route. Installed and serviced correctly the design is safe; the real risk is unqualified work or defeated safety devices.
Can I change from a vented to an unvented cylinder?
Usually, if the incoming main can supply enough flow and pressure, the work is done through the competent/notified route, and a compliant discharge run is achievable from the cylinder position. All three are survey questions before anyone orders a cylinder.
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