The short version
- Unvented cylinders deliver hot water at mains pressure to every outlet
- Cylinder size should match the household's peak demand, not the loft space
- Recovery time matters as much as capacity
- Unvented cylinders need annual checks on their safety components
Vented vs unvented
A vented cylinder is fed by a cold tank in the loft and delivers gravity pressure — weak upstairs showers are the classic symptom. An unvented cylinder is sealed and fed from the mains, so pressure at the outlet matches the incoming supply. Replacing vented with unvented is one of the most noticeable upgrades in an older house.
Sizing and recovery
Capacity covers the peak; recovery decides how soon the next person can shower.
- Count baths and showers used in the morning peak
- Allow for bath volume, which dominates demand
- Check the coil rating — it drives reheat speed
- Match cylinder insulation standard to keep standing losses low
- Leave service access around the safety valves and expansion vessel
Maintenance nobody expects
Unvented cylinders have expansion vessels, temperature and pressure relief valves and tundishes that require annual inspection. Skipping it is both a safety and warranty issue, and it's usually done at the same visit as the boiler service.
Common questions
Can I keep my existing cylinder with a new boiler?+
Sometimes, if it's in good condition and correctly sized. We assess it rather than assume.
Do unvented cylinders need a special qualification?+
Yes — they must be installed and serviced by a suitably qualified engineer.
Will an unvented cylinder improve my shower?+
If you currently have a loft tank feeding it, almost always yes.
Do I need a cylinder for a heat pump?+
Yes. Heat pumps store hot water rather than heating it instantly.
Upgrading a cylinder system?
We'll size the cylinder around your actual morning demand.