Unvented Cylinder — What It Is, When to Fit One (UK 2026)
An unvented hot water cylinder delivers mains-pressure hot water to every tap — no loft tank required. Here is the UK regulatory reality and install cost.
Step-by-step method
1. What 'unvented' means
Cylinder is sealed and pressurised from the mains via a PRV. Mains-pressure hot water — no gravity feed. Requires G3-registered installer.
2. G3 regulations
Legal requirement: installer must be G3-qualified (Part G of Building Regs). Expansion vessel, PRV, T&P valve and tundish are mandatory.
3. Discharge pipe (D1/D2)
22mm tundish + 28mm discharge in copper to a safe location — outside is easiest. Max length and drops are strictly regulated.
4. Best UK brands
Megaflo (Heatrae Sadia) — market leader. Santon Premier Plus — good value. Range Tribune — commercial quality. All 25-year warranty.
5. Install cost
180L cylinder installed £2,200–£3,000 in most UK areas. Larger sizes and remote installs (loft) add £400+.
Frequently asked questions
Can I install an unvented cylinder myself?+
No — G3 registration is a legal requirement. Uninsured, uncertified installs void household insurance.
Unvented vs combi boiler?+
Unvented + system boiler = high flow to 3+ bathrooms simultaneously. Combi = single bathroom, no cylinder space needed. Different applications.