Boilers & Heating
Unvented cylinder installation
TL;DR
An unvented cylinder swap is £1,850–£3,200 supply and fit including all safety kit (expansion vessel, PRV, tundish, T&P valve), Building Control notification, and G3 commissioning. Takes a full day. Only a G3-qualified engineer can legally fit one — it's not a job for an unregistered plumber. Common sizes: 150L for two bedrooms, 210L for three bedrooms, 250–300L for four bedrooms with two showers running simultaneously.
An unvented cylinder gives you mains-pressure hot water at every tap — no more weak upstairs shower, no cold-water tank in the loft, and enough flow for two showers running at once. Because it holds hot water at high pressure it's classed as a G3 fitting under Building Regs, which means only a registered engineer can legally install it, and the work must be notified to Building Control.
This page covers sizing, brand choice, and the safety kit that separates a compliant install from a dangerous one.
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How do I size an unvented cylinder?
Rule of thumb: 45 litres per person, per day, plus a buffer for simultaneous use. A one-shower household of two adults is comfortable on 150L. A three-bed family with one shower needs 180–210L. A four-bed with a family bathroom and an en-suite that might run together needs 250–300L. Undersized cylinders leave you with the shower going cold; oversized ones waste standing heat.
- 1–2 people, one shower — 150L
- 3 people, one shower — 180L
- 3–4 people, one shower — 210L
- 4+ people, two showers simultaneous — 250–300L
- Larger property with three bathrooms — 300L+ or a system boiler with high recovery
The safety kit that must be there — every time
An unvented cylinder is a pressure vessel. The safety kit isn't optional and the omissions on cheap installs are what kills people. Every install must have an expansion vessel sized to the cylinder, a pressure-reducing valve on the cold inlet, a tundish visible from the cylinder, a temperature & pressure relief valve (T&P) discharging to a safe location, and a Building Regs G3 certificate registered with your Local Authority.
- Expansion vessel — sized to cylinder capacity
- Pressure-reducing valve (PRV) — set to 3.0–3.5 bar
- Tundish — visible air-break to see if the T&P is discharging
- T&P relief valve — 90°C / 7 bar cut-out
- Discharge pipe — to safe termination outside, correctly sloped and sized
- Building Control notification via a G3 competent-person scheme
What we do on install day
Full-day job. Drain and remove the old cylinder (or the loft tank if converting from vented), set the new cylinder on a base, run the mains cold in with PRV and expansion vessel, plumb hot outlets, wire the immersion and thermostats, run the discharge pipe from the T&P valve to a safe termination, commission and pressure-test, register with Building Control and the manufacturer for the 25-year warranty, and hand over benchmark and G3 paperwork.
Combi vs unvented — which is right for you?
A combi heats water on demand, has no cylinder, and is limited by its flow rate — usually one shower at a time. An unvented cylinder stores hot water at mains pressure, so you can run multiple showers simultaneously. If you have one bathroom and modest hot-water use, a combi is simpler and cheaper. If you have two-plus bathrooms or a big bath, an unvented (fed by a system boiler or heat pump) is the right answer.
- One bathroom, up to 2 people — combi is fine
- One bathroom, family of 3+ — unvented starts to make sense
- Two+ bathrooms — unvented, no question
- Heat pump — must be paired with an unvented cylinder
- Solar thermal or PV diverter — needs an unvented (twin-coil)
Typical prices
Real-world ranges from recent Northwest jobs. Your final price is confirmed in writing before any work starts.
| Item | Typical price | Notes |
|---|---|---|
| 150L unvented cylinder — supply & fit | 1,850 – 2,250 | Small household, one bathroom |
| 180L unvented — supply & fit | 1,950 – 2,450 | Standard family install |
| 210L unvented — supply & fit | 2,150 – 2,650 | Most common 3-bed size |
| 250L unvented — supply & fit | 2,450 – 2,950 | 4-bed, two showers |
| 300L unvented — supply & fit | 2,650 – 3,200 | Larger property |
| Vented-to-unvented conversion | +200 – 450 | Removes loft tank, extra pipework |
| Annual G3 service | 95 – 140 | Expansion vessel recharge, T&P test |
| Building Control notification | 0 | Included via competent-person scheme |
Which brand should you fit?
We fit Megaflo Eco Plus and OSO Super S as our default choices — both come with 25-year cylinder guarantees and stainless steel construction. Also fit Telford Tempest, Gledhill Stainless Lite Plus and Heatrae Sadia Megaflo where the customer prefers. Not commission-driven; we recommend on capacity and stock availability. We install and service Worcester Bosch, Vaillant, Ideal, Baxi, Viessmann and Alpha — none of them pay us commission and we don't push a "boiler of the month".
Unvented cylinder red flags
This is one of the most safety-critical jobs in domestic plumbing. Do not compromise on these.
- Show me your G3 registration number (competent-person scheme)
- Will you notify Building Control and give me the certificate?
- Is the discharge pipe going to a safe outside termination (not a soil stack)?
- Is the expansion vessel correctly sized for the cylinder capacity?
- Are you registering the manufacturer's 25-year warranty for me?
- What's your quote breakdown — cylinder, safety kit, labour, discharge run?
- Will the annual service include re-charging the expansion vessel and testing the T&P?
Our promise on this job
Flat £50 diagnostic — deducted in full from any work booked. Fair, transparent quote before we start. If we can't fix the fault we diagnosed, the £50 still covers the engineer's home visit. All gas work is signed off by a Gas Safe registered engineer.
Talk to KJ about unvented cylinder installation
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