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, we refund the diagnostic. All gas work is signed off by a Gas Safe registered engineer.
Talk to KJ about unvented cylinder installation
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