An unvented cylinder gives you strong, mains-pressure hot water at every tap and shower without a loft tank. If you live in Chorley and are weighing one up, here is what actually matters before you commit, from sizing to the legal bits many people miss.
Unlike an old vented setup fed by a cold tank in the loft, an unvented cylinder is connected straight to your cold mains. That means water leaves your taps at close to incoming mains pressure, so a shower on the top floor feels as strong as one downstairs.
Because the water is heated in a sealed vessel, it has to cope with expansion safely. That is why every unit has an expansion vessel or air gap, a pressure reducing valve, an expansion relief valve and a temperature and pressure relief valve. These are safety-critical, not optional extras.
The single biggest factor is your incoming mains. An unvented cylinder typically wants at least 1.5 bar of pressure and a decent flow rate, ideally around 20 litres a minute or more. Parts of Chorley on higher ground or at the end of a long run can be weaker, so a proper flow and pressure test at your kitchen tap comes first.
You also need somewhere to run the discharge pipe from the relief valves to a safe outside point, and space for the cylinder itself, usually an airing cupboard or utility.
Size is about how many people and bathrooms you have. A couple in a two-bed might be fine with 150 litres, while a family of four with two bathrooms often suits 210 to 250 litres. Undersizing means running cold mid-shower; oversizing wastes energy keeping water you never use hot.
For a like-for-like replacement, a typical installed cost in the Chorley area tends to fall somewhere between roughly £1,500 and £3,000 depending on cylinder size, brand, pipework changes and whether a new heat source is involved. Treat that as a guide only, as every home differs.
Installing an unvented cylinder is notifiable work under Building Regulations, and it must be fitted by someone holding a G3 qualification. A competent installer will register the work so your Building Control paperwork is handled without you needing a separate application.
Once fitted, the cylinder should be serviced annually. The engineer checks and recharges the expansion vessel, tests the relief valves and inspects the temperature control. Skipping this is a false economy, as a flat expansion vessel is the most common cause of water dripping from the outside discharge pipe.
Published 9 July 2026 by Red Leaf Plumbing & Heating.
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