Hot water
Vented, unvented, and the litres-per-person rule that decides which one you need. Undersizing a cylinder is the mistake people live with for twenty years.
Get my fixed priceThe two types
They look similar in the cupboard and behave completely differently at the tap.
The detail
A cold tank in the loft feeds the cylinder, and the pressure at your shower is simply the height difference between that tank and the shower head. On a bungalow, or a top-floor bathroom in a house, there is barely any height — which is exactly why those showers are weak.
The advantages are real though: it is cheap, it is simple, it needs no annual safety check, and it carries on working in a mains water outage.
No tanks. The cylinder is sealed and pressurised at mains pressure, so every outlet in the house runs hard, and two showers can run at once. This is what most people want.
It needs a proper safety kit: an expansion vessel to absorb the water expanding as it heats, a pressure relief valve, a temperature and pressure relief valve, a tundish you can actually see, and a discharge pipe to somewhere safe. That kit is not optional and it must be checked annually.
It also needs the incoming mains to be up to it. Below about 20 litres a minute and 1.5 bar you will be disappointed, and no cylinder can fix a poor supply.
| Household | Bathrooms | Cylinder |
|---|---|---|
| 1 to 2 people | 1 | 120 litres |
| 3 people | 1 | 150 litres |
| 3 to 4 people | 1 plus en-suite | 180 litres |
| 4 to 5 people | 2 | 210 litres |
| 5 or more | 2 plus | 250 to 300 litres |
Roughly 35 to 45 litres per person, plus capacity for simultaneous draw-off. If you are between two sizes, take the larger one — the cost difference is small now and impossible later.
Boiler water never touches your bath water. It runs through a coil inside the cylinder and the stored water takes the heat through the coil wall. A bigger coil surface reheats the cylinder faster, which is why two cylinders of the same litres can behave very differently.
If you have a heat pump in your future, this matters a great deal — heat pumps run at a much lower flow temperature and need a far larger coil than a gas boiler does. Worth knowing before you buy a cylinder you intend to keep for twenty years.
Size and coil
Questions
Look in the loft. If there is a large cold water tank up there feeding the cylinder, it is vented. If the loft is empty and the cylinder has a small red expansion vessel and a copper discharge pipe running outside, it is unvented.
Vented cylinders give you gravity pressure, which is why upstairs showers are often weak. Unvented gives you mains pressure at every outlet.
The rule of thumb is 35 to 45 litres per person, then add capacity for a second bathroom. One bathroom and two people is about 120 litres. Four people and two bathrooms is nearer 210.
Undersizing is the mistake people regret. An extra 30 litres costs very little when the cylinder is being fitted and is impossible to add afterwards.
If you have one bathroom, almost certainly, and you get the airing cupboard back. If you have two bathrooms genuinely used at once, keep the cylinder — a combi will disappoint you every morning.
Our quote tool applies that rule automatically. Three or more bathrooms and it will not offer you a combi at all.
A modern stainless unvented cylinder should manage twenty to twenty-five years. An old copper vented one often goes longer but loses efficiency as the insulation degrades.
If yours is a bare copper cylinder with a jacket strapped round it, replacing it will save you more than most people expect.
Yes, annually, and it is a legal requirement for the safety discharge arrangement to be checked. The expansion vessel loses its charge over time, which is the most common cause of a discharge pipe dripping.
It is also a G3 activity, so whoever does it must hold that qualification. Ours do.
The quote tool asks, works out which suits your bathrooms, and prices both paths openly.
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