What Size Hot Water Cylinder Do I Need?

Hot water cylinder sizes by bathrooms and occupancy

Sizing starts with how much hot water leaves the cylinder in the busiest hour of the day, not how much the household uses in total. Nobody runs out of hot water at two in the afternoon. They run out between seven and nine in the morning, and that is the only period the cylinder has to survive.

The rule of thumb most installers work to is somewhere between 35 and 45 litres of stored water per person per day, then adjusted upward for baths, for showers with high flow rates, and for outlets that get used at the same time. Applied to real households it lands on the bands below, which are the sizes cylinders are actually manufactured in, so there is little point agonising over the gaps between them.

Where a household sits on the boundary between two sizes, the honest answer is usually that the smaller one will do if the heat source reheats it fast, and the larger one is the safer buy if it is fed by an immersion heater or an old boiler with a tired coil. That is a decision about recovery, not about litres. London hard water pushes the same decision the other way over time, because scale on a coil steals recovery every year it builds, so a household on the boundary here is often better off with the larger vessel.

• One or two people, one bathroom, showers rather than baths: 120 to 150 litres.
• Two or three people, one bathroom plus a cloakroom: 150 to 180 litres.
• Three or four people, one bathroom and an en suite: 180 to 210 litres.
• Four or five people, two full bathrooms, baths in regular use: 250 to 300 litres.
• Five or more people, or three bathrooms, or a freestanding bath: 300 litres and upwards.
• Any house running a heat pump: larger than the equivalent gas figure, usually the next size up or two.

Litres of storage is not litres of hot water at the tap

A 210 litre cylinder does not give you 210 litres of shower. It gives you rather more, and this catches people out in both directions.

Stored water sits at around 60 degrees and nobody showers at 60 degrees. A comfortable shower is near 40, so every litre leaving the cylinder is blended with roughly the same again of cold on the way to the outlet. In round terms a cylinder delivers about one and a half times its own volume as usable warm water before the temperature falls away.

Working against that is stratification and the draw off point. Hot water floats, cold feed enters at the bottom, and the two do not stay in neat layers once water is moving fast. Pull water out quickly and the incoming cold mixes upward, so the last quarter of the cylinder arrives lukewarm rather than hot. Draw it slowly and you get far closer to the full capacity. This is why the same cylinder copes with two people showering one after the other and falls over when they shower at once.

The practical version: a standard bath takes 150 to 180 litres of blended water, which is about 80 to 100 litres out of the cylinder. A ten minute shower on a normal head is around 90 to 100 litres blended, so 50 or so from the cylinder. A ten minute shower on a wide rainfall head can be double that. Add up the busiest hour in those terms and the size chooses itself.

Count the outlets and the flow rates, not the bedrooms

Estate agents count bedrooms. Cylinders are emptied by outlets, and specifically by outlets used simultaneously. A three bedroom house where everyone leaves at different times has a gentler hot water profile than a two bedroom flat shared by two people who both start work at nine.

Flow rate matters more than most people expect. A conventional shower head on a decent unvented supply runs somewhere around 9 to 12 litres a minute. A large rainfall head, or a head plus body jets, can pull 15 to 20. That is not a small difference. It roughly halves how long the cylinder lasts, and it is the single most common reason a correctly sized cylinder feels undersized after a bathroom refit.

So take stock of what the water actually goes through: how many shower heads, whether any is oversized, whether the bath gets filled or just rinsed out. If a refurbishment is planned, size for the bathroom you are going to have, because the cylinder will outlive the shower.

• Two showers that can run at the same time push you a size up, regardless of household size.
• A rainfall or drencher head counts roughly as two ordinary showers.
• A bath that gets filled daily is worth the better part of an extra person.
• A bath and a shower running together is the load that defeats undersized cylinders.
• Guests, a lodger or a teenager arriving shortly are worth planning for now rather than in two years.

Recovery time matters as much as capacity

Capacity is how much you have. Recovery is how fast you get it back. A household that empties the cylinder every morning does not need a bigger cylinder so much as a faster one, and the two are solved differently. Before assuming yours is undersized, it is worth checking how long hot water should take to heat up on a system in good order, because a cylinder that used to cope and no longer does has usually slowed down rather than shrunk.

An indirect cylinder is heated by a coil carrying water from the boiler. How quickly it reheats depends on the surface area of that coil and how much of the boiler output it can absorb. A modern high recovery cylinder with a generously sized coil can take 18 to 24 kilowatts from the boiler and bring a full tank from cold back to temperature in something like 25 to 40 minutes. An older cylinder with a short, narrow coil might only absorb a fraction of that, and take two hours or more for the same job.

An immersion heater is a different world entirely. A standard immersion is 3 kilowatts. Heating 200 litres from cold on an immersion alone is a job measured in hours, not minutes. That is why a cylinder relied on for electric heating, or one that spends winter on the immersion because the boiler is unreliable, should be sized generously: it has no chance of catching up during the day.

The consequence for sizing is simple. Fast recovery lets you buy smaller, because a 180 litre cylinder refilled inside half an hour is being topped up while it is emptied. Slow recovery means the cylinder has to hold the whole morning in one go, and then you are buying litres because you cannot buy speed.

A cylinder that has quietly lost recovery speed is one of the signs your cylinder needs replacing rather than a reason to buy a bigger one.

If an existing cylinder is slow to reheat and always has been, check the coil performance before buying a larger replacement. A limescaled coil, a motorised valve that is not driving fully open, or a cylinder thermostat clipped loosely to the outside of the tank will all masquerade as a cylinder that is too small. Where the reheat has stretched out over years rather than always been poor, the signs that a cylinder is finished are worth reading first, because buying a bigger one will not fix a coil that has stopped transferring heat.

What oversizing actually costs you

The instinct when replacing a cylinder is to go bigger for safety. It is not free, and in a small property it is often the wrong call.

Every cylinder loses heat through its insulation continuously, and a bigger cylinder has more surface area losing it. Modern foam insulated units are good, but a large tank kept at temperature for a household that uses a third of it is paying to heat water that goes cold and gets reheated, every day, for capacity nobody draws.

There is also the reheat penalty. A bigger tank takes longer to bring back to temperature from cold on the same heat source, which is the opposite of what the household wanted when they bought it. And water that sits for long periods at lower temperatures in a barely used cylinder is exactly the condition you do not want, which is why an oversized cylinder still has to be brought fully up to temperature regularly rather than run cool to save money.

Then the physical cost. A 300 litre cylinder full of water weighs over 300 kilos before the unit itself. On a ground floor that is nothing. In a first floor airing cupboard in a converted flat, or on a timber floor in a loft, it is a real load, and it eats a cupboard that was doing something useful.

Size also follows you into maintenance. An unvented cylinder repair on a larger vessel means more draining down, more time on site and more disruption than the identical fault would cause on a smaller one.

An oversized vessel also pushes more expanded water through the same discharge arrangement, and a pressure relief valve leaking repeatedly is one of the ways an undersized expansion vessel announces itself.

Weight is the constraint people forget. Before upsizing significantly in a flat or a loft, the floor structure under the cylinder position needs to be looked at rather than assumed.

Will it fit? Airing cupboard and access constraints

In a London flat this decides the answer as often as the arithmetic does. Cylinders grow in two directions and cupboards do not.

Diameter is the usual blocker. Common cylinder diameters run around 450, 500, 545 and 580 millimetres, and the step from one to the next is what buys litres without buying height. A Victorian conversion with a cupboard built around a narrow vented tank frequently will not take the diameter of the unvented cylinder the household wants. Measure the cupboard internally, boxed in pipework and door frame included.

Height is the other one, and sloping ceilings, boxed in soil stacks and the top of a door opening all interfere. Manufacturers make slimline tall cylinders and short fat ones at the same capacity for exactly this reason, so a difficult cupboard rarely rules out a size outright, it limits which model can deliver it.

Then the route in. A 250 or 300 litre cylinder is a long item, and getting it up a tight communal stair, round a half landing and through a standard door is a real consideration rather than an afterthought. Walk the route with a tape before ordering.

One unvented requirement also shapes cupboard choice: the discharge pipe from the safety valves has to run in metal with the tundish visible and terminate safely outside, so in a top floor flat with a cupboard in the middle of the plan, that route can dictate where the cylinder goes and therefore what size fits.

Access decides more hot water cylinder replacement jobs than capacity does, because a vessel that will not go up the stairs is not an option whatever the calculation says.

A G3 qualified plumber should measure the route as well as the cupboard before anything is ordered.

• Measure the cupboard internally: width, depth, height to the lowest obstruction.
• Measure the door opening and the tightest turn on the route in.
• Allow clearance above for the immersion heater to be withdrawn later.
• Allow space at the front for the controls, the expansion vessel and the valve set.
• Confirm a practical route for the discharge pipe to an outside wall.

Heat source changes the answer

The same household needs a different cylinder depending on what is heating it, and this is where old sizing habits go wrong.

On a modern gas boiler with a high recovery coil the bands above hold. On an older boiler, or where the coil is small, size up or accept slower mornings. The bands assume a sealed unvented cylinder fed from the main; on a vented cylinder fed by a loft tank the litres work the same way but the pressure at the outlet does not, and the difference between vented and unvented cylinders decides how usable those litres feel before capacity ever comes into it.

On a heat pump the rules change properly. Heat pumps deliver lower flow temperatures, so the cylinder needs a much larger coil surface area to transfer heat at a useful rate, and it needs more capacity because it reheats gently rather than rapidly. A cylinder that suited a gas boiler will disappoint on a heat pump even at the same litres. Heat pump cylinders are sold as such, with the coil area to match, and a household moving from gas should expect to go up a size.

On solar thermal, the cylinder usually carries a second lower coil and extra volume to store what the panels produce on a good day. In flats fed from a communal heating system there may be a thermal store or a Pulsacoil type unit instead of a conventional cylinder. Understanding how a Pulsacoil works matters here, because it stores heat rather than drinking water, capacity behaves differently as a result, and the replacement options are set by what the building supplies rather than by what the household would choose.

Once the size is settled the next question is which unit to buy, and the best unvented cylinder brands are chosen on coil surface area and parts support rather than on the name pressed into the casing.