What Size Hot Water Cylinder Do You Need? London Guide

Two Different Questions: How Much, and How Fast

Cylinder sizing gets discussed as a single number in litres, but it is really two problems. Storage volume determines how much hot water is available before the cylinder runs cool. Flow rate determines how quickly it can be delivered, and that is set by the incoming cold main rather than by the cylinder. A 300 litre cylinder on a weak supply will still give a weak shower, and a well-sized cylinder on a strong supply will still disappoint if three people shower in succession and it holds 120 litres. Both have to be right.

What a Household Actually Draws

Useful sizing starts from real draw-offs rather than the number of bedrooms. A bath typically takes somewhere in the region of 100 to 150 litres of blended water depending on the tub. A mixer shower commonly runs at around 8 to 12 litres per minute, so a ten-minute shower is roughly 80 to 120 litres of blended water. High-flow and multi-head showers go well beyond that.

Those are blended figures, and the cylinder only supplies part of them. If water is stored at 60°C, mains cold sits around 10°C and you want 40°C at the outlet, the hot fraction is (40 − 10) ÷ (60 − 10), which is 0.6. So a 100 litre shower draws about 60 litres of stored hot water. That arithmetic is the reason a 150 litre cylinder can cover more than it appears to on paper, and also the reason storage temperature is not something to quietly lower to save energy without thinking about capacity.

Sizes Commonly Specified by Property Type

These are the ranges typically specified in practice, not a calculation, and a survey should confirm them against how your household actually uses hot water:

• One bathroom, one or two occupants: commonly around 120 to 150 litres.
• One bathroom plus an en-suite, three or four occupants: commonly around 180 to 210 litres.
• Two bathrooms, four or five occupants: commonly around 210 to 250 litres.
• Three or more bathrooms, or a household that fills a bath while a shower runs: 250 to 300 litres and upwards, and at this point simultaneous demand rather than daily total is what drives the choice.

Two things push a household up a band regardless of size: baths rather than showers, and a schedule where everyone uses hot water within the same hour rather than across the morning.

Simultaneous Demand Is the Real Test

The question that separates a comfortable installation from a marginal one is not how much hot water you use in a day but how much you use at once. Two showers running together demand roughly double the flow rate, and the incoming main has to supply that on top of anything else running. This is where sizing and mains capacity meet, and why the survey should measure the incoming flow rate at the property rather than work from the property's postcode.

If the measured flow will not support two outlets at once, a larger cylinder does not fix it. The answers are on the supply side: upgrading the incoming pipe, or buffering with an accumulator.

Reheat Time and the Coil

A cylinder that recovers quickly behaves like a larger one. Recovery depends on the heat input and on the surface area of the indirect coil, which is why cylinders aimed at high-demand households advertise a large coil.

The physics is straightforward. Raising 180 litres of water by 45°C takes roughly 9.4 kWh. A 3kW immersion heater on its own therefore needs over three hours, before losses. A boiler putting a useful 20kW into a well-matched coil does the same job in well under an hour. That gap is why the immersion heater is a backup rather than a plan, and why an undersized coil quietly turns a well-sized cylinder into a once-a-day resource.

Space, Weight and Access in a London Home

The practical limits often decide the final choice. A 210 litre cylinder holds 210kg of water before the cylinder itself is counted, all bearing on one small footprint, so upper-floor airing cupboards and loft positions need the floor structure assessed rather than assumed.

Footprint matters too. Common unvented cylinders in the 150 to 250 litre range sit at roughly 550 to 600mm in diameter, which is more than many original airing cupboards in Victorian and Edwardian conversions were built around. Slimline models trade diameter for height, and horizontal models exist for loft spaces, but both change the pipework layout. There also has to be room to withdraw the immersion heater and to reach the inlet controls and the tundish for servicing, which is regularly overlooked until the first service visit.

Bigger Is Not Automatically Better

Oversizing has real costs. A larger cylinder loses more heat standing still, takes longer to reheat from cold for the same heat input, takes more space, and in a hard water area presents more heated surface for scale. It also does nothing at all for flow rate, which is the complaint most people actually have. The aim is a cylinder that comfortably covers your worst realistic hour, not the largest one that will fit through the door.

What a Survey Should Establish Before Quoting

• Measured incoming static pressure and flow rate at the property.
• The number of bathrooms and, specifically, which outlets could run at the same time.
• Bath against shower habits, and the shower types fitted or planned.
• Available footprint, height, floor structure and service access at the proposed position.
• Available heat input, since coil size and boiler output determine reheat.
• Whether an extension or loft conversion is planned, because it is far cheaper to size for it now than to replace later.

That last point comes up often enough to be worth planning around — our article on why an extension or loft conversion means resizing the heating system covers the same logic for radiators and pipework. When you are ready to have the sizing done properly, our central heating system upgrade service covers the survey and the specification together.