How to Choose a Hot Water Cylinder: What Actually Decides It

Start with the incoming main, not the cylinder

Measure the flow at the kitchen cold tap before you look at a single product page. Fill a bucket of known volume with everything else in the property turned off and time it. Twenty litres a minute or better and a mains pressure cylinder will perform the way the brochure suggests. Around ten litres a minute and the supply is the job, not the cylinder.

A sealed unvented cylinder does not create pressure or flow. It stores hot water and hands out whatever the street main hands it. Fit a 250 litre unvented cylinder to a supply running at twelve litres a minute and you have bought a very good vessel and the same disappointing shower. In London the cause is usually old lead or fifteen millimetre pipe from the stopcock in the pavement, or a shared rising main up a converted terrace. In a flat, test morning and evening, because what the neighbours are doing changes the answer.

If the main is poor you have three honest options: improve the supply, which changes more than any appliance you can buy, keep a vented system and pump the outlets that need it, or fit an accumulator. None of those is decided by the cylinder, which is why the test comes first.

• Run the bucket test at the kitchen cold tap, which is normally fed straight off the main.
• Twenty litres a minute upward: an unvented cylinder will do what it says.
• Mid teens: it works, but two outlets at once will feel ordinary.
• Around ten or below: solve the supply before spending money on storage.

Vented, unvented or thermal store: which suits the property

If you have a decent main and a cupboard with a route for a discharge pipe, choose unvented. If the loft cistern works, the household has one bathroom and nothing bothers you, keep the vented system and repair what breaks. If you are in a block fed from a communal heating circuit, the choice has already been made and it is a thermal store.

A vented cylinder is copper, gravity fed from a loft cistern and open to atmosphere, so nothing inside is under pressure and nothing needs a scheduled safety check. What it costs you is the loft, and pressure set purely by how high that cistern sits above the outlets.

An unvented cylinder is a sealed stainless vessel fed off the main, so hot and cold arrive at the same pressure and thermostatic showers behave as designed. It frees the loft, which is why loft conversions force the decision. In return it wants a yearly check, because its relief valves and expansion arrangement are only proven by being operated, and it needs a discharge route ending outside where near boiling water can land without scalding anyone. Installing one is notifiable work for an engineer who holds the qualification.

A thermal store inverts the logic: the vessel holds heated water open to atmosphere and your drinking water passes through a heat exchanger inside it, arriving hot at mains pressure. In purpose built London blocks that is usually a Pulsacoil or similar fed from the landlord circuit, and replacement is about matching the block rather than choosing freely.

In a leasehold flat, check the lease and the managing agent before you specify anything. Blocks often restrict what can be fitted and where a discharge can terminate.

Direct, indirect or twin coil: how the heat gets in

Indirect means a coil inside the cylinder carries hot water from your boiler, with an immersion heater as backup. Direct means no coil, and electric elements are the only heat source. Twin coil means two coils, so a second source such as solar or a heat pump can feed the same vessel. With a working boiler, indirect is the answer nearly every time.

Direct cylinders exist for good reasons: flats with no gas, blocks where a boiler is not permitted, conversions where a flue cannot be routed. They work, but a resistive element is slow, so a direct cylinder wants to be larger than the indirect equivalent and wants a tariff that lets you heat overnight.

Twin coil is the buy if a heat pump or solar thermal is plausible within the life of the cylinder, because adding a coil later is not a modification, it is a new cylinder. Heat pumps deliver heat at a much lower flow temperature than a boiler and need far more coil surface area to move the same energy, which is why a standard indirect cylinder on a heat pump reheats slowly and gets blamed for it.

• Working boiler, normal house: indirect, with an immersion as backup.
• No gas, or a flat where a boiler is not an option: direct, sized generously and heated overnight.
• Solar or a heat pump now or later: twin coil, decided at purchase.
• Communal heating in a block: usually a thermal store matched to the block.

How many litres do you actually need?

Size for the busiest hour, not the day. One or two people with a single bathroom sit around 120 to 150 litres. A family of three or four with a bathroom and an en suite wants 180 to 210. Four or five people across two proper bathrooms wants 250 to 300. Three bathrooms, or a bath anyone genuinely fills, and you are above 300.

Two things move those bands. Stored water sits near 60 degrees and nobody showers at 60, so a cylinder hands out roughly one and a half times its own volume as usable warm water. And draw rate matters: pull water out fast and incoming cold mixes upward, so the last quarter arrives lukewarm. That is why the same cylinder copes with two showers one after another and fails when they run together.

Flow at the head moves the answer more than household size does. An ordinary shower head runs nine to twelve litres a minute, a wide rainfall head fifteen to twenty, which roughly halves how long the cylinder lasts. Size for the bathroom you are going to have, because the cylinder will outlive the shower. If you land between two sizes, there is more detail on working out cylinder capacity, and the short version is that the smaller vessel is fine if it reheats fast.

Recovery time is the spec nobody reads

Capacity is how much you have. Recovery is how fast you get it back, and it is set by the surface area of the coil rather than by the size of your boiler. A high recovery cylinder with a generous coil can absorb something like eighteen to twenty four kilowatts and bring a full tank back to temperature in half an hour. An older cylinder with a short narrow coil can take two hours from the same boiler.

The arithmetic settles arguments. Raising 180 litres by fifty degrees takes a little over ten kilowatt hours of heat. Through a coil absorbing eighteen kilowatts that is about thirty five minutes. Through a three kilowatt immersion element the same ten kilowatt hours take about three and a half hours. Same water, entirely different household experience.

So when a household keeps running out, the fix is often recovery rather than volume, and a faster cylinder on the same footprint solves a morning that a bigger one would only solve by taking space the cupboard has not got. Ask for the coil rating in kilowatts and the reheat time from cold. Hard water then works against recovery for the whole life of the unit, because scale bakes onto the coil and the element first. A cylinder that coped when it was fitted and no longer copes has usually slowed down rather than shrunk.

• Ask for coil surface area or coil kilowatt rating, and reheat time from cold.
• A high recovery coil is the cheapest upgrade on the whole order.
• An immersion heater is a backup, not a recovery strategy.
• Assume the reheat gets slower every year unless the water is treated.

Will it fit in the airing cupboard?

Measure four things before you order: the cupboard footprint, the ceiling height at the cupboard, the narrowest point on the route in, and the floor. A 300 litre cylinder stands over one and a half metres tall, weighs more than 300 kilograms when full, and has to be tipped upright inside the room it will live in. On London staircases the route kills more specifications than capacity ever does.

Diameter comes first. Standard cylinders are commonly around 545 millimetres across and slimline versions around 450, and the slim one gets its capacity back by being taller. In a Victorian conversion with a boxed in cupboard beside a chimney breast, the slim tall unit is often the only thing that goes in, and then you need the ceiling height to stand it up. Under a sloping top floor ceiling the answer is a horizontal cylinder or a rethink.

Then the kit around the vessel, because an unvented cylinder is not just the cylinder. It needs room for the inlet control group on the cold feed, for an external expansion vessel where the design uses one, and for clearance above the crown to withdraw the immersion element without cutting the cupboard apart. Leave room for a spanner, because a cylinder crammed in with no working space costs money every time it is serviced.

The discharge route is what stops jobs dead. Anything a safety valve releases runs through a tundish you can see, then away at a continuous fall in the correct material, terminating outside where near boiling water cannot scald anybody. In a second floor flat over a shop, or a basement conversion, that route is often the hardest part of the job and wants settling before the order.

Finally the floor. Cylinder weight sits over a small footprint, and joists at the top of a converted house are not always up to a 300 litre vessel where a 120 litre copper one used to be. In a new build, nothing gets fixed through liquid screed over underfloor heating without knowing what runs beneath it.

• Cupboard internal width and depth, plus clearance for the valve set and vessel.
• Ceiling height at the cupboard, for standing the cylinder upright.
• Narrowest point on the route: front door, stair turn, loft hatch, cupboard opening.
• Headroom above the crown to withdraw the immersion element.
• A viable discharge route from the tundish to a safe point outside.
• What the floor is built from and what it will carry when the cylinder is full.

Stainless steel, copper, and what makes a cylinder last in London

For a mains pressure cylinder, stainless steel is the only sensible answer and it is what the market has settled on. Copper is fine on a vented system where nothing is pressurised. What decides how long either lasts here is not the metal so much as the water, the temperature it is held at, and whether anyone ever looks at it.

Duplex stainless is rolled thin, which is why a modern cylinder weighs a fraction of the copper one it replaces, and a thin vessel is unforgiving of poor water chemistry. Scale lands on everything hot: it steals recovery from the coil, burns out elements, and settles on relief valve seats, which is why a valve quiet for six years suddenly passes and puts a trickle down the tundish. Holding storage temperature sensibly slows all of it and costs nothing. What hard water does to a cylinder over its life argues for a serviceable design over a sealed pretty one. A serviceable design is also what keeps an unvented cylinder repair possible years later, because a unit whose controls can be reached is a unit that can be kept alive.

What to look at when choosing between models

Choose on serviceability and spares, not on the photograph. The question that matters in year seven is whether the parts are ones an engineer carries or can get the same day, and whether the thermostat, the thermal cut out, the element and the relief valves can be changed individually. That is an opinion and we will defend it, because everything that fails on these units is a part rather than the vessel.

Prefer an external expansion vessel over an internal air gap design where the cupboard allows it, because an external vessel can be checked and, when it fails, unbolted and replaced in one visit. Air gap designs are tidier and take less space and they work well, but restoring them is a job on the cylinder itself rather than a part swap.

Prefer a standard immersion boss over a proprietary one. A stock size element means any engineer fits a new one from van stock in an hour, where a proprietary element means waiting on a manufacturer order, and in ten years it means hoping the part is still made. The same goes for control sets: a common inlet group beats an integrated module where one failure means buying the assembly.

Then ask whether the warranty depends on an annual service being carried out and recorded, because most do.

• Published spares list, available to any qualified engineer.
• Standard immersion boss and a standard element size.
• Separately replaceable relief valves and thermostat rather than a sealed module.
• External expansion vessel where the cupboard has room for one.
• Clear stated requirements for keeping the warranty alive.

The order to settle it in

Run the decision in the order that eliminates options fastest, which is the order an engineer uses on a survey. Supply first, because it decides the type. Type next, because it decides what has to fit. Space and discharge route next, because they decide the shape and often the capacity. Heat source and recovery next. Capacity last, chosen from whatever survived. The same order applies to a hot water cylinder replacement in an existing property, with the single difference that the discharge route is usually already there and needs checking against what the new appliance requires rather than inventing from nothing.

• One: measure the incoming main with a bucket and a watch.
• Two: choose vented, unvented or thermal store from that result and the property.
• Three: measure the cupboard, the route in and the discharge route out.
• Four: decide the heat source and ask for coil rating and reheat time.
• Five: pick the capacity from what is still standing.