Hot Water Cylinder Installation and Replacement in London

Choosing a cylinder

Vented, unvented, thermal store or heat pump cylinder: which one suits your home?

A vented cylinder is fed by a cold water tank in the loft, so the hot water arrives at tap pressure set by the height of that tank. It is simple, cheap to run and forgiving, but showers often need a pump to feel strong. An unvented cylinder, sometimes called a Megaflo after one well-known brand, is fed straight from the mains, so hot water reaches taps and showers at mains pressure and the loft tank can usually be removed.

A thermal store works the opposite way round. The tank holds heated water that is not used directly: it passes through a heat exchanger to warm the mains water as you draw it off. Thermal stores suit homes with solid fuel, solar or several heat sources, but they are bulky and less common in London. A heat pump cylinder is an unvented cylinder with a much larger internal coil, because a heat pump delivers water at a lower temperature than a boiler and needs more surface area to transfer the heat.

For most London houses and flats with a gas boiler, the decision is between a like-for-like vented replacement and an unvented upgrade. Unvented is usually the better choice where there are two or more bathrooms, where you want a stronger shower without a pump, or where you are removing the loft tank to free up space. It does need good incoming mains pressure and flow, which we check at the survey before recommending anything.

Sizing

How to size a hot water cylinder by bathrooms and occupants

Size is chosen by how much hot water you use at the busiest time of day, not by the size of the property. A rough starting point is about 35 litres of stored hot water per person per day, then adjusted for how many baths, power showers or rainfall heads run at the same time. A couple in a flat with one shower often needs far less storage than a family of five with a bath and two showers in use on a school-day morning.

Oversizing is a real cost. A cylinder that is too big takes longer to heat, loses more heat while it waits and takes up cupboard space you may not have. Undersizing means cold showers for the last person in the queue. Good sizing also looks at the boiler or heat source: a small boiler heating a large cylinder will have a slow recovery time, so the two have to be matched.

At the survey we count the bathrooms, ask how many people live in the home and when they wash, measure the cupboard or loft space, and check the floor and doorway for the cylinder you are considering. You then get a written recommendation with the reasons for the size. If you would like a ballpark before we visit, see our pricing page for how packages are built.

Safety devices

G3 regulations, expansion vessels and the valves on an unvented cylinder

Because an unvented cylinder is a sealed vessel full of heated mains water, Building Regulations requirement G3 says the stored water must never exceed 100°C and any discharge must be carried away safely. Approved Document G meets this with at least two independent safety devices on top of the normal thermostat: a non-self-resetting thermal cut-out on each heat source and a temperature relief valve (usually a combined temperature and pressure relief valve) that opens if the water overheats. An expansion vessel or internal air bubble takes up the extra volume as the water heats, and an expansion relief valve opens if the pressure climbs too high. Together they keep the temperature and pressure within safe limits.

Approved Document G says an unvented system should only be installed, commissioned or altered by a person competent in unvented hot water systems. Installing or replacing one is notifiable building work, so it must be self-certified by an installer registered with a competent person scheme or notified to Building Control before work starts, and you should receive a commissioning record and the manufacturer's documents. Keep these safe: they matter when you sell or let the property, and a manufacturer may ask for them if there is ever a warranty claim.

The safety valves discharge through a tundish, a small visible air gap in the pipework, into a discharge pipe. The pipe from the valve to the tundish is the D1 pipe, which must be no smaller than the valve outlet, with no more than 600 mm of pipe between the valve and a vertical tundish fitted in the same space and lower than the valve. The pipe after the tundish is the D2 pipe, which must normally be at least one pipe size larger than the valve outlet (larger still on long runs), drop vertically at least 300 mm before any bend, then fall continuously at no less than 1 in 200, be metal or another material shown to withstand the discharge temperature, and end somewhere safe and visible. Getting this route right is often what decides where an unvented cylinder can go in a London flat.

Water safety

Legionella, storage temperature, immersion heaters and timers

Stored hot water that stays lukewarm is where Legionella bacteria can multiply. HSE guidance is to store hot water at 60 degrees Celsius or above and to reach at least 50 degrees at taps within about a minute, though this does create a scalding risk, so thermostatic mixing valves are used at baths and showers where children or older people use them. In new homes and conversions, Building Regulations also limit hot water to baths to 48 degrees. Lowering the cylinder thermostat to save energy is a false economy if it lets the water sit in the danger range.

Most cylinders also have an immersion heater as a back-up or as the main heat source. It is a good safety net when the boiler is out of action, but it should be on a timer or controlled sensibly, because heating a full cylinder by electricity at peak rates is expensive. A dual-element setup, or an element sized for the cylinder, avoids heating more water than you need.

If a property has been empty, for example a flat between tenants, flush the taps and showers and bring the cylinder up to temperature before use. Landlords have a duty to assess the risk of Legionella in rented homes, and a simple written assessment of the system, the temperature and any dead legs in the pipework is usually enough for a domestic property.

Boiler and cylinder

Combi boiler, or system boiler with a cylinder?

A combi boiler heats water as you use it and needs no cylinder, which suits flats and homes with one bathroom and light demand. The limit is flow: when two taps or showers run at once, the boiler divides its output between them. A system boiler with an unvented cylinder stores hot water in advance, so several outlets can run together at good pressure. Households with two or more bathrooms often find that stored hot water is the better fit.

Replacing the boiler and the cylinder at the same time makes sense when both are old, because the cylinder coil and controls are matched to the boiler, and one visit saves disruption. If you are weighing up a switch from a regular boiler and loft tanks to a system boiler and unvented cylinder, the two jobs are best planned together, and our pages on boiler installation and boiler servicing and replacement explain the boiler side.

If you are considering a heat pump in the next few years, tell us. A cylinder with a larger coil, or one that can later take a solar input, costs a little more now but avoids replacing it twice. Where it makes no sense we will say so and recommend a straightforward like-for-like cylinder.

Old tanks

Removing old copper cylinders and loft tanks

When you switch to an unvented cylinder, the cold water storage tank and the feed-and-expansion tank in the loft are usually no longer needed. Taking them out frees space, reduces the risk of a loft leak or frozen tank, and removes a place where debris can settle. Redundant pipework is capped back or taken out so there are no dead legs that can hold stagnant water.

Old copper cylinders are heavy and awkward to move, particularly on stairs and in flats without a lift. We drain them, cut them down if needed and take them away for recycling, so you are not left with a cylinder in the garden. Some older cylinders and lagging may contain materials that need careful handling. If you suspect asbestos in old pipe lagging, tell us before work starts and we will pause until it has been checked.

If you are keeping a loft tank for another reason, such as a vented secondary system, we leave it in place and check that it is properly covered, insulated and clean. Any loft changes can be combined with other work, such as a loft conversion, in which case the tanks usually have to move anyway.

Future-proofing

Solar-ready and heat-pump-ready cylinders, and keeping costs sensible

Some cylinders have a second coil near the bottom, which can take heat from solar thermal panels or another low-temperature source, while the upper coil is fed by the boiler. A heat pump cylinder has one large coil for the same reason: low-temperature water needs far more surface area. If either might be in your plans, fitting a cylinder with the right coils now is far cheaper than replacing a working one later.

Not everyone needs this. If a heat pump or solar is a distant idea, a good standard unvented cylinder with proper insulation is usually the sensible choice, and Part L of the Building Regulations already expects cylinders to meet insulation and control standards. Ask at the survey and we will explain which option you are really paying for, and which extras you can leave out.

What drives the final cost is the size of the cylinder, whether the new one goes in the same place, how much pipework and discharge route is needed, and whether loft tanks are being removed or the mains supply needs work. You can see how packages are built on our pricing page, or ask for a written estimate after the survey so there are no surprises on the day.

Direct or indirect

Direct and indirect unvented cylinders: what is the difference?

An indirect cylinder is heated by a coil connected to your boiler. The boiler water circulates through the coil, heats the stored water and returns to the boiler, without the two ever mixing. This is the usual choice in homes with a gas boiler, and it normally also has an immersion heater as a back-up. When people talk about a cylinder for a boiler, they mean an indirect one.

A direct cylinder has no coil and is heated only by one or two electric immersion elements. It suits homes without a boiler, such as all-electric flats, where heat comes from the electricity supply. It is simple and compact, but the running cost depends entirely on your electricity tariff, so it is worth checking whether an off-peak tariff or a timer helps.

Some cylinders are sold with both: a boiler coil as the main source plus immersion elements, and sometimes a second coil for solar or a heat pump. The right type depends on what heats your home now and in the near future, so we check the boiler, the controls and the electrical supply at the survey. If you are upgrading as part of a wider heating job, our gas boiler and heating page lists the other services we can combine.

More answers

More questions about cylinder replacement in London