Twin Coil Cylinder: Do You Need One?

What a twin coil cylinder is

Most hot water cylinders heated by a boiler are indirect. Hot water from the boiler runs through a coil of pipe inside the tank, heats the stored water through the coil wall, and goes back to the boiler. The water you draw at the taps never mixes with the boiler water.

A twin coil cylinder has two of those coils, one above the other, each with its own pair of connections. The lower coil sits in the coolest part of the tank and is meant for a source that delivers lower temperature heat, which is why solar thermal panels are almost always connected there. The upper coil is connected to the boiler, which tops the water up to a safe storage temperature whenever the lower source has not done enough. There is normally an immersion heater as well, as a backup.

Twin coil cylinders are made in both vented and unvented versions. An unvented twin coil runs at mains pressure with the same safety controls as any other unvented cylinder, and it has to be fitted by someone qualified for unvented work under Part G3 of the Building Regulations.

How to tell if you have one

Count the pipe connections on the side of the cylinder, not counting the cold feed at the bottom and the hot draw off at the top. Two connections means a single coil. Four means two coils. A small box on the wall nearby with a pump, a pressure gauge and a digital controller, usually labelled for solar, is the other giveaway.

The data label on the cylinder will also say. Look for the words twin coil, solar or duo in the model name, and a second coil listed in the specification. Our guides to direct vs indirect hot water cylinders and to identifying brands such as Megaflo, Joule or Gledhill show where the label usually sits.

When a twin coil cylinder is worth it

The reason to decide at purchase is simple: a coil cannot be added to a cylinder later. If a second source is realistic in the next ten to fifteen years, the twin coil is the cheaper route over the life of the tank.

A heat pump on its own is a different case. It delivers heat at a much lower flow temperature than a boiler and needs far more coil surface area to heat the tank in reasonable time. A cylinder built for a heat pump has one very large coil for exactly that reason, and the lower coil on a solar twin coil cylinder is usually sized for solar panels, not for a heat pump. Check the maker's data before assuming it will work. Our heat pump hot water cylinder guide explains what to look for.

A wood burning stove with a back boiler is another heat source people ask about. Solid fuel is an uncontrolled heat source, because you cannot switch a fire off the way you can a boiler, and manufacturers generally do not allow it to heat an unvented cylinder directly. That setup needs a vented cylinder or a thermal store designed for it.

• Solar thermal panels are fitted now, or are genuinely planned within the life of the cylinder
• A hybrid setup where a heat pump and a boiler both heat the water, and the cylinder maker specifies twin coils for it
• Another heat source the manufacturer approves for the lower coil

When a single coil is the better buy

If the boiler will be the only heat source, a twin coil gives you nothing for the extra cost. The unused coil still takes up room inside the tank, the cylinder has two more connections to keep watertight, and some twin coil models are taller than the single coil equivalent, which matters in a tight London airing cupboard.

The same applies to homes heated only by electricity. A direct cylinder with immersion elements, or an electric hot water cylinder on an off peak tariff, is the right choice there. If you want to use the sun without a second coil, solar PV with a diverter feeding the immersion heater does it with no extra plumbing at all.

An unused solar coil: what to do with it

Many twin coil cylinders we find have a solar loop that no longer works. The panels leaked, the fluid cooked, the pump failed, or the controller died and nobody replaced it. The boiler coil has carried on heating the water, so the household often has no idea.

An unused lower coil does no harm on its own, but a half working solar loop can. If the pump station gauge reads zero, or there is fluid staining around the pipework, the loop should be properly decommissioned: drained, the old fluid disposed of, and the pipes capped at the cylinder. Our guide to solar thermal hot water covers whether a dead system is worth repairing instead.

Owners sometimes ask whether the boiler could be connected to both coils to heat the tank faster. On some cylinders that can be done, but it depends on the maker and the system design, and it is not a quick fix for slow recovery. If the hot water is slow to come back, the cause is more often the boiler settings, the controls or a scaled coil. Our guide to why hot water runs out quickly works through those first.

Replacing a twin coil cylinder

When a twin coil cylinder fails, the choice depends on the solar system. If it still works and is worth keeping, replace like for like with a twin coil of a suitable size, and reconnect the solar loop, the pump station and the controller sensor. If the solar is dead and not coming back, decommission it and fit a standard single coil cylinder. That is usually simpler, sometimes smaller, and the space saved can matter.

Either way, it is worth checking the size. A lot of solar cylinders were oversized on purpose so the sun had a large volume to heat. Without solar, that extra volume is just more water to heat and keep hot. Our guide on what size hot water cylinder you need helps you size the replacement for the people in the house now.

The cost of a replacement is set by the cylinder model, the pipework changes, whether the solar loop is reconnected or decommissioned, and the access. Our hourly rate and call-out are on the pricing page and agreed before we attend. For the wider picture, what a cylinder replacement actually costs explains what goes into the total.

Talk to an engineer

If you have a twin coil cylinder with a solar loop that has stopped working, or you are replacing a cylinder and are not sure whether to keep two coils, call 020 4652 5111. We will look at what is connected, tell you whether the solar side is worth keeping, and fit the cylinder that suits how your home heats water now. We have 20 engineers across London.