Choose a single coil hot water cylinder if the boiler is the only heat source feeding it. Choose a twin coil cylinder if a second source, almost always solar thermal, heats the same stored water. The second coil sits low in the vessel where the water is coolest, and the boiler reheats the upper zone through the other coil. Without a second circuit connected, the extra coil stays cold.
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Single coil and twin coil hot water cylinders compared: coils, heat sources and stored volume
A single coil hot water cylinder has one heating coil, fed by the boiler. A twin coil cylinder has two coils, so a second heat source such as solar thermal can heat the same stored water.
| Criterion | Single coil cylinder | Twin coil cylinder |
|---|---|---|
| Heat sources | One coil, so one wet heat source, normally the boiler. An immersion heater can sit alongside it as electric backup. | Two coils, so input from two sources, most often solar thermal supported by a gas or oil boiler. |
| Where the coils sit | The single coil is fed by the boiler flow and return. | Lower coil takes the solar flow and return, upper coil takes the boiler. |
| Coil sizing | One coil, sized for boiler flow temperatures. | The two coils are not equal: on one 500 litre model the upper coil is 2.3 square metres and the lower 1.0 square metres. |
| Stored volume | The boiler heats the whole stored volume. | Volume is split into a dedicated solar zone and a boiler zone, sized to Building Regulations and SAP. |
| Adding a source later | The coil is built into the vessel, so a second coil cannot be added afterwards. | The second coil is already in place when a solar circuit is connected. |
The second coil only earns its place if a second heat source is actually connected to it. With no solar circuit on the lower coil, you have paid for a heat exchanger that stays cold.
What the second coil in a twin coil cylinder actually heats
The second coil takes heat from a source other than the boiler, in most homes a solar thermal circuit. A pumped closed circuit carries heated fluid from the collectors to the lower coil, which sits in the coolest water at the bottom of the vessel. Those low temperatures keep the heat transfer working even when sunlight is weak. The boiler feeds the upper coil and reheats the warmer zone nearer the draw-off.
Solar input rises and falls with the weather. Manufacturer instructions for solar cylinders therefore require a backup heater, either a conventional boiler or an immersion heater. On twin coil unvented models the upper coil is connected to the boiler with a dual thermostat and a high limit thermostat, which is a cut-out that stops the stored water overheating.
HSE guidance for employers, people in control of premises and landlords in Great Britain is to store hot water above 60°C and distribute it above 50°C. Legionella bacteria multiply between 20°C and 45°C and do not survive above 60°C. The boiler side of a twin coil cylinder is what brings the stored water up to its set temperature, whatever the collectors have contributed.
When a single coil hot water cylinder is the right choice
A single coil cylinder fits any home where the boiler is the only wet heat source. That is the standard arrangement with a system or regular boiler, and manufacturers build indirect single coil ranges around it in sizes from 70 to 300 litres. Typical situations:
- Heating and hot water come from a gas or oil boiler and there are no solar collectors on the roof.
- You live in a flat with no rights over the roof, so a solar circuit is not available to you.
- An old cylinder has failed and the rest of the system stays exactly as it is.
- You want an immersion heater as electric backup rather than a second wet circuit.
- You would rather have the boiler heat the full stored volume than split it into two zones.
Volume and coil surface matter more here than the number of coils. Manufacturer data for one cylinder range pairs 150 litres with a 1.0 square metre coil, 200 litres with 1.4 square metres and 300 litres with 2.8 square metres, and a larger coil surface shortens reheat time. Our guide on what size hot water cylinder you need covers how the size is worked out for a household.
When a twin coil hot water cylinder is the right choice
A twin coil cylinder is the right choice when a second heat source will genuinely be connected to it. Manufacturers describe these cylinders as designed to accept input from two heat sources, most commonly a solar input backed by a controlled appliance such as a gas or oil boiler. Situations where that applies:
- Solar thermal collectors are already on the roof and the old cylinder is being replaced.
- Collectors and cylinder are going in as one project, so both circuits are piped at the same time.
- A solar installation is costed and dated, not just an idea for some future year.
- A larger cylinder has to serve two sources at once, such as a heat pump with solar thermal, which is a heat pump system and not something we install.
Timing is the practical argument. The coil is welded inside the vessel, so the decision is made when the cylinder is bought, not later. If solar is years away and undecided, a single coil cylinder plus an immersion heater usually answers the same need without splitting the stored volume.
How to tell whether your existing cylinder is single or twin coil
Count the pairs of primary pipe connections on the side of the cylinder: a twin coil has two flow and return pairs, a single coil has one. The data badge on the casing gives the model name, and solar models usually carry the word solar in that name. A separate pumped circuit running up towards the roof is the other giveaway.
A single coil cylinder cannot be converted. The coil is part of the sealed inner vessel, so a second one only comes with a new cylinder. Manufacturers answer the same need two ways: a model with a second indirect coil, or a model with electric heating elements for reheating. That second route is what solar PV ready cylinders use, and it is not the same thing as twin coil.
Coil count is separate from whether the cylinder is vented or unvented. Both types exist with one coil and with two. If you are not sure which system you have, our guide on telling a vented cylinder from an unvented one walks through the signs.
What the rules require when an unvented cylinder is replaced in England
An unvented hot water cylinder, including Megaflo unvented cylinders, may only be installed by a competent person, whether it has one coil or two. Requirement G3 of Schedule 1 to the Building Regulations 2010 applies in England, with Approved Document G, 2015 edition incorporating the 2024 amendments, as the guidance. The vessel must have at least two independent safety devices in addition to the thermostat that sets the normal temperature. Temperature relief valves are fitted at the factory and should only be disconnected for replacement.
Unvented systems up to 500 litres and below 45 kW should be installed as a proprietary unit or package. Schedule 3 to the Building Regulations 2010 lists the work an installer registered with a named scheme can self-certify, which includes heating and hot water systems and their controls. Work on the gas appliance side falls under the Gas Safety (Installation and Use) Regulations 1998 as amended and belongs to a Gas Safe registered engineer. Our guide on who can install an unvented hot water cylinder sets out the qualifications.
Nothing on a cylinder's safety devices, and nothing on gas or the flue, is householder work. What you can safely check yourself is limited to boiler pressure, the thermostat and programmer settings, the reset button and bleeding radiators.
Single coil or twin coil: which to specify for your home
Specify a single coil cylinder if the boiler is and will stay the only wet heat source, if you are replacing a failed cylinder on an unchanged system, or if a flat gives you no access to a roof. Specify a twin coil cylinder if solar thermal collectors are already fitted, or if collectors and cylinder go in as one project with a date against them.
Where solar is only a possibility, the honest answer is usually single coil with an immersion heater as backup. You keep the full stored volume on the boiler, and a twin coil cylinder can still be fitted later if the collectors ever go ahead. Where the collectors are real, twin coil avoids buying a cylinder twice.
Cylinder choice also depends on mains flow, cupboard space and how many bathrooms draw water at once, which is what a survey is for. We carry out a free home survey and give you a written fixed price before any work starts, and you can ask about the cylinder alongside a new boiler installation.
Sources
- Worcester Bosch — Greenstore TC Cylinder Series, installation and maintenance instructions
- Worcester Bosch — Cylinders
- Vaillant — auroSTOR 500 litre twin coil cylinder
- Vaillant — Solar heating system with auroSTOR (PDF)
- Heatrae Sadia — Megaflo Eco SolaReady installation manual (PDF)
- Heatrae Sadia — Megaflo Eco Solar
- GOV.UK — Approved Document G, 2015 edition with 2024 amendments (PDF)
- legislation.gov.uk — Building Regulations 2010, Schedule 3
- HSE — Hot and cold water systems
- Baxi — Assure Indirect Cylinders