A twin coil hot water cylinder features two internal coils to use the heat from more than one heat source to heat up the water contents of the cylinder. Twin coil cylinders are commonly installed in homes where there is a primary heat source, for example an oil boiler or heat pump, and a solar thermal system also installed. If your heating system comprises of a single heat source, you will not require a twin coil cylinder. However, if you are planning to add solar thermal panels to your existing heating system or if you are upgrading your heating to a new system which combines more than one low-carbon technology, such as an air source heat pump and solar thermal, then you will require a twin coil cylinder.
A twin coil cylinder incorporates two stainless steel internal coils each for connection to a separate heat source. The top coil (also known as the primary coil) connects to the main heat source, such as a boiler or heat pump. The bottom coil (also known as the solar coil) connects to an additional heat source, typically a solar thermal system.
Each internal coil and its corresponding heat source are on separate circuits with each heat source contributing to heating the water stored inside the cylinder. With twin coil cylinders, such as those supplied by Grant UK, the solar thermal system will heat up the water as much as possible while the primary heat source provides additional heat when required.
A twin coil cylinder operates within the wider heating system in a similar way to a single coil hot water cylinder (deigned to work with just one heat source). Cold mains water enters the cylinder. Each internal coil transfers heat from the connected heat source into the water, heating up the water to the desired temperature. The heated water is stored inside the cylinder until there a hot water demand, when the water will then exit the cylinder and flow to the taps or showers.
Grant’s Quick Recovery Twin Coil cylinders are supplied with two dual thermostats. These dual thermostats are a combined control and high limit thermostat which are designed to be fitted in the sensor pocket inside the cylinder. One dual thermostat will control the primary heat source and the second controls the solar thermal system. Typically, the solar thermal system will be prioritised to heat the cylinder water with the main heat source being used when additional heat input is needed.
A twin coil cylinder forms part of the domestic hot water circuit in a heating system. The space heating circuit(s), which feature radiators and/or underfloor heating, will be separate as the space heating demand will be fulfilled by the main heat source, not the solar thermal system.
A twin coil cylinder is needed when two separate heat sources are being used to heat the same stored domestic hot water. The examples below provide the most common scenarios where a twin coil hot water cylinder would be required.
Homes with a gas or oil boiler and solar thermal panels will require a twin coil cylinder to use heat from both sources to fulfil the hot water demand:
A twin coil cylinder combines an air source heat pump with a solar thermal system to allow both heat sources to contribute to the domestic hot water production. In these types of the systems:
Please note, air source heat pumps operate at lower flow temperatures so twin coil cylinders connected to heat pumps must have a suitably sized top (primary) coil which is typically larger than the solar thermal coil. All of Grant’s twin coil cylinders are suitable for low temperature heat pumps.
Homeowners planning future low-carbon upgrades can choose a twin coil cylinder to allow for a second heat source to be added at a later date. If your home is currently using a boiler and you are intending to install solar thermal panels in the future, you could upgrade your hot water cylinder to a twin coil option. The twin coil cylinder can be connected to your existing boiler using the top coil. Your installer would leave the lower coil unconnected until required. Then, when your solar thermal panels are installed, this system can be connected to the lower cylinder coil, combining the two heat sources with ease.
Homes that use only one heat source, such as only a boiler or only a heat pump, do not need a twin coil cylinder. In these instances, a correctly sized single coil cylinder is used instead. The single heat source will operate to fulfil both the space heating demand (via the space heating circuit) and the domestic hot water demand (via the hot water circuit featuring the single coil hot water cylinder). A twin coil hot water cylinder is only required when more than one heat source is being used to meet the domestic hot water requirements of a property.
Air source heat pumps operate most efficiently at lower flow temperatures compared to traditional boilers. An air source heat pump installation will always require a separate cylinder for the domestic hot water production. If the heat pump is the only heat source, a single coil cylinder will be sufficient but if the heat pump is being installed with solar thermal panels, a twin coil cylinder will be required with the heat pump connected to the primary or top cylinder coil.
Any hot water cylinder installed with a heat pump must be compatible for use with this low carbon heating system. Heat pump cylinders feature larger internal coils to maximise heat transfer. Single coil and twin coil heat pump cylinders are also heavily insulated to minimise heat losses and maximise system efficiency.
Solar thermal collectors provide low-carbon heat to the lower coil of a twin coil cylinder. The solar coil’s primary function is to optimise solar energy by heating the water contents from the bottom up. On sunny days, the solar coil can heat the whole cylinder, while on overcast days when there is less solar gain, the main heat source, connected to the top coil, will add additional heat towards the top of the cylinder when required.
A twin coil cylinder working alongside solar thermal collectors and either an air source heat pump or boiler can reduce the overall demand on the main heat source. The solar thermal system will heat up the cylinder water throughout the year, even in diffused sunlight on cloudy days, and the heat pump or boiler will be used to provide additional heat when the cylinder temperature falls below set points that the solar thermal system cannot achieve. Consequently, a twin coil cylinder connected to solar thermal can help lower energy bills because the demand on the main heat source is reduced.
Twin coil cylinders allow two separate heat sources to heat up and provide domestic hot water to a home. The range of benefits twin coil cylinders offer include:
Twin coil cylinders must be correctly sized, installed in a suitable location inside the home, and set up with correct controls to ensure their efficient performance. The size of cylinder required will be determined when your installer completes the system design for your heating system. While every home has different requirements, a four-person household with an average hot water usage will typically require a 250L cylinder.
Grant’s twin coil cylinders are designed to be floor standing, installed vertically, and must be fitted in a frost-free location such as an airing cupboard or plant room. When planning the installation, consideration should include:
During installation and commissioning, the installer will set up the appropriate controls and sensors to ensure both two heat sources supplying the twin coil cylinder are correctly managed. Heating systems using more than one heat source will have a series of controls which must be accurately set up and should not conflict with one another. This is why it is important that a new system and cylinder is designed, installed and commissioned by a suitably qualified and trained installer.
If you are considering a new hot water cylinder for your home heating system, your next step is to contact a local G1 heating engineer. Discuss your heating and hot water requirements with an installer who will advise on whether a twin coil or single coil cylinder will best meet your needs. In the meantime, here’s a quick checklist to summarise when a twin coil or single coil cylinder will typically be needed:
Grant UK supplies a wide range of package heating solutions, including biofuel compatible oil boilers, air-to-water heat pumps, heat emitters, solar thermal systems and hot water cylinders. The Grant hot water cylinder range includes twin coil cylinder options designed specifically to work with a boiler or heat pump which is installed alongside a solar thermal system. Three Quick Recovery (QR) Twin Coil Cylinders are available in 210 litres, 250 litres and 300 litres capacities. The QR Twin Coil Cylinders provide an efficient, reliable hot water storage solution for homes where a Vortex oil boiler or Aerona heat pump is paired with a Grant solar thermal system
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