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Which Cylinder For My Heat Pump?

There are many different cylinder manufactures and they each have their own standard product range. On top of this; most manufacturers will ‘tailor make’ you a cylinder to your to your exact specification and requirements, and they have it down to a fine art. In some cases a cylinder can be ordered in the morning and collected in the afternoon!

There are plenty of variables when choosing a cylinder and it is not one size fits all; the key is to select the right cylinder for your job. People refer to them as a battery because they are a store of ‘charged’ hot water. It is important to find the balance between storage capacity, demand, and heat input energy required for your battery. These questions all relate to size and occupancy level of your property including the amount of outlets within kitchens, bathrooms and en-suites, and in particular; mixer showers. Further tips and measures about containing hot water flow rate are available further down the page.

We then have to consider the heat source for our cylinder or battery, which in our case is an air source heat pump. We recommend using a thermal store although unvented cylinders can also be used. Thermal stores are popular on the continent and particularly in Germany where hot water regulations are more stringent than ours. Their popularity is due directly to their efficient nature, versatility of connecting to multiple energy sources and quality of mains pressure supply.

Within air source heat pump systems a buffer vessel or buffer tank is required, a thermal store will eliminate the need for a buffer tank and provide a host of other benefits.

Explained firstly;

Used in conjunction with air source heat pumps, a buffer tank is installed in addition to the hot water cylinder, within the primary heating circuit. It increases the volume of water a heat pump has to heat, allowing excess heat to be stored within the buffer tank until it is required. Buffer tanks increase response times when the heating is turned on due to this store of heat. Buffer vessels reduce the short cycling of the heat pump, and increase COP and lifespan of the heat pump. The diagram below shows a buffer tank in conjunction with a domestic hot water unvented cylinder.

The diagram above shows a heat pump coupled with an unvented cylinder. Unvented cylinders are full of potable sanitary water, safe for us to drink and supplied at mains pressure. This is a key benefit of an unvented system and means we can, within reason supply multiple hot water outlets (like showers) at the same time. A combination boiler or gravity based system would not produce enough pressure to do this.

The primary water in the flow from the air source heat pump is used to heat up the water in the cylinder through an indirect method; it is pumped through the coil at the bottom of the unvented cylinder. In the diagram above, the unvented cylinder is labelled as ‘the tank’ while the coil is labelled as the ‘heat exchanger’ which is exactly what it is.

As heat pumps run at lower temperatures, it is imperative that the coil within the cylinder is sized correctly in terms of exposed surface area in order to achieve the correct transfer of heat. Failing this the sanitary water within the cylinder will not get hot enough. All the cylinders we fit come with a built in immersion heater in order to boost temperatures when required and also for the legionella protection cycle.

The unvented cylinder in the diagram above has only a single coil. Twin coil cylinders are available should you wish to combine your heat pump with solar thermal panels. Doing this will increase your systems efficiency, heat pump COP and electricity consumption. Solar thermal panels can produce 80% of your hot water annually, and even in winter months they can act as a pre heat for your heat pump. Again the second solar coil will need to be sized correctly to your systems needs, taking into consideration amount of solar panels and cylinder size.
The size of cylinder you require is linked to your property size and occupancy level. The recommended amount of daily hot water per person per day is 45L. Generally, bigger houses will require bigger cylinders and consideration should be taken in respect to cylinder location; whether or not it will fit in the cupboard, through the door way and whether the floor is strong enough to take the weight when full. Early planning to a whole system approach is recommended. This will help prevent problems that could arise at a later stage.

A thermal store is similar in comparison to an unvented cylinder, not least in appearance.

They are both coated in a super insulated spray foam casing and they both contain a body of water.

However, they are in fact opposites; the main difference is within the water they contain and how they function.

An unvented cylinder contains a large amount of sanitary water which is heated when primary water from the heat pump is fed through a heat exchanger coil

A thermal heat store is a large amount of primary water kept warm by the heat pump.

The heat pump feeds water directly into the cylinder. Sanitary water is fed through a large coil inside, from which it draws the heat out of the primary water.

Water enters the cylinder cold and exits as hot.

Sometimes, instead of a large coil through the centre of the cylinder a plate heat exchanger is located on the outside of the cylinder and hot primary water from inside the cylinder is used to heat the cold mains water.

Some manufacturers say this is better for stratification within the cylinder.

Underneath all the insulation, thermal store cylinders are generally constructed of copper unlike the stronger stainless steel casing required for the pressurised unvented cylinder. The reason for this difference is that a thermal store (unlike a an unvented cylinder) is not a pressurised store, it is in fact a ‘vented system’ all be it a ‘sealed vented system’ meaning it is not pressurised but also meaning neither the hot water circuit or the heating circuits are open to the atmosphere because they are sealed. There is no chance of any air entering the pipe work to cause problems; so there is no danger of damage or irritation to any of the system that is usually associated with the ‘old type’ vented systems.

There are a lot of benefits to a thermal store that make them appealing in certain situations:

A thermal store allows us to ‘dump heat’ and save it for later when we need it.

Contribute to both the supply of heat hot water and the supply of heat to the heating system without the need for a buffer vessel, unlike an unvented cylinder.

With a thermal store we can connect to multiple sources of heat, where as with an unvented cylinder you can only have two coils (up to cylinders of a very large size) before the heat exchange would not be effective. This means that with only two coils we are limited to only two inputs of heat, for example the heat pump, and solar thermal panels.

Due to the pressures within the unvented system it is dangerous and against regulation to connect an unvented cylinder to a biomass boiler or wood burning stove etc. However, with the thermal store this is not a problem due to it being vented low pressure.

The thermal store will however deliver domestic hot water at mains pressure because the hot water coil or exchanger is connected directly to the mains.

They offer huge versatility and many options of energy input; we can connect oil, gas, and bio gas boilers, wood chip, wood pellet or wood burning stoves, agars, solar thermal panels, air source heat pumps and ground source heat pumps. Basically anything that can generate heat, you can plumb in!

There is no danger of lime scale build up within the cylinder in hard water areas as the water within the cylinder is not been constantly replaced (like with an unvented cylinder.)

There is no danger of legionella as we are not storing any hot water we are to draw off.

Schematic Layout Of Thermal Store With Heat Exchanger On Outside

Hot Water Savings

When deciding on a thermal store, or any system, try and think of it as a whole house approach and address all the issues, making sure you have all the ‘pieces of the jigsaw puzzle in place.’

Especially important with a thermal store is mains pressure and flow rates at the end point of use. This needs to be controlled to achieve the right balance: Too high a flow rate and not enough energy will be taken from the exchange within the cylinder to warm the water adequately.

We control this by; fitting pressure reducing valves before the thermal store, so as not to exceed the limit recommended by the manufacturer.

We also need to watch our water consumption at the end point of use.

Using more hot water than is actually needed not only wastes the water we pay for but also the energy we use from the thermal store to heat it (which we also pay for).

Now you may be thinking ‘I agree but when I turn the tap on I want hot water and I want enough of it! In this situation you can ‘have your cake and eat it’

At each hot water outlet, shower head or tap we are able to regulate this consumption without compromising on performance, we fit flow regulators and aerators: The flow regulator slows the flow of hot water.

The benefit of this is that in turn the pressure is increased; brilliant for showers.

The aerator then mixes this high pressure jet of water with air to create a nice creamy flow with a fuller coverage.

So whilst less water is actually supplied it feels like more, due to the increased pressure and mix with air. This mix with air also has the added benefit of preventing splashing.

Flow regulators and aerators will limit flow and prolong heat reserves, especially in peak demands. They are the final piece of the puzzle to your complete energy efficient hot water system using a thermal store or unvented cylinder. They are essential when buying new and can be retrofitted to existing taps.

When you purchase your shower, you should be aware of the following:

A fully operational power shower should use no more than a maximum of 11 litres per minute
An Eco Shower uses 7 litres per minute
A Norwegian Eco Shower uses 4.5 litres per minute

If you have a 500 litre tank (of which 320 litres is hot water), you can have:

3 (fully pressurised) showers of ten minutes' duration, or
5 Eco showers of nine minutes' duration, or
7 Norwegian Eco Showers of ten minutes' duration

Building and insulation standards have improved considerably over the past number of years which has meant that it's possible to reduce boiler and heat pump sizes. However, simultaneously, our demand for hot water has increased (more bathrooms, shower rooms, ensuites, wet rooms, swimming pools) and this has resulted in driving the size of boiler and heat pumps back up again. It is vital that you consider your hot water requirements and that your domestic water system is designed properly. You need to install the appropriate size tank and pipe to ensure efficiently delivering enough hot water.