Most people think that the modern way to provide mains pressure hot water is using either a combination boiler or an unvented hot water cylinder.  This is not usually the case however.

Combination boilers will almost always be the preferred option for properties where low flow rates are acceptable, and there is no need to run fast baths or two outlets at once.  However, for properties with more than the most basic requirements, house builders and local authorities use thermal stores to provide mains hot water.  Unvented cylinder do have their place, however for the vast majority of installation, a thermal store will provide everything that an unvented will, as well as more, and with none of the disadvantages of an unvented cylinder

A Thermal Store can overcome a number of  problems, providing very high flow rates of hot water at FULL mains pressure (no pressure reducing valves).  The utilisation of higher mains pressures also means that higher flow rates can be obtained to taps.

The question has to be, therefore, how to achieve mains pressure hot water from a hot water cylinder that is under no pressure at all ? 

The water in the cylinder is used as a battery for heat. As such it never goes anywhere, certainly not to the taps, but is instead circulated through a 'heat exchanger' that transfers the heat from the hot water in the store, to heat up high pressure mains water.  The water in the store and the mains water never mix,  but are kept separated by stainless steel metal plates in the heat exchanger that transfer the heat.  The heat exchangers are pressure tested to 72bar, far higher than will ever be encountered, and are approved. 

As stores are filled up once only, there is no need for a permanent connection for filling the store, and hence no need for an overflow or discharge pipe.  As such, systems can be as simple as cold in and hot out, with no other pipework connections.

They can also be DIY installed by anyone competent - you don't need to be a registered installer.  Nor do they need regular preventative servicing - the only component that may need servicing is the pump, a standard circulator that should last many, many  years (and is simple to change).

As the water in the store remains the same, there is no introduction of limescale or other salts.  The water in the store should be protected with standard corrosion inhibitor for central heating systems, as this will ensure continued operation of the system, keeping it clean years to come.  Anti-freeze can also be added, making this system suitable for dwellings that are left unheated in the cold weather.

The picture to the left shows a standard indirect Stowaway Heat Bank Thermal Store.

Plate Heat Exchanger 

The plate heat exchanger transfers heat from the stored water to heat mains water, as outlined above. It is recessed through the casing, into the foam insulation.  The heat exchanger has four connections: mains in/out, and store water in/out.  Two models are available as standard.  The 14 plate version will provide 100kW of hot water, while the 28 plate version provides over 150kW.  Heat exchangers can be twinned up (more than one) giving total outputs of over 300kW.

Heat Exchanger Pump  

The heat exchanger pump circulates the water in the store through the heat exchanger.  Setting 1 is enough for most properties, with setting 3 providing maximum output.  One should remember that it is not the water going to taps that runs through this pump, and as such it has no affect on water pressures.  The pump is fitted with unions and isolating valves for easy servicing.

Flow Switch 

The flow switch turns on the heat exchanger pump when a hot water outlet is opened.  The flow switch is fitted onto the incoming mains cold water pipe, and uses an externally mounted sensor to detect movement of a magnet inside the pipe.  It is capable of detecting flow rates less than 1 litre/minute, and can cope with flow rates as high as 45 litres/minute.  To ensure reliability, we have the switches made to our own specifications, using Tectite push fit fittings for fast, reliable connection to pipework.

Thermostatic Blending Valve 

To control the hot water temperature supplied to taps, the unit is fitted with a 1" thermostatic blending valve.  This mixes hot water generated by the heat exchanger with cold water to obtain a set output temperature.  The valves are factory set to approximately 55C, and locked in position to prevent tampering.  The setting can be adjusted by simple removal of a screw in the valve head, freeing the head. The valve is fitted with unions to allow rapid servicing.  Pipework allows standard valves to be upgraded to other types if required.

Filling Loop 
9  54 31

To initially fill the cylinder with water, the system includes a filling loop.  This comprises of a flexible hose that connects between the cold mains and the store, and allows filling.  The fill level valve (56) must always be opened when filling, and the filling loop must always be removed after filling.  A captive cap is provided to blank off the mains supply valve.

On indirect systems, filling loops may also be used for filling of the boiler system if required.

Water Level Switch  55

The level switch provides controls with an indication of whether the unit is full of water.  Normally, the level switch is only use to drive the low water level neon on the wiring centre, to provide a clear warning that the unit is not full. The level switch can also be used to protect immersion heaters against being turned on without water in the store.

Low Water Level Warning Neon  55

To provide a clear warning that the unit requires filling with water, a neon light is fitted clearly visible on the wiring centre, along with instructions on how to re0fill.

Wiring Centre 

All units are fitted with the most suitable form of wiring box.  Usually this is a standard 12 terminal wiring centre, with larger wiring centres used as controls demand.  Controls will require a 3 Amp fused permanent mains supply.

Fill Level Valve 

To indicated if the unit is fully filled, the fill level valve should be opened.  It can be used to provide a visual indication is the unit is full, and to drain off excess water after filling.  The valve should also be opened when draining down the cylinder.

Overfill Chamber and Drain 

Should the unit be overfilled, some water may be discharged on the first heat-up.  The top of the unit contains a chamber that can hold this water.  A drain cock is provided to drain off any water easily.  Small amounts of water will evaporate away in time.  A removable lid is provided to cover the unit.

Expansion Vessel 

Inside the top chamber is located the expansion vessel.  This takes up the volume changes as the stored water expands and contracts with heating and cooling.  This is typically by 3% of store volume.  The expansion vessels DO NOT contain any pressurised air, but instead allow water to freely move in and out of the vessel.

Automatic Air Vent  7

To release air that comes out of suspension (from the stored water) during the first few weeks of operation, the unit is fitted with an automatic air vent.  The cap of the vent can also be removed to provide a handy funnel for adding inhibitor.

Drain Cock 

To assist in servicing, we fit units with a simple drain cock that can be connected to any normal hose.

Y-Pattern Strainer 

To protect all controls from grit in mains water supplies, we supply a Y-pattern strainer loose, for installation onto the incoming mains water pipework.  This should be fitted down-stream of a visible and accessible isolating valve.

The following items are concerned with getting heat into the thermal store, as well as to do with central heating controls.  Select the various systems from the picture selection box at the top of page.

Primary Heating Coil
 21  22

The unit shown here is indirect, heated by passing hot water through a copper coil in the cylinder. The coil is high duty, being larger than the size required by British Standards. Standard connection sizes are 22mm, with the coil itself being made from 28mm copper tube and suitable for pressurised primary (boiler) systems. 

Cylinder Thermostat 

To set the temperature to which the store should be heated, we fit an immersion type cylinder thermostat.  This is factory set to 75C, and labelled with a calibration sticker.  Tamperproof versions available to prevent accidental resetting.

Backup Immersion Heater 

In case the primary heat source be unavailable (boiler problems), a backup 3kW heating element is fitted to heat the top half of the unit electrically. We use a standard Incaloy immersion heater, fitted with control and overheat thermostats.  Heater require site wiring to a supply, with a neon power switch in the cylinder cupboard. 

Boost  Immersion Heater 

In situation where the store is not hot enough for use, a 3kW boost immersion heater is used to heat the top half of the store. This heater is usually powered from a permanent 16A fused power supply, making it always available for use. 

Care should be taken not to over use the boost heater as it can make use of peak rate electricity (almost 3 times the price of the economy).  It is preferable to heat the unit with economy rate electricity as much as possible, only using the boost to top up where necessary.  The heater is positioned to only heat up the top half of the store as this is quicker and uses less electricity than heating the whole store.

Economy  Immersion Heater 

To heat the entire store using cheap rate electricity, a 3kW lower immersion heater is fitted.  We use a low noise heater for economy use, as the heater will be used most often at night. 

The heater is wired to an economy power supply, typically provided at night (Economy 7), and sometimes certain hours of the day (Economy 9, Warmwise).  As this heater is the principal source of cheap heat, it should be used as much as possible to avoid boosting.  Stores are very well insulated and lock in the heat for a considerable period.

Economy  Immersion Heater 

To heat the entire store using cheap rate electricity, a 3kW boost immersion heater is fitted.  We use a low noise heater for economy use, as the heater will be used most often at night. 

The heater is wired to an economy power supply, typically provided at night (Economy 7), and sometimes certain hours of the day (Economy 9, Warmwise).  As this heater is the principal source of cheap heat, it should be used as much as possible to avoid boosting.  Stores are very well insulated and lock in the heat for a considerable period.

Relay Wiring Centre 

Units with immersion heaters as the primary heat source, we take the power supplies to the heaters via a relay wiring centre.  This ensures power supplies are isolated until the unit is full, or if overheat is detected.  A simple overheat reset button can be used to reset the system after overheat has occurred, although control thermostats should be checked.

Immersion Heater Controller 

To enable electrically heated units to make use of a single permanent 16A immersion heater supply, the fitted immersion heater timer allows complete control.  The controller allows times to be set when the economy heater 15 is used, typically set to when electricity is cheap.  It also has a run-down timer for controlling the boost heater, that can be set to anything up to 2 hours.  Simply twist to set and press the boost button and the boost heater will power up for the set time.

Two Channel Programmer 

To set both hot water and central heating times, the fitted Danfoss CP715 provides a simple and reliable solution. The programmer provides 3 on/off settings for automatic control, on constant, on all day, or 1 hour boost settings.  It can also be set to work as a 24 hour, 7 day, or 5/2 day split.

Room Thermostat 

Temperature control for the central heating is provided by the room thermostat. It is supplied loose or site fixing and wiring back to our wiring centre.  The stat tells the system when the boiler needs to fire up to drive heating.

Primary Circulating Pump 

Movement of water around the boiler, heating system, and store heating coil is achieved by the primary pump.  Various sizes and types are available, including modulating.  The pump is fitted with isolating valves and union connections for easy servicing.

Hot Water Zone Valve 

This valve opens and closes to allow boiler water to flow through the store heating coil.  It is fitted with a manual override facility.

Central Heating Zone Valve 

This valve opens and closes to allow boiler water to flow to the central heating.  It is fitted with a manual override facility.

Primary Automatic Air Vent 

To assist in venting air from the primary system, the highest point on the primary pipework is fitted with an automatic air vent.  The vent can be left open while filling to purge air, and it will not let water pass. It can also be left open for the first few weeks of operation to help purge any remaining air in the system.  A backup manual vent is also provided.

Primary Pressure Relief Valve & Gauge 

Sealed primary systems require a pressure relief valve in case the system is overfilled, or the boiler starts producing steam.  This is set to 3 bar, and is fitted with a pressure gauge to show the system pressure.  The relief valve needs to be taken to a visible safe discharge point.  If required, the relief valve can be relocated nearer the boiler.

Primary Expansion Vessel 

To allow for expansion and contraction of the water in a sealed primary system, a 12 litre expansion vessel is provided.  The vessel is attached via 750mm flexible hose, allowing easy movement for servicing.  Normally, the vessel is simply sat on top of the store, however it can be wall mounted if preferred.

By-Pass Connection 

To allow for systems with TRVs, as well as pump overrun boilers, a by-pass is usually required.  The pipework on the store is fitted with a tee to make a connection to the by-pass.  If required, units can be fitted with by-passes, as well as manual or automatic by-pass valves.  By-passes are suited to supplying towel rails, as they will heat the rails when the store is reheated, or the central heating is on.

Heating Coil Balancing Valve 

To limit the flow of water through the store heating coil, a lock-shield balancing valve is fitted.  If required, this can be adjusted to as per boiler requirements, usually to achieve a 10C temperature difference between the coil flow and return.














The disadvantages of an unvented cylinder:
Heat Bank Thermal Stores overcome ALL these problems.

Safety requirements:
Pressure Reducing Valve to limit incoming water pressure.
Pressure Relief Valve to prevent pressure from building up to high.
Overheat protection including isolation from boiler.
Temperature and Pressure Relief Valve to protect during overheat.
Only safety controls approved for use with a particular store are allowed.
Must be installed by a recognised unvented qualified installer.
Must be serviced at regular intervals to ensure operation of safety controls.
 Relief valves required use of visible discharge pipe to outside, that often ends up dripping.
 Over time, there is a build up of limescale within the cylinder, and on heating elements.

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