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Thermal Store Installation

Buffer Tanks

Cordivari 100 Litre Buffer Tank

This section is titled "Buffer Tanks" but what exactly is a Buffer Tank?

A Buffer Tank primarily serves two purposes, firstly it is a "neutral point" in a hydraulic system and secondly, it guarantees a minimum heating load for particular heating appliances.

They are used differently depending on the application and heating appliance, this will be covered in great detail on this page. Although we can give advice on the methods used, the detail characteristics are very installation specific and we would generally recommend a form of Data Logging for accurate optimisation of your system, details of which can be found here;

If you are interested in combining Multiple Heat Sources, we would suggest visiting our specific page on this specialist application.

Buffer Tanks are generally used to reduce "short cycling" of appliances, although the success of their use depends on various criteria from controls, capacities, heating loads, flow rates and how they are connected to systems.

The Buffer Tank pictured to the left is 100 litres, it serves a heating load of 4.3kw and is being heated by a 5kw appliance. In the data logging section, we have actual information from the system and clearly demonstrates the use of the Buffer Tank.

Sizing of the buffer tank is very important and unfortunately, there is no golden rule. Too large and the system becomes unresponsive. Too small and the heating appliance will short cycle.

Buffer Tanks can be used for any heating appliance and for some they are a good addition, for others they are vital.

The information below is not specific to one installation and does not apply to a single heating technology, the information can however be applied to almost all heating technologies. The main installation requirement with a buffer tank is to ensure flow temperature is optimised, stratification within the buffer tank is maximised and firing periods of the appliance are as long as possible whilst delivering the required comfort levels.

If you wish for your Buffer Tank to be repaired, replaced or serviced, simply call us on 01206 432 999.

Do you own an existing Buffer Tank and are interested in Heat Lab Ltd improving the performance of your system?

Operation of the buffer tank is very dynamic and one configuration will not yeald best results. Fortunately, we have experience of improving installations over the UK and have developed several products to automatically adjust to optimum conditions;

All of these products are available from ourselves and have a thorough description in their relevant section of this website or just click on the description above.

To appreciate the how a Buffer Tank operates, we have information below outlining the operation of the equipment and the controls involved.

The Technical Side of Buffer Tanks

This section will explain the key differences between various types of conditions and will also show illustrations of the dynamic flow through a buffer tank, understanding this will increase your chances of success.

Heating Only Buffer Tank Schematic

Modulating Boiler Heating Only Schematic

The schematic to the left is for an appliance which can modulate and is therefore tolerant to the heat load of the installation as the appliance can "modulate", adjust the power output.

Due to the modulation, we size the buffer tank to 20 litres per kilowatt of the minimum heat output or 20 litres per kilowatt of heating load, which ever is the greater.

The reason for using the minimum heat output of the appliance to allow the modulation to adjust to the heat load and once on the lowest heat output, any excess energy is then stored in the buffer tank.

Our control systems ensure appliances are generally operating at the lowest heat output possible, the appliance modulating to the heat load is much more energy efficient and is to be used as a preference to Buffer Tanks.

Buffer Tanks are essential when modulation does not enable the boiler to run for a period of time without either overheating or acheiving the required water jacket temperature. Therefore, there will always be a situation where a heating appliance can not modulate to the heat load (perhaps due to mild weather) and this is why a buffer tank is required for optimal efficiency.

The water flowrate is also effected postively by the installation of a Buffer Tank and if low water flowrates in your heating circuit are a problem, it is worth considering a buffer tank or low loss header to "spilt" the heating circuit.

Modulating Appliances typically only require a capacity of less than 200 Litres.

Heating and Domestic Hot Water Buffer Tank Schematic

Modulating Boiler Heating and Domestic Hot Water Schematic

As you can see from the schematic to the right, it is possible to add an indirect Domestic Hot Water Cylinder but only if the Domestic Hot Water Cylinder Coil can dissipate 100% of the minimum heat output of the appliance and is therefore only advised for modulating appliances, not Ground Source Heat Pumps, Oil or Biomass Boilers for example.

The heating system can generally be of any configuration, "S", "Y" or "W" plan are all potentially suitable.

Typical examples of this application are modulating Gas Boilers and Air Source Heat Pumps, both gaining by the additional firing durations and particularly with Air Source Heat Pumps, the required flowrates are acheived with lower duty circulation pumps.

The "index" heating circuit generally has the pressure loss reduced as water only flowing through a partial circuit.

Modulating Boiler Heating and Domestic Hot Water with Thermal Store Schematic

To the left is a superior option as it utilises a Thermal Store for Domestic Hot Water and therefore, we do not have the problem of a relatively small heating coil in the cylinder to dissipate the heat output of the appliance.

The addition of a Thermal Store to this installation would make it suitable for appliances which do not modulate, such as Ground Source Heat Pumps, Oil Boilers and Biomass Biolers, although these appliances do generally feature limited modulation.

It should be noted that the heating appliances which do not modulate, should have the Buffer Tanks sized at 20 litres per kilowatt of the maximum heat output and in the case of Biomass Boilers, 25 litres is the general recommendation.

Although the addition of the Thermal Store does not appear to be a major alteration, it should be noted that they can perform quite differently to an convention indirect Domestic Hot Water Cylinder and it does have an potential to reduce in temperature when being heated simultaneously with the heating circuits, especially with Biomass Boilers as they can take 10 minutes to ignite.

Electric Control circuit for Domestic Hot Water Priority

The solution to the problem is use a standard "W" Plan wiring configuration or alternatively, "interlock" the central heating circulation pump as shown in the electrical circuit to the right.

This control circuitry ensures that the Domestic Hot Water has priority and therefore, Domestic Hot Water can not be effected by an potential Heating Demand.

If your Thermal Store is being heated indirectly from a Buffer Tank, this priority is particularly important as the volume of water being heated is potentially very large and in the case of Biomass Boilers, can be several thousands of litres.

The Heat Lab Cascade Control has this functionality as standard.

Buffer Tank Sizing and Dynamic Flow

There is further technical information on Buffer Tank sizing and dynamic flow characteristics available here;

Flow Injector Modification to Buffer Tank

In addition to ensuring that the circulation pump speeds are controlled to enusre that unrequired mixing does not occur within the Buffer Tank or Thermal Store, we can also install "Flow Injector" to help reduce the mixing within the buffer tank.

Effectively this delivers the water flowing from the boiler and rather than let water become turbulent within the Buffer Tank, the water is "Injected" into the Heating Flow Connection.

Although not essential, this modification will ensure flow temperature to the heating circuit/s are optimised.

In the "Buffer Tank Sizing and Dynamic Flow Characteristics", some effort has been made to explain the dynamics of the Buffer Tank or Thermal Store as a high percentage of systems fail on this overlooked subject.

We would also suggest that this Flow Injector is installed before any electronics are installed as a passive solution to a heating flow temperature problem will always be preferable.

Depending open the Buffer Tank used, it is sometimes preferable to use a 3 Port connection to a Buffer Tank.

We have had some recent success with this configuration and when fully tested, details will follow.

Tel. 01206 432 999