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Thermal Store Installation

Buffer Tank Sizing and Dynamic Flow

This article is applicable to Buffer Tanks and Thermal Stores, with their relevant sections found here;

The graphs below compare cycle durations for indentical boiler outputs to identical heating requirements of an installation with different size buffer tanks.

It also assumes identical generic single point controls and ideal loading/unloading of the buffer tank. The boiler does not modulate and does operate a weather compensated system.

Note – Steady state running is not shown. ie Heat input equal to demand

Buffer Tank Comparison Chart

This graph shows very clearly the following;

Buffer Tank Volume and Control Comparison Chart

This graph is displaying the following;

With appliance modulation, the 50% duty can be avoided, increasing cycle duration.

Firing at 85% with 15kw/litre and ideal controls, will equal the cycle duration of an appliance firing at 50% with 25kw/litre and average controls.

Effectively firing the appliance on high fire whilst the buffer is as cool as practical and modulate to low fire, before target temperature is achieved is desirable.

The appliance will remain at low fire, until the maximum return temperature is achieved or the flow temperature is insufficient, causing high fire. For example, the “Heat Lab Ltd Cascade Control” pre-esbe temperature will fluctuate between high and low set-point without the appliance becoming satisfied. Unless it becomes heat dissipation limited and then it will become satisfied.

It should become clear that the area inside the box is not desirable, the cycle times are short in the 30 to 70 range and over 70, the “Satisfied” conditions are becoming increasingly short.

Buffer Tank Comparison Chart

Below is a sample of the calculations for creating the above graph.

Buffer Tank Calculations

The graph below shows the following;

Buffer Tank Comparison Chart

That the cycle duration is effected equally by both the volume and controls.

The 15 litres/kw with ideal controls is equal or better to the 25 litres/kw with poor or average controls. It is also indicating that 15 litres/kw is equal to 50 litres/kw with poor single thermostat controls.

The 15 litres/kw with ideal controls should even match 75 litres/kw with some modulation.

Understanding how Buffer Tanks Work

The important aspect to consider about Buffer Tanks, is that they are "just" a tank and do not feature any internal heat exchange coils, with the exception of Solar Thermal applications.

If a coil is being considered for any other application, we would suggest considering Plate Heat Exchangers as they are relatively inexpensive and have much better thermal heat exchange.

Generic Boiler – “P1” Pump fixed speed and no “Loading Valve”

How a Buffer Tank Heats Illustration One

No Combustion, temperatures all equal and flow identical through the system

How a Buffer Tank Heats Illustration Two

Appliance at “High Fire”, resulting in 10 Deg C Delta t and warmer, less dense, water remaining in the top portion of buffer. The cooler water in the bottom of the buffer is displaced by the water entering the top of the buffer

How a Buffer Tank Heats Illustration Three

As the buffer is heated, proportionally more thermal energy (heat) reaches the bottom of the buffer and subsequently returns to the heating appliance, increasing the flow temperature

How a Buffer Tank Heats Illustration Four

As above

How a Buffer Tank Heats Illustration Five

As above

How a Buffer Tank Heats Illustration Six

As above

How a Buffer Tank Heats Illustration Seven

As above, although the flow temperature is at the maximum design threshold and any further increase will result in the appliance modulating, overheating or turning off

Generic Boiler – “P1” Pump fixed speed and “Loading Valve”

How a Buffer Tank Heats Illustration One

No Combustion, temperatures all equal and flow only occurring through the heating appliance up to the first “tee” in the pipework. There is no flow through the buffer tank.

How a Buffer Tank Heats Illustration Two

Appliance at “High Fire”, resulting in 10 Deg C Delta t with flow only occurring through the heating appliance up to the first “tee” in the pipework and returning to the appliance.
There is no flow through the buffer tank.
This stage will only increase the temperature of the heating appliance and flue system

How a Buffer Tank Heats Illustration Three

As above

How a Buffer Tank Heats Illustration Four

As above

How a Buffer Tank Heats Illustration Five

Any further increase in return temperature will result in the “loading valve” mixing the cooler return water from the bottom of the buffer tank

How a Buffer Tank Heats Illustration Six

The buffer tank is starting to be heated, although the water flow rate to the buffer is low. This low flow rate will increase the opportunity for the hot, less dense, water to remain buoyant and remain in the top portion of Buffer Tank.
The cooler water in the bottom of the buffer is displaced by the water entering the top of the Buffer.

How a Buffer Tank Heats Illustration Seven

As the buffer is heated, proportionally more thermal energy (heat) reaches the bottom of the buffer and subsequently returns to the heating appliance, via the “Loading Valve.
As the temperature of the water returning from the buffer tank increases, the loading valve mixing reduces and the flow through the “Tee” reduces.

How a Buffer Tank Heats Illustration Eight

As the buffer becomes heated, proportionally more flow returns from the buffer tank and less flow through the “Tee” to the loading valve

How a Buffer Tank Heats Illustration Nine

As above

How a Buffer Tank Heats Illustration Ten

The temperature of the water returning from the buffer tank is now equal to the control temperature of the loading valve and the loading valve has no further mixing requirements

How a Buffer Tank Heats Illustration Eleven

The flow temperature is at the maximum design threshold and any further increase will result in the appliance modulating, overheating or turning off

Dynamic Flow through a Buffer Tank, distribution system connected

How a Buffer Tank Heats with no Esbe or Mix Illustration One

Flow rate equal through pumps P1 and P2 - The Flow Temperature from the Heating Appliance will be equal to the Heating Flow Temperature

How a Buffer Tank Heats with no Esbe or Mix Illustration One

Flow rate higher through pump P1 than P2 - The Flow Temperature from the Heating Appliance will be equal to the Heating Flow Temperature

How a Buffer Tank Heats with no Esbe or Mix Illustration One

Flow rate lower through pump P1 then P2 - The Heating System Flow Temperature will be COOLER than the Appliance Flow Temperature

Dynamic Flow through a Buffer Tank, distribution system connected & mixing valve

How a Buffer Tank Heats with no Esbe and Mix Illustration Zero

Flow rate equal through pumps P1 and P2, blending valve not mixing - The Flow Temperature from the Heating Appliance will be equal to the Heating Flow Temperature

How a Buffer Tank Heats with no Esbe and Mix Illustration One

Flow rate through pumps P1 and P2 remain unaltered, although flow down/through the buffer increases as the blending valve starts to mix as Buffer becomes directly heated.

How a Buffer Tank Heats with no Esbe and Mix Illustration Two

Flow rate through pumps P1 and P2 remain unaltered, although flow down/through the buffer increases as the blending valve continues to mix as Buffer becomes directly heated.

How a Buffer Tank Heats with no Esbe and Mix Illustration Three

Flow rate through pumps P1 and P2 remain unaltered, flow down/through the buffer increases as the blending valve further increases mixing and the buffer use is maximised.

How a Buffer Tank Heats with no Esbe and Mix Illustration Four

Flow rate through pumps P1 and P2 remain unaltered, flow down/through the buffer is at maximum as the blending valve is fully diverted and only heating the buffer.

The effect of a Loading Valve on a Buffer Tank

How a Buffer Tank Heats with Esbe and Mix Illustration One

The Loading Valve returning flow directly to return as the minimum return temperature is not achieved - The Heating Appliance Flow will NOT be delivered to the Heating System

How a Buffer Tank Heats with Esbe and Mix Illustration One

The Loading Valve mixes a low flow rate into the Buffer Tank - The Heating System Flow Temperature will be substantially COOLER than the Appliance Flow Temperature

How a Buffer Tank Heats with Esbe and Mix Illustration One

The Loading Valve allows a higher flow rate into the Buffer Tank - The Heating System Flow Temperature will be COOLER than the Appliance Flow Temperature

How a Buffer Tank Heats with Esbe and Mix Illustration One

The Loading Valve is not mixing - The Flow Temperature from the Heating Appliance will be equal to the Heating Flow Temperature

How a Buffer Tank Heats with Esbe and Mix Illustration One

The Loading Valve is not mixing and mixing valve starts to operate - The Buffer Tank is now being heated by directly as the flow rate to heating system decreases

How a Buffer Tank Heats with Esbe and Mix Illustration One

The Loading Valve is not mixing and mixing is requiring less energy - The Buffer Tank is now being heated by directly as the flow rate to heating system decreases

How a Buffer Tank Heats with Esbe and Mix Illustration One

Flow rate through pumps P1 and P2 remain unaltered, flow down/through the buffer is at maximum as the blending valve is fully diverted and only heating the buffer.

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.

Tel. 01206 432 999