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Buffer or no buffer? With weather compensation?

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Buffer or no buffer? The detail

To understand whether or not a buffer is required on a system, it is important to appreciate the daily heating cycle in a typical household, as shown on the chart below (click on it to enlarge it).


  • The green line shows the load required from a boiler throughout the day. The load is zero over night, and typically reaches a peak in the morning at about 7am and 7pm.  If the boiler is correctly sized, on a very cold day this peak will be close to 100% of the rated output of the boiler. On a less cold ‘average winter day’, the peak requirement might reach 80% of the required output.


  • The biomass boilers that we install can all modulate their output down to about 30% of the maximum. Thus a 25kW boiler can modulate its output down to around 7.5kW. A 15kW boiler can modulate its output down to about 4.5kW, and so on.


  • Thus our biomass boilers can track the required load unless the required load falls below 30% of the rated output of the boiler. This is only likely to happen for a very small percentage of the time (eg at the end of the heating season or in summer).  In the diagram the blue triangles represent the times when the boiler cannot accurately track the load.  At this point the boiler is inefficient, supplying more heat than is required in the ‘on cycle’ and turning off at regular intervals to compensate.


  • In order to ‘track the load’ effectively the boiler needs sophisticated ‘weather compensated’ controls. Weather compensated controls measure the outside temperature and reduce the temperature at which fluid flows through the heating system accordingly. If it’s 10C outside there is no need to blast heat at 80C through the radiators… it is much more efficient to reduce the flow temperature through the radiators to about 50C. In addition some weather compensated controls can adjust the time at which the boiler turns on in order to meet the required room temperatures at the correct time.


  • In contrast an oil or boiler has only two states: FULL ON or OFF.  Oil and gas boilers blast heat out at 70C or 80C regardless of the temperature outside. For all but the coldest days (which need heat 24 hours a day if the temperature in the house is to be maintained), the only way they can maintain the temperature in the house is to cycle on and off, on and off, on and off.  So short blasts of 80C, followed by a rest. This is inefficient and increases fuel consumption by about 15%.


  • In larger systems or in systems where the biomass boiler is over-sized or is feeding multiple loads (eg two different buildings), the minimum load required might be well below the minimum 30% threshold of the boiler. Thus the percentage of time where the boiler cycles ‘on and off’ will be increased. Cycling a biomass boiler on and off is very inefficient. Indeed cycling any boiler on and off is inefficient, but with biomass boilers the problem is exacerbated by the fact the operational temperature of the burner is about 800C and reaching this temperature can take 20 minutes. In these circumstances a buffer tank is used.


  • The purpose of the buffer tank is to reduce cycling. A buffer store will typically be around 1000 litres or more and can hold about 60kWh or more of heat energy. To heat the buffer from cold will take at least two or three hours. Heat is supplied to the load (radiators, hot water cylinder) from the buffer. The buffer only switches the boiler on when it needs replenishing. In effect, the buffer tank allows the system to track the required load without wasting heat.


  • Note that all oil and gas boilers could be run more efficiently with a buffer tank; the principle of a buffer is not limited to biomass boilers. It is only more pertinent because of the time required for the boiler to get to temperature.


Buffer or no buffer? The quick version


The question of whether to use a buffer or not is thus answered by the following:


There is no need for a buffer if all three of the following conditions apply:

  • The boiler feeds a single property, is rated under about 30kW and is tightly sized with the maximum heat loss equalling the rating of the boiler.


  • If the minimum load is likely to be around 30% of the boiler rating. Note that in summer the only load is likely to be hot water. A typical domestic tank stores 12kWh of energy, and will therefore take just under 40 minutes to heat through with a 20kW boiler. The boiler will therefore spend 20 minutes getting to temperature to provide 40 minutes of heat. This is acceptable, but efficiency could be improved by using a solar thermal system to provide hot water and letting the boiler remain idle in summer.


  • If weather compensated controls are used. In order to work efficiently the boiler needs to be TOLD to modulate down below its maximum output. This will only happen if weather compensated controls are in operation that tell the boiler to modulate to give a lower flow temperature through the radiators.


In all other circumstances, use a buffer.  The green load line in the above graph can therefore be tracked without wastage.

Weather compensated controls

When ‘retro-fitting’ a biomass boiler it is possible either to use the existing controls or to install new weather compensated controls.

Weather compensated controls adjust the flow temperature through the system in accordance with the outside temperature. They can also adjust the time at which the heating system is turned on to achieve a given target temperature at a set time.  For example the flow temperature might be reduced to 50C when the outside temperature is 10C, since there is no need to blast heat through the radiators at 80C in such comparatively mild conditions. See the above section Buffer or no buffer (plus weather compensation).

Weather compensated controls typically increase the installation cost by £1500- £2000 per building (the precise cost depends on how many heating zones there are). However they reduce the on-going running costs by about 15%.

Weather compensated controls are particularly beneficial when the installation includes external (‘district heat’) pipework. The standing losses from a district heat pipework can be quite high. The losses are significantly reduced if the flow temperature through the district heat pipework is reduced and therefore it makes sense to use weather compensated controls whenever district heat is involved. Solar thermal is also worth considering with a biomass system. For more information see “District heat, solar thermal”.

Our buffers

We typically install AKVATERM buffer tanks, ranging in size from 300 litres to 4,000 litres.

You can download their brochure here:  AKVATERM-Accumulator-Tanks

AKVATERM was founded in 1993 and is now the market leader in Finland for buffer tanks as well as one of the leading suppliers of tanks for the mechanical equipment industry. Their tanks are robust, simple and have a life expectancy of over 50 years.