Understanding the 3Cs can lead to more efficient boilers

Published:  03 August, 2011

Understanding and appreciating the differences of boiler dry cycling, short cycling and short circuiting can lead to significantly higher boiler efficiency, says Tony Willis

It is well understood by building services engineers that boilers can fire unnecessarily or inefficiently under certain load conditions. Common causes include boiler dry cycling, boiler short cycling and boiler short circuiting and it is essential to understand these phenomena and how to control/prevent them.

Firstly, therefore, it is important to understand how these can occur.

Boiler ‘Dry Cycling’

When no further heat is required from the system load and the boiler is at set point it goes into stand-by mode. Due to the ambient radiated heat losses, together with the flue draught heat losses (‘standing losses’), the boiler cools down over time. This may cause the boiler temperature controls to instruct the boiler to re-fire and return the boiler back to set point. In this case the heat is not being used by the system base load; it is only replacing the ‘standing heat loss’ and therefore wasting energy.

Dry Cycling 

Boiler ‘Short Cycling’

This is caused when the boiler minimum firing/ boiler capacity exceeds the current system load. For example, the boiler has a minimum 100kW output value but the current system base load is at 50kW. This causes the boiler to fire for very short periods, as the heat generated (100kW) cannot be used and the boiler will reach set point very quickly - and cool down slowly. On some boilers this can increase standing losses caused by increased burner purging, where cool air is blown across the boiler combustion chamber each time the boiler starts.

This problem can also occur on fully modulating boiler/burners – for example those with limited turn down ratio. In the example above, with a base load above 100kW, the boiler will modulate without turning off and ‘short cycling ‘ can occur. However, with a base load below 100kW the boiler will reach and exceed the set point and the burner will turn off and ‘short cycle’.

As a consequence of dry cycling and short cycling, boilers can suffer premature damage due to the increased thermal shock each time the boiler fires unnecessarily.

Short Cycling 

Boiler ‘Short Circuiting’

This is caused when lead and standby boilers are not being automatically hydraulically isolated via motorised ‘back end’ valves. The result is that some of the heat from the lead boiler will be pumped across to the standby boilers, increasing the level of ‘standing losses’.

A common misconception is that an existing BMS is addressing all of these inherent problems, but this is often not the case. To overcome the specific issues, additional retrofit controls may be required to optimise each boiler, such as boiler load optimisation. Such controls can be retrofitted to existing plant and fully integrate with and complement other controls, such as BMS, sequencing and weather compensation etc., without compromise .

Short Circuiting 

Tony Willis is Technical Sales Director with Sabien Technology.

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