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Clamping down on waste

19 September 2012

It’s often surprising to discover that even a modern boiler fitted with a building management system and weather compensation could still be wasting significant (10-25%) amounts of energy.

This is because of boiler ‘dry cycling’, which happens in majority of low and medium temperature hot water boilers – and ‘standard’ controls aren’t configured to manage it.

Unless measures are taken to control it, boiler dry cycling occurs when a boiler loses heat to its surroundings (acting like a radiator) so that the water temperature in the boiler falls below the current set-point of the boiler’s internal thermostat / load control. When this happens, the boiler may fire simply to recover the boiler’s standing loss/temperature of the wasted heat – even though this energy is not being used to meet demand for heat from the building.

So why doesn’t the BMS control it?

Boiler dry cycling is a phenomenon that occurs at individual boiler level. A BMS is typically programmed to optimise the building’s energy requirements as a whole. In the case of boilers, the BMS measures the blended flow and return temperatures to/from all the boilers, rather than monitoring what each boiler is doing.

Thus, in a plant room with three boilers the lead boiler could be firing continuously to meet the heating loads, while the other two are dry cycling and wasting energy. A BMS measuring the blended temperatures will be unable to identify that dry cycling is occurring.

‘Demand control’ is sometimes cited as a solution for boiler dry cycling. This enables the boilers to fire until all of the system’s demands – e.g. domestic hot water, radiators, and air handling units – are all satisfied. Unfortunately, in a typical building such loads are not typically satisfied simultaneously for any length of time. For the rest of the time there is a strong likelihood of dry cycling occurring during transient loads.

In fact, dry cycling has been a recognised problem for many years, and early attempts at retrofit solutions were an abject failure (sadly, some have reappeared recently in an attempt to exploit the ‘green pound’). These ‘failures’ either delayed the boilers’ firing, or artificially reduced the boiler set points, both of which allowed temperatures to fall in the spaces being heated. In addition, they can cause direct conflicts with the BMS strategy by artificially changing the boilers’ set point temperatures.

An alternative, using Sabien’s patented M2G boiler load optimisation technology, is to carry out real-time analysis of each boiler’s flow and return temperatures every 10 seconds and measure the decay of the flow and return every second. This provides a true load profile for each boiler and can therefore differentiate between a genuine call for heat and one that is simply firing due to standing losses.

The beauty of this approach is that it recalculates the values every time the boiler reaches its required set point temperature, so it adapts to variable set-points and does nothing to conflict with the BMS operation or other controls such as weather compensation or sequencing.

With over 5,000 M2G units installed, this technology has been tried and tested in a wide range of applications and frequently subjected to independent verification. Energy savings range from 10% to 25%, depending on the exact nature of the system, and the payback period is typically less than two years.

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