Taking the temperature of energy efficiency

13 December 2012

Boilers respond to changes in temperature, so trying to control them in relation to time or by lowering their set point can result in uncomfortable workspaces and missed KPIs, says *Tony Willis

It’s probably true to say that facilities managers are spoilt for choice when it comes to retrofit solutions that promise to save them energy. The real challenge, though, is to understand enough about how they work to judge which ones are worth further investigation.

A case in point is the control of boiler dry cycling, which can occur in most commercial boilers, potentially wasting considerable amounts of energy. This is a well-known phenomenon and the building management system (BMS) and other controls aren’t typically configured to identify and resolve it.

So there are sound financial and engineering reasons for introducing additional control, especially when that additional control delivers a fast return on investment. There are several products on the market that claim to provide a solution but how best to sort the wheat from the chaff?

The first thing is to be clear what they will do. As noted earlier, the basis of controlling a boiler is to respond to changes in temperature. So if they work by delaying the firing of the boilers or by artificially lowering the boilers’ set points, they won’t deliver the energy savings or payback they promise and can compromise comfort conditions in the building.

Of course, a boiler that has its firing delayed will fire less frequently. However, this delay allows the temperature of the hot water to fall so that more heat energy is required to restoreit to the required temperature. Thus, when the boiler does fire, it fires for longer periods.

Nevertheless, such a strategy might reduce the firing time by, say, 30 minutes over an eight hour period. However, the burners that deliver heat to the boilers generally modulate between 30% and 100% of their full capacity - or they may have two stages of firing. Either way, they are unlikely to be firing at 100% for most of the time. Therefore, simply measuring the firing time is meaningless. It’s the volume of gas that’s used that is important.

The other issue is lowering the set point of the boiler, i.e. the temperature at which the boiler will fire to restore the temperature of the water inside it. Lowering the set point will certainly delay firing but when the boilers come on they will take longer to heat the building to the required temperature. Consequently people get cold and where there are KPIs relating to maintaining space temperatures, these may be breached.

Also, any lowering of the set point will conflict with the operation of the BMS or other controls. So instead of adding to the energy savings these other controls achieve, it will probably reduce the total savings that are achieved.

The alternative is to use a boiler load optimiser such as Sabien’s M2G, which works on the basis of temperature, allows the boiler to fire when it needs to and is incapable of changing the boilers’ set points. Instead, it works by analysing each boiler’s flow and return temperatures every 10 seconds, and measuring the decay of the flow and return every second. This provides a true load profile of each individual boiler, adapts to BMS/optimiser variable set-points and does nothing to conflict with other controls such as weather compensation, demand control or sequencing.

Sabien’s M2G has been evaluated and deployed by a number of FM service providers and end users, with typical paybacks of less than two years. These include Interserve, Vinci Facilities, EDF Energy, Schneider Electric, Babcock, Carillion, John Laing, G4S, Serco, Jones Lang LaSalle, BT, Aviva, Royal Mail, central government departments such as Defra, and CLG, along with many local authorities and universities.