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Addressing the myths of boiler controls

building management systemThe BMS takes care of everything

Typically a building management system (BMS) is programmed to manage the heating system (i.e. the building) as a whole, rather than controlling the performance (fine tuning) of individual boilers. Typical BMS strategies may include, optimum start/stop, weather compensation or boiler sequencing. This is usually achieved by monitoring the blended water temperatures from all of the boilers via the common return header to the boilers rather than measuring the individual boiler/s so any energy wastage through boiler dry cycling at an individual boiler level is not always detected or prevented by the BMS control.

Of course it is possible to configure a BMS to monitor dry cycling but this requires expensive bespoke programming for each boiler and will require on-going commissioning. Customers that have explored this option report this cost greatly exceeds that of introducing purpose-designed retrofit boiler load controls that work along side and compliment the existing BMS.

boiler controls
Dry cycling does not occur with new boilers

Boiler dry cycling can occur with virtually all types of boilers. When the boiler reaches the required designed temperature and is left in standby and no longer firing, the boiler will cool down due to the natural “standing losses”, the temperature of the water in the boiler eventually falls below the set point on the boiler’s internal temperature controls. This triggers the boiler to fire, even when there is no requirement for heat/ heating load from the building, thereby the boiler is only firing to overcome the “standing loss” and wasting energy.

Well-insulated and low water content modern boilers will cool down more slowly, compared to older less-insulated boilers, so the dry cycling may be less frequent – but it will still occur, leaving opportunities for cost savings.

 Retrofit controls interfere with the existing control strategy

It is certainly true that some attempts to prevent boiler dry cycling conflict with the existing control strategies, often resulting in compromised comfort conditions and possibly even an increase in energy consumption. However, this situation can be avoided by using Dynamic Efficiency “Stop-Go” boiler load control technology.

Sabien’s patented M2G boiler load controller constantly measures and analyses the temperature profile of each boiler in real time. This enables the M2G to identify and prevent the boiler from dry cycling and, more importantly, allow the boiler to fire immediately if there is a genuine demand for heat. The intelligent software within the M2G enables it to dynamically manage the boiler to ensure dry cycling is prevented.

If a BMS is in place, the M2G integrates with it taking its “Stop-Go” signal directly from the BMS. Just as importantly, it recalculates the values every time the boiler reaches its required set point temperature. This means it adapts to BMS/optimiser variable set-points and does nothing to conflict with other existing controls such as weather compensation, demand control or sequencing. The boilers’ designed set points are never altered.

Reduced firing time = reduced gas consumption

Some retrofit boiler controls delay the “time” firing of the boiler in an attempt to prevent boiler dry cycling and reduce energy consumption. They are often able to demonstrate that the burners have fired for shorter periods of “time” and the assumption is made that less firing equals lower fuel consumption.

This is not the case, most modern boilers use modulating burner or multi stage firing. In this case the “time” element of a boiler control does not equate to the energy used i.e. fuel used.

For example: a modulating boiler firing for 5 minutes at low fire, will use much less fuel, than if it was operating for 5 minutes at high fire.

It is therefore very misleading to state that the energy saved is based on a “time reduction” of boilers operating.

 Energy bills are an accurate guide to energy savings

Comparing energy bills from before and after an energy-saving initiative may seem like a good guide to the success of such a project, but there are several reasons why this is not the case.

Firstly, energy bills will typically detail the gross consumption of energy (e.g. gas) to the building(s). Typically this does not equate solely to the gas consumed by the boilers, as gas may also be used for other purposes in the building, such as catering.

Secondly, the energy consumption of heating plant needs to relate to weather conditions. Comparing gas consumption from a cold winter with that for a mild winter is meaningless. The answer is to use degree day data provided by the Meteorological Office to adjust gas consumption figures accordingly and provide an accurate picture of performance based on the heating requirements during variable weather conditions.

Thirdly, there are a number of other factors that can contribute to a change in heating patterns, such as improvement to the building’s insulation, changes in building usage etc. It is therefore important to put a methodology in place that will provide accurate and transparent measurement and verification of any energy-saving project. We recommend working to the International Performance Measurement and Verification Protocol (IPMVP).

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