Boiler control is based on ‘temperature’, not ‘time’

Industry comment

Published: 26 November

Author: Tony Willis

Type: Industry comment

Year: 2012

Boilers generate ‘heat’ to meet the required space and hot water temperatures. This may be determined by a simple thermostat or indeed a BMS,  so there’s no sense in trying to introduce a control that uses a simple ‘time’ value or delay.

Energy managers are faced with a number of options regarding retrofit boiler controls. The real challenge is to understand how they actually work and 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 other controls (e.g. BMS) aren’t typically configured to identify and prevent it. So there are sound financial and engineering reasons for introducing controls to prevent it. Several products claim to provide a solution but how do you know what works and what does not?

First, establish how these products actually control the boiler. If they work by delaying the firing of the boilers based on an historical firing pattern / time delay, or by artificially lowering the boilers’ set points, they could compromise comfort conditions and conflict with existing sophisticated building controls.

The underlying principle is that boilers are controlled by temperature not time, so attempting to control them on the basis of time alone doesn’t make sense. Of course, a boiler that has its firing delayed will fire less frequently. However, this allows the temperature of the hot water to fall so that more heat (energy) is required to restore the required temperature and the boiler will therefore fire for longer periods.

For example, such a strategy could reduce the firing time by, say, a total of 30 minutes over an eight hour period. If installed on modulating burner/ boilers the period of time a boiler is firing does not directly relate to the energy consumed or saved. The burner may operate at high fire for five minutes or at low fire for five minutes - the time is the same, the energy consumed is not!

If savings are based on artificially lowering the overall boiler set point temperatures, this could be done by manually adjusting the boiler’s own thermostat control at no cost or indeed by adjusting the BMS strategy. However, this is not as simple as it first appears as the majority of boilers set points have been set at the optimum levels to achieve savings and meet required temperatures. Artificially lowering the set point will therefore conflict with the operation of the BMS, weather compensation, boiler sequencing etc. Consequently, instead of adding to the energy savings achieved by existing controls, retrofit controls that artificially lower the set point will potentially reduce the required comfort levels and jeopardise the KPIs for the building.

An alternative is to use a boiler load optimiser that works dynamically on the basis of the boilers’ temperature load profiles, ensuring the boilers’ set points are always maintained. This is done 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.

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