Could do better

Industry comment

Published: 17 March

Author: Alan O'Brien

Type: Industry comment

Year: 2009

Not enough is being done to optimise the performance of boiler plant and some very obvious measures are being overlooked, argues Tony Willis, Technical Director with Sabien Technology.

Many of our legislation-driven energy saving measures are designed to improve the energy performance of new buildings and major refurbishments – Part L being an obvious example. Consequently there is also a very strong case for taking a closer look at the performance of existing plant.

I would argue that, as an industry, we have a responsibility to our end customers to make them aware of these issues and help them save as much energy as possible. However, there are some very obvious measures that are currently being ignored, usually through a lack of awareness or misconceptions about how such systems are controlled.

Boiler plant is a case in point. All of us in the industry know that around 80% of boiler plant is oversized and subject to frequent stops and starts. Many of us are also aware of the problem of ‘dry cycling’ where the boiler frequently fires just to compensate for standing heat losses in the system itself, rather than actually heating the building. The trouble is we’re not doing enough about it.

Part of the reason for this is a common perception that the BMS will take care of this. The reality, though, is that a BMS is designed to control building comfort levels by zone control and weather compensation via mixing valves – it doesn’t address the issue of standing losses from the boiler plant itself. And these standing losses can be very significant.

When a boiler is standing idle it acts like a large radiator and even a modern well-insulated boiler will lose 1-2% of its heat to the surroundings. A further 2-3% may be lost due to convection currents in the flue drawing ambient air through the boiler; and this situation is exacerbated even further when pressure jet burners purge the combustion chamber before firing.

Clearly, then, significant savings can be achieved by addressing this issue of standing losses – and this has to be done without compromising on comfort levels in the building. The thermostat set-point still needs to be the key reference point.

For example, one traditional remedy has been to introduce a timed delay so that boiler firing is delayed for a pre-set time when the system calls for heat. The trouble is this will happen whether the call for heat is the result of a genuine need for more heat in the building or the result of standing losses.

A smarter and far more adaptable approach is to introduce intelligent boiler load optimisation that analyses the temperature differentials of the flow and return water, references the thermostat set point and determines the underlying reason for the call for heat. Applying this approach to over XX buildings in the last few years we have found it is possible to achieve energy and emissions savings of 10-35%, depending on conditions, while maintaining comfort levels.

The principle is actually very simple. If the temperature of the flow or return water falls rapidly this will be due to a demand for heating; if the temperature falls slowly, this will be the result of dry cycling – the exact rates having been determined by extensive field trials and pilots. So in the latter case boiler firing can be safely delayed without compromising comfort levels.

Crucially, the system also references the thermostat set point each time it takes measurements, so it will constantly adapt to any changes that are made to the system parameters. In this respect, the practice of boiler load optimisation is wholly complementary to the BMS, rather than duplicating any of its actions.

As with any complex building services system, there is a need to take a holistic view of all of the interacting elements of that system to achieve maximum efficiency. So rather than being a stand alone issue, dry cycling is just one example of how we, as an industry, can still do more for our customers.

As featured in Heating & Ventilation Review - March 2009