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Boiler plant - Beasts in the basement

Most boilers waste a significant proportion of their input energy for much, if not all, of their operating lives - a fact that is largely unrecognised and unaddressed by those who specify, install and manage them. Tackling boiler inefficiencies is a top priority if you want to reduce energy consumption and cut costs with an excellent Return on Investment.

This article explains the key reasons for this sorry state of affairs and what you can do to improve the efficiency of your boilers.

  • Oversizing
  • Standing losses
  • Purging
  • Sequencing and isolation
  • Combustion efficiency
  • What you can do

You might also like to read these related articles:

Are boiler controls a good idea?

Is your BMS wasting energy?

Managing Energy for Comfort

Oversizing

The vast majority of buildings have boiler plant that is significantly over-sized for the load that it is required to satisfy. One reason for this is that the plant load varies according to the ambient temperature and, for most of the time, this is fairly close to the temperature above which the building doesn’t need heating at all – due to solar again and the heat emitted by all other energy consuming equipment in the building and its human occupants. In fact the amount of other equipment has often greatly increased since the building and its boiler plant were designed.

A second key reason is that boiler plant is usually designed to include substantial additional capacity for such things as excessive safety margins, plant breakdowns and possible future building extensions. Thus the majority of plant is actually over-sized throughout its operating life, usually by a very significant margin.

An example of this is a 9,000 kW (output) set of boilers which during its first year of continuous operation used just over 3,000,000 kWh of gas, an average load of just 350 kW or 3.5% of the input capacity! As boiler standing losses are often quoted as being about 3% of the installed capacity the inefficiency of this installation is apparent. The operators of this plant came to the bizarre conclusion that it was actually not large enough and installed a fourth boiler – at considerable expense!

Standing losses

Standing losses are usually described as “radiation” losses from the external surfaces of the boilers.  Whether these are actually 3% will depend on the boiler: many older boilers have considerable un-insulated or poorly insulated surfaces, whereas some more modern units, particularly in the larger sizes, are quite well insulated. The degree of insulation on smaller boilers is extremely variable, with some older ones being well insulated, whilst some modern ones (particularly “modular” type units up to about 300 kW) are completely un-insulated!

Purging

Radiation losses are only part of the equation. Larger boilers with pressure jet (as opposed to atmospheric) burners will invariably perform pre- and post-fire purges. The forced draught fan runs for up to a minute before the burner is lit to ensure that the boiler is clear of inflammable gases, and for a further shorter period immediately afterwards.  

These purges have the effect of blowing air at ambient temperature through the boilers, and the more frequently this occurs the more energy is lost. A recent investigation identified that two boilers in one installation were completing nearly ninety starts in a single day, but only running for about 5 minutes after each start. This is not very efficient!

Sequencing and isolation

The optimisation of boiler efficiency requires that only the boiler capacity needed to provide the necessary output is connected at any one time. “Connected” in this context includes hydraulic connections: if stand-by boilers remain in circuit they will continue to suffer from radiation losses, and the ability of the lead boiler to achieve the required set-point will be reduced by “dilution”. If the set-point is not achieved, the BMS/control system will almost certainly enable additional boilers, resulting in short cycling. There are various ways of identifying the actual boiler power required for a building, and the completion of this analysis will identify the opportunities for rationalising the boiler plant.

Combustion efficiency

This is usually checked by the engineer after completing boiler maintenance. The reported figures are rarely checked by anyone else, and on some occasions they are complete rubbish. Ideally, combustion efficiency should be checked between maintenance periods, both as an independent check on the quality of maintenance and in order to determine the rate of deterioration of the plant. A few per cent loss of efficiency can represent a substantial annual cost and may also be a symptom of health and safety issues.

What you can do

You can reduce energy consumption and costs and deliver more comfortable conditions for building users by:

  • Accurately determining the maximum load, removing and selling any surplus boilers
  • Controlling the remainder appropriately
  • Hydraulically isolating those not in use at any one time
  • Ensuring correct maintenance and boiler efficiency

With a bit of effort and the right expertise the rewards are high.

Your Independent RISK FREE Solutions

A Green Consultancy Boiler Health Check will investigate all of the above issues and more to ensure that the efficiency of your boiler plant is maximised. Alternatively, our Investment Grade Energy Efficiency Audit will identify and prioritise all cost-effective energy saving opportunities for reducing your energy consumption. For an investigation of all equipment controlled by your BMS you might like to consider BMS Health Check or a BMS Audit.

Put us to the test NOW! To discuss your requirements please call John Treble on 01761 419081 or email John@GreenConsultancy.com

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