Kevin Stones, technical director of Hoval, explains why a good understanding of building services plant is the key to working with meaningful life-cycle information
It has become very fashionable to talk about life-cycle costs, and this is certainly a more accurate way of assessing the overall, whole-life cost of an item of plant to the end user. Doing this properly and producing truly meaningful information, however, requires an in-depth understanding of each item and often requires some input and guidance from the manufacturer.
Boilers are a case in point. The headline figure, when considering running costs and efficiencies, is nearly always the combustion efficiency – but it could cost the end user dearly if the selection were made on this figure alone. The purpose of this article is to explore the other aspects of boiler plant that contribute to life-cycle costs, and to provide some guidance on the sorts of questions you should be asking boiler manufacturers.
The cost of a pump
One obvious example is the amount of electricity that will be required to pump water round the system. There is a common misconception that a low water content boiler will have lower pumping requirements because there is less water to move. In fact, the opposite is often the case.
This is because low water content boilers have narrow waterways with higher resistance, and require minimum flow rates, so a shunt pump is often required to deal with the resistance and maintain the flow rate.
And it is surprising how much an extra pump can add to the life cycle costs, above and beyond the extra up-front cost and an additional item to maintain.
In a recent project, the cost of non-condensing boilers which required a primary header pump and two secondary circuit pumps was compared with condensing boilers that just required two circuit pumps.
The extra cost of buying condensing boilers was around £120,000 but the predicted savings on pump electrical costs alone were in the region of £50,000 a year - so combined with the extra efficiencies of condensing the payback period was very acceptable.
It is also worth bearing in mind that a steel shell non-condensing boiler requires a minimum return temperature of 55°C to prevent condensing, which may also necessitate extra kit in the form of a three-port valve or a shunt pump. For condensing boilers, on the other hand, the lower the return temperature the better, as this will allow more condensing and more heat to be extracted from the flue gases.
On the subject of flue gases, another thing that a lot of people overlook is that the lower flue gas temperature with condensing boilers means that a single skin flue can be used without excessive surface temperatures, so there are potential savings here.
Larger boilers using 3-phase pressure jet burners may also require a gas booster, leading to higher capital and maintenance costs with increased control cabling and commissioning, especially if a standby booster is required for security. Again, this needs to be borne in mind when considering whole-life costs.
Of course, the maintenance of the boilers themselves is another important consideration through the life of the plant, so boilers that are easy and quick to maintain, sharing common parts between different sizes of boiler, will lead to lower life-cycle costs.
Understanding the system
When considering the characteristics of the boiler it is clearly important to view these in the context of the whole system. For instance, it has been common practice in the UK to design the system for an 11°C difference between flow and return water. However, modern boilers are able to operate with a 20°C difference, and this greatly reduces the hydraulic resistance of the system, so that pumping costs can be reduced further.
Control freaks save money
Achieving real efficiencies and life-cycle cost savings requires good control – something that applies to all building services plant – so it is essential to choose equipment that can provide the level of control required for optimum performance.
In the case of boilers one of the critical areas to address is how often the burners switch on and off, as each on/off wastes energy and increases emissions. Modulating burners not only help to minimise on/off switching, they also ensure the boiler matches the load more closely to optimise performance. However, most traditional boilers with pressure jet burners allow only a turndown of 3:1, whereas some condensing boilers enable a 6:1 turndown with a single boiler or a 12:1 turndown with twin boilers – providing much greater flexibility to minimise stopping and starting.
With any boiler installation, the quality of the commissioning is vital in getting the best performance from the plant and the control regime. If the boiler manufacturer sub-contracts commissioning work there is a danger that the quality of the commissioning may vary so this is another factor to explore with the manufacturer.
When all of these areas are considered, the true cost of running a boiler installation begins to emerge. So it is important to get beyond the combustion efficiency and explore every aspect of operation.
Hoval T: 01636 672711