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Life cycle costs on the boil
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.
In recent years there has been increasing recognition of the need to consider
the life-cycle costs of plant, as this is certainly a more accurate way of
assessing the overall, whole-life cost. 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
prove expensive 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 that
should be asked of 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’s 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 to
condensing boilers that just required two circuit pumps. Clearly the
condensing boilers cost more that non-condensing boilers but the substantial
savings on pump electrical costs, by not having the extra pump, gave a
payback of just under two years.
It’s 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 equipment 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 difference sizes of boiler, will lead to
lower life-cycle costs.
Understanding the system
When considering the characteristics of the boiler it’s 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’s 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 only allow 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’s important to get beyond the combustion
efficiency and explore every aspect of operation.
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