Seeing the full picture
For a whole range of reasons, sustainability has become very high profile and is beginning to influence decision-making at all levels, from corporate policy at board level right down to the heating plant equipment selected for a new build or refurbishment project.
At the ‘granular’ level of plant selection the contribution of these items to the whole life costs (financial and environmental) of the building has taken on greater importance. To understand these costs fully, however, it’s vital to look beyond the headline performance figures and capital cost of main items such as boilers to take in related parts of the system.
In the case of the boilers, the headline performance figure will usually be combustion efficiency which, of course, is very important. But basing a decision on this alone could prove very costly for the end client because there are several other factors that need to be considered.
One of the key factors to take into account is the amount of electricity that will be required to pump water round the system, something that many people are either inclined to underestimate or have misconceptions about.
For example, it would seem logical that there is less water to move around in a system using a low water content boiler, so the pumping requirements will be less. In fact, the opposite is frequently the case because low water content boilers have narrow waterways with higher resistance, and require minimum flow rates. As a result, a shunt pump is often required to deal with the resistance and maintain the flow rate.
Again, many people are inclined to dismiss the importance of this extra pump. After all, it’s only one pump. However, the additional electricity costs can be substantial and there are also increased maintenance requirements to take into account.
In a recent project, the building services engineers compared the cost of non-condensing boilers which required a primary header pump and two secondary circuit pumps to 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 per annum – so combined with the extra efficiencies of condensing the payback period was very acceptable.
It’s also worth bearing in mind how boilers have changed in recent years and to design accordingly. 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.
Another thing to bear in mind is 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, many people overlook the fact that a lower flue gas temperature means 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.
Similar considerations need to be given to refurbishments, especially where it’s just the boiler being replaced and other elements of the heating system remain in place. In particular, the characteristics of the new boiler may have a knock-on effect on other parts of the system which, in turn, impinge on the whole life costs of the installation.
For instance, an existing system that has been designed for use with non-condensing boilers will probably heat emitters that have been sized to provide flow and return temperatures that are too high to provide condensing. This isn’t necessarily a reason not to use condensing boilers, as these will still give greater seasonal efficiencies even when they’re not condensing, but it is something to be aware of.
An additional benefit of condensing boilers is that they offer a better turndown than non-condensing boilers, resulting in fewer starts and stops. As a result, NOx emissions will be reduced which has a beneficial effect on overall environmental impact and will help to gain a higher BREEAM rating. As a rule of thumb, NOx emissions from condensing boilers are generally around half those from the best atmospheric boilers.
If a condensing boiler replaces a pressure jet oil boiler the end client will also benefit from reduced emissions, as well as lower noise and electricity consumption. As low sulphur fuel oils become more readily available it will be possible to achieve substantial improvements even when fitting a new pressure jet boiler, by adding a condensing economiser to provide condensing.
As noted above, servicing is important to ensure the original performance is maintained. When an old boiler is replaced, therefore, it’s vital to take a fresh look at the maintenance regime as this will almost certainly be different with the new boiler in place, in terms of performance and legislative compliance.
In both new build and replacement projects, achieving real efficiencies and life-cycle cost savings necessitates choosing equipment that can provide the level of control required for optimum performance. 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.
Replacing plant may also require changes to the existing control system as modern controllers may require more information from a wider range of sensors. Or the existing controls may not interface easily with the new plant without a little tweaking. If biomass and fossil fuel boilers are being mixed – perhaps with other renewables such as solar heating – the controls will be particularly important to get the sequencing correct.
As with any building services installation, the quality of the commissioning can make all the difference to the ongoing performance of the system. Given the complexities discussed so far, it’s clearly worth checking whether the boiler manufacturer uses in-house staff or sub-contracts commissioning – the latter increasing the possibility of variable quality.
It is only by considering all of these factors that an accurate picture of life costs emerges and a system is specified that truly addresses the end client’s desire for a cost-effective solution with a minimal carbon footprint.