Low water content: it doesn’t take a genius

Optimising energy efficiencies from commercial plant can be a complex operation, but first and foremost, it comes down to specifying the best boilers for the job. Peter Gammon, Technical Manager at ELCO UK (formerly MHS Boilers), explores the issue of boiler efficiencies and how water content can play a key role.


Reducing energy usage makes perfect business sense; it saves money, enhances corporate reputation and helps everyone in the fight against climate change. So there is a strong argument for building owners and facilities managers to constantly evaluate their day-to-day running costs. One of the cost centres that has the greatest effect on the bottom line is heating, with considerable benefits from upgrading commercial plant.


Depending upon the age of the boiler, it is common for the appliance to be non-condensing type with a relatively low efficiency compared to the latest models on offer. There will clearly be some cost benefit to any replacement of ageing plant, but with efficiencies put to one side for a moment, there are other factors at play. One aspect in particular which requires careful consideration is water content.


Primary concerns


For facilities managers and building owners, the water content of the boiler plant might not be a primary concern. But as consultants and specifiers will well know, there can be a considerable difference between the low water content (LWC) and high water content boilers (HWC). Of course, there has been plenty of conflicting information from manufacturers which have blurred the lines between the two types.


So first of all, it’s only fair to point out that there will always be a project and system design that is better suited for one type over another. By simply considering the overall efficiency levels of the boilers, there is unlikely to be much difference. Likewise, carrying out a seasonal efficiency calculation for both boiler types (based upon the calculation methodology given in the Non-Domestic Building Services Compliance Guide), there is little difference. However, when you take into account the weight of water contained in a HWC appliance, there are some stark differences.


Recognising the advantages and disadvantages of each type is vital for the correct specification of any new equipment, especially as water content can have a significant effect on the day-to-day running costs of a commercial property.


To illustrate this point, let’s take a typical HWC unit with an output of 1,048kW by 80/60°C, an average water content of around 1,100 litres (kg equiv.) and a gross heat input of 1,190kW. The time taken for the water mass in the boiler to reach operating temperature is 264 seconds with an energy input of some 88kWh. Consequently, the carbon emission per heat-up occasion equates to 16.03kgCO2 and, more notably, the cost to heat the water content from 20ºC to 80ºC at a rate of 4.64p/kWh is an uneconomical £4.05.

Keeping in mind the cost analysis above, we can perform the same calculations for a typical, modern premix gas-fired LWC condensing boiler of similar gross heat input and output. With a heat input at maximum rate of 1,183.2kW and a far lower water content of 117 litres (kg), the time taken to bring the mass of water from an ambient standby temperature of 20°C to a flow temperature of 80°C is 28 seconds. That’s 236 seconds less than the high water equivalent. But it isn’t until you look at the carbon emissions and cost per heat-up occasion that the benefits really start to show.


In terms of energy input, the LWC boiler requires 9.2kWh, which produces 1.7kgCO2, and costs an astonishing £0.431 (at typical current gas costs). Now, remembering the previous figure of £4.05 (almost 10 times more), the disparity in heat up consumption over the lifetime of the boiler plant (usually 15 to 20 years) show dramatically higher running costs. Furthermore, carbon emissions will also be some 89% higher per cold start heat-up occasion.


Limiting factors


Calculations aside, one suggested argument against LWC boilers is that they suffer when under low loads. This assertion is due to a high volume of freshly heated water circulated back to its return port via the low loss header (boiler-system interface). As HWC boilers don’t require a minimum water flow, they are often seen as a superior option. However, since 1 January 2013, the Ecodesign Directive requires high efficiency pumps to be used and, to comply, manufacturers of LWC appliances now supply speed controlled pumps. These match the heated water mass delivery to the load requirement – thereby avoiding recirculation and impairment of the boiler’s capability to operate in condensing mode.


Another limiting factor to consider with HWC boilers is the associated weight from the unrestricted water flow. They are considerably heavier than LWC alternatives, placing additional weight loadings on a building’s structure. In addition, HWC boilers can cause slower response times, considerably higher costs to heat-up from a cold start, greater heat-up time emissions, a potential increase in water treatment chemicals and the requirement for larger system expansion vessels. Any of these factors can make LWC a more favourable choice for the majority of commercial applications.


Flexible heat generation


The heat loads of buildings vary constantly, and a well-planned installation of LWC boilers provides a flexible heat generation service that is quick to respond to increasing or decreasing changes in load. This allows very high, real load-driven operating efficiencies that are not dampened by the effects of high water content inertia.


There are, of course, plenty of considerations to take into account when specifying low or high water content boilers. Although there will not always be a ‘one type fits all approach’, the overall costs of running a boiler have to be high on the specification list. As the Carbon Trust points out, when refurbishing or installing a new heating system: “Always select the most appropriate system for your requirements” and “prioritise energy efficiency in the design and installation stages”. With the cost per heat-up clearly demonstrated, as well as the ability to overcome low loads, LWC boilers are sure to remain the most popular option for the majority of commercial installations.


Ultimately, installing LWC boilers provides greater potential for energy savings over their lifetime and can reduce carbon emissions, while also enhancing the reputation of a corporation (and its balance sheet). Now that’s a combination that certainly makes good business sense.


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