As high efficiency and low emissions continue their rise up everyone's agenda, Stokvis sales director Paul Sands flags up effective ways to more than meet the ever-more strident demands for sustainability
TO maximise heating and hot water efficiency, it is advisable to incorporate condensing technology, minimise the quantity of stored water and provide full modulation down to the lowest levels. In this way, standing losses are minimised, boiler cycling is reduced and part-loads can be matched.

A good solution, therefore, is to combine a low-water-content, fully modulating condensing boiler with a low-water-content, fully modulating plate heat exchanger.

The use of a packaged plate heat exchanger, rather than a calorifier or direct gas-fired unit, is more efficient and cost effective.

· Installation is quicker, saving labour costs.

· Less space is needed, saving valuable area within the building.

· Extra flueing is not required, saving both materials and labour costs compared with gas-fired units.

· Insurance pressure-testing is not needed as with calorifiers, saving costs.

With all these savings in view, it is clear the payback period can be greatly reduced.

Buildings vary, of course, both in size and purpose, and the most important principle is to consider the maximum and minimum heating and hot water loads required.

If we look at two ends of the size and purpose spectrum - offices and hotels - some idea of the variables involved can be given. Other types and sizes of building fall elsewhere on this spectrum and of course on-site specifics need to be looked at for every project.

First, offices: These tend to require a higher heating than hot water load. Therefore, it might be thought that, with an off-peak hot water load, the boiler would cycle excessively. However, with an instantaneous load, a couple of taps would need an input of 42kW. (42kW = 0.2l/s x 4.2 x 500C; M x CP x delta T)

Thus, provided the boiler selected can modulate down to 42kW, the boiler and plate heat exchanger option is ideal. In this example, the boiler could be rated at 168kW with modulation down to 25%, or 210kW with modulation down to 20%.

Next, hotels: These generally have a higher hot water than heating load.

If the hot water load were simply added to the heating load, the selected boiler load and perhaps even the number of boilers would be excessive, which would be wasteful, both of initial and running costs.

However, since the peak periods of hot water demand tend to be very short-lived, it is better to let hot water take priority over certain heating circuits, so that only a percentage, if any, of the hot water load needs to be added to the boiler load.

In some cases it may still be desirable to reduce the hot water load, so a relatively small quantity of buffer storage can be incorporated. Here, too. the boiler and packaged plate heat exchanger option wins through.

With a condensing boiler, efficiency is highest when the system design maintains the boiler return temperature below 56ºC (flue-gas dew-point) for most of the heating season. In other words, if a condensing boiler does not condense it rather defeats the object.

While a weather-compensated system can contribute significantly to seasonal efficiency, the return temperature at full load should be below 56ºC during peak heating conditions, because this is when the largest volume of gas is used and the greatest fuel savings can be realised.

It follows that 70CF/50CR systems at full load have the greatest potential for condensing and thus peak efficiency and savings, the efficiency being increased during milder conditions through weather compensation. The market can offer a full range of plate heat exchangers designed specifically to work in conjunction with condensing boilers.

Building regulations part L stipulates minimum boiler efficiency at high and low fire, and the best boilers exceed these levels. Part L revisions are expected this year, with greater emphasis on ensuring that high boiler efficiency is not compromised by poor boiler or system controls.

The market can provide good controllers which link in with zone controls, and some boilers can be found on the government's energy technology list website for enhanced capital allowances.

To make life easier for consultants and contractors, with recommended methods for control, hydraulics, layout etc, there are bespoke packaged plant-rooms which include boilers, plate heat exchangers, pressure sets, booster sets, pumps, controls and all mechanical and electrical elements.

In fact there is now a standard range of skid-mounted, frame-hung, fully modulating condensing boilers with sequence controls and packaged plate heat exchangers for hot water services. These come complete with all pipework manifolds, low-loss header and flue kits, and their modular form enables them to be bolted together on site.

Good equipment supplier relationships, both with consultants and contractors, are critical to ensure the best solution to any client's heating and hot water requirements.

Attention to detail is paramount throughout the entire process, from design to installation, commissioning and maintenance.

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