Getting the best performance from commercial boilers
Modern boilers are very efficient, but that efficiency can be compromised by poor system design and control. Paul Sands of Stokvis Energy Systems considers the options for improving boiler efficiency in new and refurbishment situations.
Within the commercial market today, about 70% of boilers with outputs of 30 kW and higher are wall hung. To what extent these modern condensing boilers actually deliver higher efficiencies compared to their old counterparts is, however, dependent on a number of factors — including whether they are correctly sized and specified, as well as how they are controlled.
So what are the main principles to be respected in the creation of an efficient heating and hot water system?
Although there is a definite desire on the part of specifiers to provide for future flexibility in terms of additional demand, they should resist the temptation to oversize. Aside from the increased capital outlay, a boiler's capability to modulate at low loads is reduced. It is also important to design for achieving low return temperatures in order to maximise the time when a boiler condenses. A larger ∆T or a return temperature of 56°C or below is generally considered desirable.
Wall-hung boilers are often installed as a cascade of three or more units, enabling maintenance to be carried out without interrupting service and allowing the duty to be shared.
Interestingly most condensing boilers offer slightly better efficiency when running below their full output, which can be taken advantage of in a cascade arrangement. While figures vary between different boiler ranges, a model which delivers 102% net efficiency at 100% capacity might see this rise to 106% net when running at 25% capacity; this increase in efficiency results from the lower volume of flue gas passing over the heat exchanger.
With an appropriate sequencing controller, one boiler would modulate up until the demand requires output from a second unit — at which point the duty will be shared equally between the pair. The same applies when a third boiler has to contribute. Meanwhile, the main controller also ensures ‘first-up' duty is rotated between the individual boilers to even out wear.
Weather-compensation of condensing boiler plant directly utilising outside temperature sensors will provide higher efficiencies and further savings, since a lower flow temperature will also give a lower return temperature and therefore increase condensing possibilities. In practice, for a part-load situation, the boiler controller reduces the fan speed, in turn causing a pressure drop which is utilised to modulate down the pressure operated gas valve. By incorporating weather-compensation control, the premixed air and gas is modulated down further. So, introducing weather compensation directly to the boiler plant is one of the key measures that can be used to improve the performance of new and existing heating systems.
Another way to improve performance is to replace hot-water storage vessels with modern plate-heat-exchanger (PHE) technology. When a boiler constantly cycles on and off to maintain the temperature a large amount of stored domestic hot water, it is generally firing inefficiently — normally well below its modulation range.
The way to eliminate these cycling and standing losses is to replace the calorifier with a PHE that heats domestic hot water on demand at much higher levels of efficiency. This approach combats the possibility of legionella contamination. Essentially, the PHE set up sees higher loads being met instantaneously, resulting in the boiler mainly working within its part load or modulation capabilities. The plate heat exchanger also provides primary return temperatures as low as 35°C which is ideal for condensing boilers. Further savings would be achieved by making use of a preheat vessel where the hot water is heated from a solar-thermal system.
Given the Government's environmental agenda, the proportion of smaller, wall-hung boilers is likely to remain as the larger share of the market as the uptake of renewable energy increases and, therefore, the ‘top-up load’ provided by the boiler plant requires smaller boilers that can provide good turn down/modulation with condensing capability that match the required load.
Paul Sands is sales and marketing director at Stokvis Energy Systems.