When a heating system is being designed, one of the primary considerations is clearly to source heating plant that is inherently efficient. The selection process should also take account of operational parameters such as variable heating loads, ensuring the plant is able to adapt efficiently to the changing demands for space heating and domestic hot water from the building(s).
Indeed, this latter consideration has become even more important in recent years as new buildings have become more thermally efficient and many existing buildings have been refurbished to improve their thermal performance. This leads to set point temperatures being achieved more quickly, so that the heating plant is operating under part-load conditions for longer periods of time.
A tried and tested approach to managing variable heat loads has been to use a cascade arrangement of modular boilers, each of which is able to modulate. For example, if there are several boilers in a cascade – each capable of 5:1 turndown the result is a highly adaptable and responsive system. Clearly, the effectiveness of the control of the system is paramount in delivering maximum performance and energy-saving benefits.
A further benefit of cascade systems is that they can be supplied as a kit complete with wall-hung boilers, low temperature hot water (LTHW) and gas pipework header(s), inter-connecting flexible LTHW and gas pipework, pumps and valves. Not only does this make installation easier, it also uses less space in the plant room.
In recent years there has been a tendency to include other, low carbon, heat sources alongside oil or gas fired condensing modular boilers, such as biomass boilers, heat pumps or solar thermal. Where different heat sources are mixed in this way, the control aspects become even more important.
A key purpose of the control strategy in a cascade is to ensure the design temperature differential (ΔT) between flow and return water temperatures is maintained. If not, the boilers will switch on and off (cycle) far too frequently so that energy is wasted, emissions increase and the life of the plant is reduced.
To maintain the design ΔT the boilers should come on at low-fire initially, or modulate to meet the heat load, depending on the type of burner being used.
Many building operators are keen to make wider use of renewable heat sources in the design of their heating systems to support their own sustainability programmes, as well as compliance with planning regulations. Of the various renewable heating technologies available, the most popular and cost-effective choice in the UK is proving to be use of cascade systems that use biomass boilers to meet base heat loads and are supplemented by gas-fired boilers.
In these cascade configurations, the biomass boiler is the lead boiler and it is essential the control strategy recognises that biomass boilers are not designed for rapid on/off firing. They require some time to stabilise, so the control parameters need to allow sufficient tolerances for the biomass boiler to meet the set-point temperature. A buffer vessel should be specified to be fitted between the biomass boiler and the heating system to ensure optimum boiler run times are achieved.
Also, if the gas-fired back-up boilers are brought in too quickly this may cause the biomass boiler to switch off, so that the full heat load is then being met by the gas-fired boilers.
Adapting to Change
When the system is first installed, correct commissioning will ensure that the controls enhance the inherent efficiency of the heating plant and the design of the distribution system. Over time, though, building usage will almost certainly change and if efficiency is to be maintained the control strategy must adapt.
For example, a system that is first commissioned to meet the heat loads of a new shell and core building should be re-commissioned in line with changes to heat loads as tenants move in.
In older buildings similar challenges apply as work patterns and occupancy change, or improvements are made to the thermal performance of the building fabric. Routine maintenance should therefore include regular assessment of the control configuration.
This is facilitated by the use of remote monitoring, which can be easily implemented for both gas or oil-fired and biomass boilers. This would also be beneficial for those maintenance service providers that have guaranteed the energy performance of the plant they maintain.
Consequently, it is very rare that a control strategy can be ‘cast in stone’ and continue to ensure efficient performance that is aligned to the heating requirements of the building. Rather, it makes more sense to think of a control strategy as ‘work in progress’ that continues to evolve through the life of the building.