How to operate condensing boilers most efficiently

Legislation and an industry-wide desire to tackle CO2 emissions have led to enormous growth in the use of condensing boilers in commercial heating applications. Unfortunately, the vast majority of those installed will never provide the energy savings or CO2 reductions they should.

So, how can their true potential be harnessed in both new-builds and refurbishments? The answer is to rethink system design temperatures and control philosophy, reappraise hydraulic system design and harness the benefits of system integration.

System design temperatures

Substituting a condensing boiler for a conventional boiler is not enough, in itself, to create optimum savings. The system conditions must be right, particularly the return water temperature.

In the UK, 82°C supply, 71°C return is the tried-and-tested favourite, but this is far from ideal in condensing applications. Condensing mode requires a return temperature of 54°C or lower. So maximum system temperatures of 70°C supply and 50°C return are preferable or, to condense continuously, 50°C supply and 30°C return.

Higher temperatures have traditionally been specified to account for winter weather but, in fact, they are required only when it is -2°C outside. The average UK daytime winter temperature is about 10°C. It is therefore better to work at lower temperatures for the majority of the year and use the weather compensation curve simply to increase the supply temperature on the rare days when outside temperatures drop below freezing.

An ideal design temperature for a new condensing boiler commercial heating system would be 65°C supply, 45°C return. The 20°C DT means pipe and pump sizes are reduced (minimising electrical load and system noise) and the lower return temperature means we can operate in part-condensing mode all year round. Radiators, underfloor heating systems and even air-handling units can all be selected to operate effectively in this range. A stand-alone system or integrated thermal interface units will provide the most efficient HWS solution.

How about refurbishments? An effective approach is to incorporate weather compensation and resize the pump to give a DT of 20°C. Very cold days, when the system operates below maximum efficiency, will be few, with overall efficiency for the year greatly improved. By virtue of the consequential improvements that Part L stipulates, the existing radiators will be oversized so a reduction in supply temperature should not be noticed.

Control philosophy

The performance of condensing boilers has been severely limited by the fact that control philosophies have failed to adapt to the capabilities of new equipment. Conventional boiler sequencing ignites each boiler in series until the load is met. This was fine in the days when boilers were at their most efficient at full load. Modern condensing boilers, however, are at their most efficient at low load (see Figure 1, above), and by sequencing them in the conventional way we are, in fact, switching them at their least efficient point.

To achieve maximum efficiencies, all boilers should be run at their lowest possible load. They ramp up and down in unison to maintain the load. This means less wear and tear on the boilers and also reduces the often overlooked start-up losses as the pre-purge sequence runs.

Hydraulic system design

Most systems incorporate a constant flow primary circuit (see Figure 2, overleaf). While this ensures that boilers are never starved of water, it wastes energy by mixing flow and return circuits, and reducing DT in part-load conditions.

Whenever heat is generated, we must strive to dissipate it usefully in the space. Control devices such as thermostatic radiator valves and two-port zone valves should ideally be used as high-limit devices with the space temperature regulated, as far as possible, by modulating the supply temperature and the flow rate.

So how can we protect modern, low water content boilers and still achieve excellent overall system efficiencies? One answer is to incorporate intelligent inverter driven pumps configured to provide variable primary flow (see Figure 3, overleaf). When applied to integrated condensing boiler systems, variable primary configuration automatically modulates flow rate to match building load while maintaining minimum flow rate across the boiler.

By reducing flow rate and supply temperature, terminal control devices stay open, reducing system pressure drop and wasted electrical input.

Note, however, that this system can only be used where there is a common system temperature regime in place.

Benefits of integration

Finally, we need to harness the benefits of integrated subassemblies for quicker installation, improved quality, lower price, lower energy consumption and fewer risks of defects.

The conventional way of designing and specifying HVAC systems has endemic shortcomings. Per-formance features are frequently replicated on different system components, leading to unnecessary upfront cost. Also, there is nothing linking the intelligent subsystems to ensure they work in harmony to optimise performance.

The Armstrong MBS integrated heating solution solves the problems associated with maximising efficiency condensing boilers, and recent expansion of the range means that systems are now available for applications from 120kW to 690kW.

The MBS incorporates fully modulating boilers, variable primary pumps, automatic fill/pressurisation unit and integrated controls, all pre-specified for optimum efficiency, and pre-assembled for rapid installation on site. It enables excellent boiler efficiencies to be achieved and exceeded without time-consuming mixing and matching of system components.

In summary, by rethinking system design temperatures, control philosophy and hydraulic system design, and by exploring the huge opportunities presented by integration, we can begin truly to harness the benefits.