| GAS BOILERS 100 Figure 2: Example potential efficiency of a commercial condensing natural gas boiler with a range of inlet (return) water temperatures and loads (Source: CIBSE Guide B1, Fig 1.40) 98 96 Efficiency (gross) % Figure 2 indicates the operating efficiency of a typical commercial condensing boiler at different loads and return water temperatures. Unlike non-condensing boilers, the performance will increase at part-load this would allow the boiler to operate typically below 20% full load several per cent more efficiently than at full load. Similarly, as the temperature of the return water reduces, the efficiency of the boiler will increase. There are inflexion points in the efficiency characteristic of Figure 2 at around 54C and 27C respectively, the points at which condensation commences and the temperature where condensation will be at a maximum. So, to maintain a high level of thermal performance, a condensing boiler should not only operate in condensing mode (that is, a return water temperature lower than approximately 54C), but also be controlled to deliver a heat output that continuously matches the demand of the building, while aiming to operate with a return water temperature as low as practicable. Return temperatures at the lower end of this range should be readily achievable from (properly controlled and configured) domestic hot water calorifiers, air handling unit (AHU) heating coils, and, potentially, underfloor heating and ceiling radiant panels. Radiator heating systems can be readily selected so as to return water that is below the condensing temperature. In refurbishment projects, the inherently oversized existing systems typically allow lower mean water temperatures and are still able to meet heating loads and so enable operation in the condensing range. This might suggest that the most thermally effective option is to use one large well-sized boiler to serve a building, knowing that for practically all the time of operation the boiler will be at part-load, and so be able to operate at a higher efficiency. (This is the opposite of the technique employed for non-condensing boilers, where the efficiency will decrease as the load reduces.) However, the use of a single boiler will increase the risk of total system failure (as there is no redundancy), limit the opportunity for planned maintenance, and also restrict the minimum possible turndown to that of a single boiler module. Installing multiple boiler modules will not only increase redundancy and so reduce the problems arising from failure or maintenance but also increase the available turndown potentially to a very few per cent of maximum load. Boilers should not cycle excessively, as this increases thermal losses and increases the output of NOx.6 The efficiency benefits of continuously operating multiple condensing Load 94 20% 92 50% 90 75% 88 100% 86 0 10 20 30 40 50 60 70 80 Inlet water temperature oC boiler modules at reduced load rather than cycling fewer boilers at higher outputs will mean that, as discussed in more detail in section 10.1.2 of CIBSE Guide F,7 it is generally more efficient to modulate all boilers in unison, compared with a step control (which would attempt to minimise the number of operating boilers). Unison control would, for example, control three separate boiler modules as in Figure 3. As the load drops below the minimum turndown for a single module, the control could sequentially switch off modules or, to prevent such cycling, make use of a buffer load. Most modern boilers will accept a 0-10V control input to modulate the boiler output directly, or to adjust the setpoint of the boiler or incorporate their own master/slave control system. The flow water temperature feeding the building heating system may be dynamically controlled with weather compensation. This will reduce the boiler flow setpoint and/or the temperature in separate zone circuits as the outdoor (ambient) temperature increases, as illustrated in the example simple optimiser schedule in Figure 4. This can provide feed-forward control of the space conditions as well as reduce the thermal losses from the distribution systems. Historically, this was principally delivered through separately pumped secondary mixing heating distribution circuits, while maintaining a notionally constant boiler supply temperature in a primary header circuit. Since modern gas boilers are able to modulate output temperatures rather than simply being on/off the compensation can be usefully applied to condensing gas boilers with a 0-10V control input to reduce the boiler supply temperature at higher outdoor temperatures. As discussed in section 5.9 of CIBSE AM14, a fully integrated zone and boiler control system will determine the highest zonal water temperature at any time and then directly compensate the boiler 100 67 % full duty CPD PROGRAMME 3 nd 2a , 1 les odu m r le Boi 33 son uni n i g rkin wo Figure 3: Unison control of a three-module boiler (Based on CIBSE Guide B1 Fig 1.41) 0 0 33 % of full load demand 67 100 52 July 2019 www.cibsejournal.com CIBSE July19 pp51-54 CPD v4indd.indd 52 21/06/2019 14:52