Objetivos

Effective control of boilers is a significant factor in achieving good energy efficiency. Inadequate or incorrect application of boiler control can add 15–30% to fuel consumption. The following points should be noted: — the control of multiple boilers is often poor — the reduced standing losses and improved part

load efficiency of modern well insulated, low water content boilers normally allows very simple hydraulic arrangements to be used for multiple boilers

— simple layouts do not require the use of individual boiler pumps, or automatic isolation valves, which are the cause of many problems associated with the control of multiple boilers

— boiler controls must be considered at an early stage in the design; adding controls after plant layouts are designed can often result in uncontrollable systems.

10.3.2.1 Avoiding boiler dry cycling

Preventing excessive boiler cycling saves energy. Small (domestic) boilers should have boiler inhibit so that their operation is interlocked with the main heating and hot water thermostats(8)_{. Interlocking in this way ensures that} the boiler and pump only operate when there is a real demand for heat.

Figure 10.9 Directly compensated single boiler system plant (reproduced from General Information Report GIR 40(7)_{; Crown}

copyright) TE TE TS 1 TC Boiler Load Common header Key

= Compensator – step output TC

= Temperature sensor TE

= Burner control and high limit thermostat TS1 TE 2 TE 4 Common header MCV 1 TE 6 TE1 TE2 TE3 TE4 TE5 TE6 TSW1 OPT1 TC1 Key

= boiler return water temperature sensor = zone 1 compensated flow temperature sensor = zone 2 compensated flow temperature sensor = zone 1 space temperature sensor

= zone 2 space temperature sensor = outside temperature sensor = HWS timeswitch

= heating optimiser

= boiler sequence proportional temperature controller TC2 TC3 SC1 SS1 MCV1 MCV2 DPE1 DPE2 DPC1 DPC2 = zone 1 compensator = zone 2 compensator

= boiler sequence control step controller = boiler sequence selector switch = zone 1 compensator valve = zone 2 compensator valve = zone 1 differential pressure sensor = zone 2 differential pressure sensor = zone 1 differential pressure controller = zone 2 differential pressure controller

TC 2 DPE 1 DPC 1 TSW 1 Module 1 Load Load Module 2 Module3 Module4 TE 3 MCV 2 TC 3 DPE 2 TE5 DPC 2 SS 1 SC 1 TC 1 TE 1 OPT 1

Figure 10.10 Pumped primary circuits with multiple secondary circuits plant (reproduced from General Information Report GIR 40(7)_{; Crown copyright)}

Heating and hot water design 10-11

Boiler anti-cycling controls

Stand-alone boiler anti-cycling controls that delay boiler firing to reduce unnecessary cycling by increasing the ‘off’ time of the boiler are available. These devices provide little or no improvement to boiler efficiency compared with good, standard controls(8,10)_.

Demand-based boiler control and system inhibit

Standing losses and excessive part load operation can be reduced where boilers and associated primary pumps are only enabled when there is a demand. This can signif- icantly reduce energy consumption where boilers and systems have high standing losses. However, response times must be adequate to meet demand, particularly where the boilers serve hot water services systems. For modern high efficiency, low standing-loss, boilers the benefit is reduced but still worthwhile.

Sophisticated strategies can be developed with advanced control systems and some lower cost systems (such as boiler energy managers) are now available with demand- based control of central plant dependent upon zonal demand. Boiler energy managers often combine opti- misers and compensators alongside system inhibition and daytime optimisation. These can be very successful for small buildings but can cause problems in larger buildings due to response times. Demand-based control strategies should be used for larger buildings.

10.3.2.2 Boiler sequence control

Controlling multiple boilers in sequence:

— matches the number of boilers firing to suit the load

— minimises the number of boilers firing, thus, maximising overall efficiency

— avoids short cycling of burner operation and, therefore, enhances energy efficient and stable operation

— is normally carried out with respect to boiler circuit return temperature, although flow temper- ature can be used.

Sequence control will not operate correctly where the flow varies as a result of individual boiler pumps or automatic isolation valves. Systems must have:

— a single primary pump, see above

— a common header or buffer vessel to ‘decouple’ primary and secondary circuits, see above

— a margin between the boiler thermostats and the sequence control setting to prevent interaction; this is normally 8 °C to allow for boiler thermostat switching differential and dilution effect of flow through off-line boilers.

Individual boiler thermostats must be set in accordance with HSE Guidance Note PM5(32)_{. Therefore, systems} must have an adequate head of water or be pressurised to permit boiler thermostat settings higher than 82 °C(33)_.

Where boilers have individual pumps, or automatic isolation valves are used, the primary circuit flow will vary. The flow/return temperature is therefore not repre- sentative of the load on the system and boiler sequence control will not operate correctly.

Where boilers do not have separate primary and secondary heating circuits (e.g. domestic), TRVs will reduce the flow through the boilers and hence boiler control is lost at low load. To give near constant flow through the boiler(s), a differential pressure regulating valve should be installed in a by-pass. At periods of low heat load, this will circulate the flow back to the boiler, maintaining the required flowrate through the boiler. TRVs in this situation will slightly reduce the savings possible with a condensing boiler but they should still be used as they provide important zone control. These problems can be avoided by using a more energy efficient system, in which the primary boiler circuit is separately pumped and connected to the secondary circuit via a common header. A variable flow compensated secondary circuit can also be used with variable speed drives for energy efficient operation. Sequence control related to outside air temperature can provide reasonably stable sequencing of boiler plant. However, it is a totally open loop control and does not respond to actual system load. It should only be used, therefore, where other methods are not possible.

Sequence selection can be manual or automatic, based on time or usage with more sophisticated control systems. Condensing boilers should always operate first; and modular boilers with a common combustion chamber normally require a fixed sequence of operation.

10.3.2.3 Burner controls

Single-stage, two-stage and modulating burners are available. Two-stage high/low burners provide more stages and therefore improved part load efficiency compared with single-stage burners. Where high/low boilers are more efficient at low fire (e.g. with a flue damper) then the sequence should gradually bring them all on at low fire before bringing any on at high fire. The sequence is therefore ‘low–low–low’ then ‘high–high–high’.

Modulating burners provide the most efficient part load operation as air/fuel ratios can be maintained across the output range, ensuring high combustion efficiency. LTHW boilers with modulating burners are now becoming more widely available, providing a means of matching the load more accurately across the full output range. In larger boilers this offers the opportunity for oxygen trim control to further optimise the air/fuel ratio. Oxygen trim control can provide savings from 2% for a well maintained boiler, up to 5% for older boilers with hysteresis in linkages, etc.

10.3.2.4 Directly compensated boilers

Direct compensation of non-condensing boilers is normal- ly of little benefit with modern high efficiency boilers due to their low standing losses and the need for separately pumped secondary circuits (see 10.3).

However, directly compensating the primary circuit is an ideal way of achieving low return water temperatures and therefore high efficiencies with condensing boilers (see

10-12 Part A: Designing the building

10.3.1 and Figure 10.9). Where heating and hot water are being supplied from a common plant then compensation must be overridden when hot water is required, possibly resulting in overheating of the space. Use a separate hot water supply where possible.

10.3.2.5 Reducing boiler standing losses

Significant energy savings can be achieved by minimising the standing losses in older, heavyweight poorly insulated boilers. Modern boiler standing losses are typically 0.75% of rated output for recent designs (but can be as low as 0.2% for high efficiency and condensing boilers). Older boiler designs with higher water content can have standing losses up to 7% of rated output. Whilst these older boilers may need back-end shut-off valves to minimise standing losses, modern boilers will not. The Boiler (Efficiency) Regulations(17)_{stipulate minimum effi-} ciency levels for new boilers at full and part load efficiency from January 1998. Manufacturers have therefore improved their plant to meet these standards, resulting in low standing losses. Building Regulations Approved Document L*(14)_{suggests even higher standards at 30%} load, which will force standing losses even lower. Losses on modern plant are therefore too small to justify instal- ling shut-off valves.

Automatic boiler isolation valves can be used in multiple boiler installations to isolate the flow through off-line boilers and reduce losses from individual boilers. However, this adds to the complexity of multiple boiler systems and often leads to control problems. Using boilers with low standing losses alleviates the need to isolate flows through off-line boilers. Isolation of flow through off-line boilers is rarely effective and the additional cost of automatic isolation valves is wasted, particularly when they leak between ports or are set to part-open.

When boiler output is not required, boilers and primary circuits should be inhibited to minimise boiler and pipework standing losses, as well as pump energy. Inhibition should occur when compensator valves are on full re-circulation and time delays are required to prevent rapid cycling. The complete heating system should shut down when outside temperatures are high.

It is difficult to achieve satisfactory standing losses with large high water content boilers. Effective automatic isolation is normally complex and losses from associated distribution systems are often high. Distributed systems with smaller boilers located nearer the loads reduce distribution losses and, with well designed control and monitoring systems such as a BMS, can often be operated more efficiently.

Boilers with forced, or induced, draught fans should have the fans interlocked with burner operation to minimise standing losses. Large boilers should have air inlet or flue dampers interlocked with burner operation to prevent standing losses from natural draughts.