CONTROLS HEATING AND COOLING
The most misunderstood and difficult-to-resolve cause of poor control is the controllability of the HVAC services
cause of poor control is the controllability of the HVAC services: no matter how good the control system or BMS is, lack of controllability means lack of effectiveness for all load conditions. A very simple example of an uncontrollable system is an oversized DX Cassette Unit with on/ off control. This will never achieve a stable room temperature, and can result in cold draughts, dissatisfied occupants and poor efficiency of operation. Changes to system design or plant selection may be required to achieve acceptable controllability – but such changes can be expensive and disruptive, and are avoidable by designing controllable systems. Alternative control strategies can sometimes be used, although not normally without some compromise of efficiency or operation.
No matter how good a building management system is, if the HVAC system is uncontrollable it cannot control effectively
Heating Many controllability issues are associated with multiple boiler and chiller systems, which are normally less noticeable to the building user but are of increasing importance to reduce energy consumption and carbon emissions (and can be expensive and disruptive to rectify). CVT032 states that inadequate or incorrect application of boiler control can easily add 15% to 30% to fuel consumption, compared with a well-controlled system. Here are some of the most common heating-system controllability issues. Poor header design (see Figure 1): Headers should be designed with flows at one end and returns at the other. If one circuit returns prior to the take-off for another circuit, the second circuit flow temperature will be affected – in this case the hot water supply (HWS) primary, which could affect HWS temperature. This should be obvious, but I’ve seen it on a major project within the last couple of years. Split headers (see Figure 2): Individual flow and return headers will cause interaction between primary and secondary pumps, varying flows and potentially affecting stability of boiler control. It can be relatively easy to resolve by making the split header into a common header. A combined header/ de-aerator/dirt separator can be very useful for this purpose. Individual boiler pumps (see Figure 3): Individual boiler pumps and a common header are desirable to eliminate dilution of flow through off-line boilers, but can be the source of controllability issues. The most common issue is that boiler sequence cannot be controlled from return
www.cibsejournal.com December 2011 CIBSE Journal 23
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