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STANDBY FOR ACTION Rethink your pumps strategy

Shane Leather, Business Development Manager – Tall Buildings, Armstrong Fluid Technology, suggests rethinking the longterm feasibility of installing 100% duty standby in projects where it is dicult to jusfy the higher carbon impact and addional costs.

n recent decades there have been pivotal moments when the HVAC industry moved away from outdated thinking towards more efficient and sustainable approaches. Examples include the transition from constant to variable flow systems, and the migration from analogue to digital control technology. I would argue that our industry has now arrived at another pivotal moment, with the acknowledgement that doubling pump capacity, ‘to be on the safe side’, could be holding us back. The practice of specifying full duty/standby pump installations is so widespread that it has not traditionally been questioned. On a day-today basis, one pump (sized for full duty) is operational while another of the same size is out of action, simply there to accommodate 100% redundancy in the event of routine maintenance or pump failure.


In high reliability installations, such as hospitals and data centres, where HVAC is fundamental to the effective operation of buildings, full duty- standby is necessary for peace of mind. However, in medium reliability installations (such as hotels, offices and retail stores) or sites considered as having a low risk (such as schools and apartment blocks) there are far more sustainable, efficient and cost- effective ways of safeguarding pump operation than having pumps standing idle ‘just in case’.

‘ Adopng a

parallel pumping strategy

(specifying two smaller pumps, each sized for 50% of the design flow) in preference to a 100% standby approach, can reduce first installed and lifecycle costs, free up space in the plant room or energy centre, and potenally halve the embodied carbon for the installaon.

Adopting a parallel pumping strategy (specifying two smaller pumps, each sized for 50% of the design flow) in preference to a 100% standby approach, can reduce first installed and lifecycle costs, free up space in the plant room or energy centre, and potentially halve the embodied carbon for the installation. In this article I will look at key factors when adopting a parallel pumping strategy: meeting design flow; determining control strategy; and product selection.

Parallel pumping

The key obstacle that system designers anticipate when considering the move from 100% standby to parallel pumping is that, if a pump should fail, or require routine maintenance, the remaining pump (sized for 50% of the duty) will not be sufficient to maintain adequate comfort conditions for the building. This is not the case, however, for a number of reasons.

Firstly, the amount of days throughout the year on which the building will actually require 100% pumping capacity is typically very low. Research shows that today’s variable speed HVAC systems operate at between 10% and 60% of design load, for around 90% of the time. In fact, a detailed analysis of actual demand carried out by Armstrong Fluid Technology for offices in Canary Wharf, identified that demand only went above 80% of load on six days in a year. So, for the overwhelming majority of operating requirements, a maximum design flow of 80%, rather than 100%, is sufficient. In addition, the latest generation of

Figure 1. Figure 2.

pumps can deliver far in excess of the presumed 50% of design flow when necessary. As Figure 1 illustrates, maximum design flow from one pump is around 80% rather than 50%, as Armstrong Design Envelope pumps meet a wider range of operational conditions than traditional models. So, if one pump was to be shut down for routine maintenance, requirements can be comfortably met with the single remaining pump. Even on the few days a year when demand exceeds 80% flow, we can identify, from cooling and heating coil characteristics, that approximately 95% or greater output from the emitters would still be delivered, in real terms, from a single pump. So whilst this parallel pumping strategy may not be suitable for all applications, there are many instances in which specifying two smaller pumps will deliver considerable first installed and lifecycle cost reductions, as well as reducing physical and carbon footprint. Full duty/standby installations, for example, typically double the embodied carbon for this aspect of the project. A 37kW 100% standby installation involves around 2,520 kg of embodied carbon, versus 1,214 kg (less than half) for two smaller parallel pumps.

Control strategy

When adopting a parallel pumping approach, however, the staging methodology is key. Typical pump staging scenarios involve turning on the next pump when the existing or current pumps reach 95% of maximum speed. Pumps are typically staged-off when the existing or current pumps slow down to 55% of maximum speed. Both of these scenarios (staging pumps on too late, and off too early) involve lost efficiency.

By contrast, adopting efficiency- based staging points (as opposed to staging points based on pump speed) avoids these lost efficiencies. Throughout the day the system “surfs” across the top of the efficiency curves (see Figure 2). This improved strategy for staging pumps can achieve energy savings of over 30%.

An increasingly popular product selection strategy is to specify integrated 22 BUILDING SERVICES & ENVIRONMENTAL ENGINEER DECEMBER 2017 VISIT OUR WEBSITE:

solutions, purpose-designed for parallel pumping (for example Armstrong’s new Tango range, which automatically optimise energy efficiency. Each Armstrong Tango solution features two pump heads with built-in parallel sensorless pump control integrated as standard. This optimises hydraulic performance, providing 3% to 6% improvement in efficiency, whilst new high efficiency iECMTM permanent magnet motors provide another 2% to 7% efficiency improvement. As Tango modules are supplied with the option of a blanking plate, maintenance can be carried out effectively on one of the pumps while the other remains in operation. Single pump operation continues to deliver 82.5% of capacity (based on a Tango module with 2 x 4kW motors each supplying 50% of the duty) while work is carried out.


To conclude, the decisions we make as system designers and installers have long-term environmental impacts, and it is this desire to improve sustainability of the UK’s buildings that drives our adoption of new approaches. Following on from new industry-wide approaches such as variable speed pumping and digital relational control, we should now be rethinking the long- term feasibility of installing 100% duty standby in those projects where it is difficult to justify the higher carbon impact and additional costs.

uArmstrong’s new Tango range automacally opmise energy eciency. Each Tango has two pump heads with builtin parallel sensorless pump control integrated as standard.

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