﻿Enjoying the benefits of variable flow
Variable-slow systems can save serious amounts of energy. Peter Rees review how to exploit their benefits.
According to the Carbon Trust, poorly performing HVAC systems can increase a building’s energy consumption and associated carbon emissions by up to 100%. With fluctuating climatic temperatures; exemplified by the recent Winter freeze, followed by a long hot Summer, the design, maintenance and operation of effective and efficient hydronic heating and cooling systems should be high on any building services manager’s agenda.
In the current environmental landscape a variable-flow system represents an attractive option, largely due to its energy-saving credentials over more traditional constant-flow systems. Savings in both cost and carbon are made possible by reducing pump speed.
However, whilst this measure alone can deliver initial energy savings, it is necessary to ensure that the system is installed and commissioned professionally — as well as continuing to operate at the design flow. If the design and commissioning parameters drift over time, the savings associated with the efficiency of a variable-flow system will gradually reduce as it ages.
A well-designed and commissioned system can reduce energy consumption by up to 50%. This is achieved by reducing the flow rate by up to 20% to generate significant savings in the long term.
Without regular maintenance and manual service checks, the system will not run at its optimum level of efficiency — with negative effects not only on the cost and carbon efficiency of the HVAC system but also on the internal ambience of the building. For example, further problems can occur that will affect the internal ambience of the building. If the pump is forced to work harder as a result of a poorly balanced system it will generate noise that travels the circuit, infiltrating rooms throughout the building.
Newer buildings present fewer challenges to building managers than older buildings, which can present a real headache— with plenty of areas to consider in the introduction of energy saving measures. Tired, out-of-date systems are ripe for replacement, while retrofit solutions can be costly, challenging to incorporate into existing building fabric and disruptive to the businesses operating there.
For those undertaking refurbishment as a means of upgrading and introducing energy-saving measures there is lots to think about. In altering the layout of an existing building (to accommodate a different use for example) problems with the heating and cooling system can occur. In most cases the waterborne system will not be altered in accordance with the new layout, leading to rooms overheating or becoming too cold. This is because tempered air cannot flow through the space as it previously would have done.
Taking refurbishment and system upgrades into account it is vital to ensure that terminal units consistently operate at the desired flow, delivering a consistent level of performance throughout the building, even those areas furthest from the pump. If the system is not commissioned and regularly checked for flow accuracy the temperature in different areas of the building is likely to vary. This will cause discomfort for staff and building users, resulting in them reaching for the thermostat in an attempt to counteract differences in internal temperature.
This ‘cost of discomfort’ is a phenomenon that, left unrecognised, can cause severe damage to the components of a variable-flow system, impacting longevity and life costs. An unbalanced heating and cooling system will quickly become unregulated. Water will work its way through the system via the quickest route, finding shortcuts throughout the system, resulting in an unregulated flow, which can cause severe fluctuations in temperature. This problem means that rooms not favoured by the irregular flow will not receive the desired temperature until late in the day or, in some cases, not at all.
When demand on the system is increased, the boiler and pump must work harder to meet the greater load requirements, which, over time will degrade the efficiency of the system. The problem is exacerbated following sustained periods of downtime such as an extended seasonal break, a bank holiday or even just first thing on a Monday morning. Extreme weather conditions can also affect the system and the demands placed upon it for heating and cooling.
Whatever the circumstance under which extra demand is placed on the boiler and pump, it is likely that a system working outside of its set design parameters will be working less efficiently as a result.
To avoid these problems, systems must be installed and accurately commissioned with hydronic balancing methodologies. A pressure-stabilising methodology must be introduced. A differential-pressure control valve (DPCV) or a pressure-independent control valve (PICV) can be used to solve this problem. These valves will divert excess pressure where there is an unwanted pressure drop or open to increase flow where there is insufficient pressure to maintain design flow.
A PICV, rather than a DPCV, can help to reduce installation costs and space needed onsite. This is because only one valve is needed instead of three. A PICV incorporates differential-pressure control alongside flow limitation and control functionality.
After commissioning, the DPCV prevents the flow and balance in the sub-circuit being affected by pressure changes caused by other sub-circuits opening and closing or pump speed changes. DPCVs are designed to alter their position in response to changes in pressure so as to maintain pressure equilibrium.
During maintenance it is important to identify the index circuit to ensure the pump is set to deliver enough pressure to maintain the flow, place pump pressure sensors deep within the system so that the pump can reduce its speed accordingly and, as a result, save energy whilst still generating enough operating pressure. Pump pressure must be sufficient for flow to continue uninterrupted. Finally, flow-measurement devices should be installed to ensure that the system continues to work efficiently.
Building-services designers and installation engineers using a system that employs DPCVs or PICVs can see benefits beyond occupant comfort. The projected energy savings from a variable-speed pump will actually be achieved, and commissioning time will be reduced.
Modern HVAC systems can provide excellent environmental control without the need for excessive energy consumption. However, the absence of an appropriate means of measuring the final balanced system could result in time lost by the engineer in identifying the cause of poor distribution, due to pressure drops in system terminals. Installing either DPCVs or PICVs should never lead to removing balancing valves at major pipe junctions round the system, leaving a system with no means of troubleshooting balancing problems.
Peter Rees is technical director with Tour & Andersson