﻿More comfort for less energy
Variable-flow systems can achieve improved comfort using less energy in many buildings — but not unless they are carefully designed and commissioned, as Peter Rees explains.
When comparing waterborne heating and cooling systems in the current environmental landscape, a variable-flow system presents a far more attractive option, largely due to its energy-saving credentials over more traditional constant-flow systems. Savings in both cost and carbon for the end client (building or facilities manager, tenants and owner) are made possible by lower pump speeds. However, whilst this measure alone can deliver initial energy savings, it is necessary to ensure that the system is installed and commissioned to operate at the design flow, otherwise these savings will gradually reduce as the system becomes less efficient over time.
Without regular maintenance and manual service checks, the system will not run at its optimum level of efficiency. As such, further problems can occur that will affect the internal ambience of the building. For example, a pump forced to work harder as a result of a poorly balanced system will generate noise that will travel throughout the circuit and infiltrate the rooms.
Additionally, when large open-plan areas are refurbished, problems can occur as the layout is altered. This can lead to problems with the system as, in most cases, the system itself will not be changed to accommodate the new plan. As such, rooms can overheat or become too cool as tempered air cannot flow through the space as it would have done previously.
It is important to ensure that terminal units are operating at the desired flow, delivering a consistent level of performance throughout the building, even in areas that are located furthest from the pump itself. If the system is not commissioned and regularly checked for flow accuracy, the temperature in different areas of the building is likely to vary — and the thermostat will constantly be altered to try and counteract differences in indoor temperature.
The ‘cost of discomfort’ can prove to be extremely detrimental on the components within a variable-flow system and can dramatically impact their longevity. As the thermostat is altered throughout the day to meet occupants’ needs, the system, over time, will continue to run at lower and lower efficiency as greater load is placed on the boiler and pump. This can prove to be especially problematic after sustained periods of downtime, such as first thing in the morning or after the weekend. Extreme weather conditions can also affect the system, such as those experienced during summer heat waves and freezing winter conditions. As a result, the boiler and pump will be forced to work harder to heat or cool the building as required, using more energy and, therefore, cancelling out any initial savings made.
To avoid such problems, the system must be installed and accurately balanced at the time of installation, using valves that can regulate the flow and ensure an even temperature level is achieved across all terminals.
Where this is not the case, or where proper measuring and maintenance of the system is not carried out, 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 system and resulting in an unregulated flow, which sometimes causes sometimes severe fluctuations in temperature. This means that rooms not favoured by the irregular flow will not receive the desired temperature until much later in the day, or maybe not at all.
To maintain a properly functioning variable-flow system, a method of stabilising pressure must be introduced. A differential-pressure control valve (DPCV) or a pressure-independent control valve (PICV) can be used to solve this problem by diverting excess pressure where there is an unwanted pressure drop or opening to increase flow where there is insufficient pressure to maintain design flow.
By incorporating a PICV into the design rather than a DPCV, it is possible to reduce installation costs and space needed on site, as only one valve is required instead of three. This is due to the core function of a PICV — incorporating differential pressure 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 changes in pump speed. DPCVs are designed to alter their position in response to changes in pressure, to maintain pressure equilibrium.
A well designed and commissioned system can help reduce flow rates by up to 20%, resulting in a pump energy consumption saving of up to 50%, which long term can generate significant savings.
During commissioning it is important to adhere to a few basic principles that can be applied to all variable-flow systems, to help make sure the system runs as efficiently as possible.
Broadly speaking, these principles include the following.
• 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 and, as a result, save energy, whilst still generating enough operating pressure for the system.
• Ensure that the pump pressure is sufficient so the flow can continue uninterrupted.
• Install flow-measurement devices to ensure that the system continues to work efficiently.
Building-services designers and installing engineers using a system that employs DPCVs or PICVs will see benefits beyond occupant comfort. The projected energy savings from a variable-speed pump will be achieved, with a reduction in commissioning times.
Modern HVAC systems provide excellent environmental controls, 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, leaving a system with no means of troubleshooting balancing problems.
Peter Rees is technical director with Tour & Andersson.