A BMS cannot perform well without a correctly designed HVAC system. And, as energy efficiency gains in importance, how can you ensure you are in control of your building?
1 October 2009
All too often a building’s heating, ventilation and cooling (HVAC) system gets overloaded, resulting in a lack of control both in temperature and running costs. There are a number of reasons for this.
The FM or building owner is in charge of indoor climate and staff comfort. Where the required climate is not achieved, the thermostat is altered and increased demand is placed on the pump and boiler. The latter has to work harder and ultimately will cost more to run.
Fluctuations of just one degree above 20ºC can increase heating costs by at least 8 per cent, while one degree below 23ºC can increase cooling costs by up to 15 per cent. Room temperatures can change at low and medium loads, even with sophisticated controllers.
Proper controllability of the heating and cooling system will eradicate the costs associated with staff discomfort, reduce time spent troubleshooting and dealing with staff complaints and lower the energy consumption of the system – with the associated benefit of reductions in cost and carbon emissions.
Three steps, known as hydronic balancing, will help you achieve a stable and accurately controllable system that allows the building management system (BMS) to deliver tangible energy savings.
1 Design flow
Because water will naturally find the easiest and quickest course through a system, the design flow must be available at all terminals. If this is not the case then the indoor climate will encounter several problems:
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Temperature fluctuations
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The installed power will not be deliverable at higher loads
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There is likely to be a long delay before the desired room temperatures are obtained at start-up – for example, first thing in the morning, or after a setback.
These can all result in higher than expected energy costs. Full control via the BMS is only possible if the required water flows are available. For this to happen the flow must be measured and manually adjusted for complete precision.
It can be done by careful sizing of the plant, however this is not so feasible for existing buildings where the FM has to deal with older pipework. If a link in this chain is not properly sized, the overall system will not function to optimum, designed performance.
Hydronic balancing prevents overflow in some circuits from causing underflows in others, detects the degree of pump oversize and generally verifies that the plant works as intended.
2 Differential pressure
The differential pressure across the control valves cannot vary too much.
If it does it can lead to the following problems:
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Continuous oscillation at room temperature.
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Maintenance problems with control valves and actuators due to fatigue.
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Room temperatures not reaching the required set point at low loads.
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Higher energy costs than expected due to unfavourable control settings to avoid instability.
Variable flow systems are a popular alternative: pumping costs are reduced, and the return temperature is minimised in heating systems and maximised in cooling systems.
However, in order for these systems to work as intended they need to be able to control differential pressure across the control valves.
There are various methods available to the designer, but only a few that deliver both controllability and energy efficiency.
3 Flow compatibility
Flow incompatibility at system interfaces can result in the design supply temperature being too low or too high. This means the installed power is not available when it is required, especially at high loads. In other words, it will take a long time for all rooms in the building to reach the correct temperature.
In many systems, the installed power exceeds the maximum that is required by 50 per cent, yet the distribution circuits still do not receive enough – the power that the boilers and chillers produce simply does not reach the heating or cooling circuits.
This dilemma is particularly common in systems with several chillers or where boilers are working in sequence.
The reason is usually a lack of compatibility at the interfaces between production and distribution. In most systems, production and distribution circuits are in direct contact with each other and this can cause serious and often mysterious disturbances, unless adequate measures are taken to avoid them.
Operating a system that complies with the three conditions for hydronic balancing, results in stable and accurate control for the best possible indoor climate with minimum energy expenditure.
Nigel Huggins is managing director of valve manufacturer Tour & Andersson
Achieving balance
It is estimated that one in three buildings experience significant problems with their indoor climate.
Corrective solutions such as sizing the plant, adding extra boilers or chillers, increasing the pump head or changing the set point, may well only aggravate the problems.
The correct solution is simply to balance the flows on each side of every interface, which will give the best operation and the lowest costs.
When designing, installing, operating and maintaining a waterborne system, the indoor climate is of paramount concern for all within the building. To avoid the many problems of inadequate temperature and high energy costs, hydronic balancing is essential.
Identifying the right contractor for the design and maintenance of a fire detection and alarm system is a challenge for any facilities manager – but make sure they are certified.
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