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The five key principles for energy efficient HVAC control... We have prepared five key principles for identifying and realising opportunities to make savings in HVAC systems, as you will see they are all statements of the extremely obvious, however you can be assured that they are all very common in practically every building that we see. |
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 On the right is a live example of data taken at one minute intervals over a 24 hour period from a building in central London and is typical of what is going on in the 90% of buildings where the control systems are inadequate. Click the image to view full size.  Only Supply Heating or Cooling if there is a demand. Demand may be identified by the delta (differential) between flow and return temperature, as you can see from this graph there is no delta on the heating circuit and minimal on the chilling. Supporting standing losses on the system, when there is no demand for a supply of energy has got to be a waste.  React to External temperatures by Compensating Internal SettingsDuring the 24 hour period under investigation there is an eight degree variance between maximum and minimum external temperature, however this is having no effect on the amount of heating or chilling being supplied. Clearly there is no compensation on either circuit, energy therefore is being wasted!  Never Operate Heating and Cooling in the Same Place at the Same Time
It can also be clearly seen that the heating and cooling come on at exactly the same time and have absolutely no effect on the space temperature of the building which stays at a constant 23ºC throughout the 24 hour period! Arguably it would be possible to maintain the target temperature of the building without either heating or chilling suggesting a potential 100% energy saving. In reality some heating may be required early on in this situation and some parts of the building may need cooling during the day.  Before illustrating the final two points it is necessary to have a closer look at the zone control of this site, there are three variable temperature zones and no DHW. Click on the graph to the right to view full size.  Control Separate Zones with Different Needs Independently Flow temperature data from three zones from the site and a little more clarity as to what is going on, zone A does appear to have some compensation, but is a little unstable, whilst zones B and C appear poorly compensated and unstable, the worst of both worlds. It is also interesting to note from this graph that the area between the zone flow temperatures and the main header flow temperature is indicative of 5ºC waste even if the three zones were compensated correctly, further more the main header actually rises in temperature as outside air temperature rises.  Provide Necessary Heat with Minimal Boiler CapacityFinally you should only use enough boiler / chiller power to satisfy the demand and not as usually happens allow your plant to “dry cycle”. This is often the biggest single give away of poor plant control from a BMS, where an enable signal is sent to each boiler and they run on their own internal stats instead of being properly demand sequenced. |
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