HEVAC’s Fan Coil Group has entered the debate over fan coil systems versus passive and active chilled beams following the publication of the CBCA’s Technical Fact Sheet 2.
Fan Coil Group claimed that “while the report claims to be a fair comparison, the CBCA have failed to use identical design criteria which cause an unfair comparison to be drawn”.
Fan Coil Group provided the following examples of to back up its claim:
- The CBCA have used 6 deg C flow, 12 deg C return temperatures for the fan coil system and 14 deg C flow, 17 deg C return temperatures for active and passive chilled beams, making the system comparison very biased towards chilled beams, when in fact to produce a fair comparison 14 deg C flow, 17 deg C return temperatures could have been used for all three systems.
It should be borne in mind that the only reason designers use 14 deg C and 17 deg C water flow and return temperatures for chilled beams is because they have to keep the water temperature above the Dew Point of the room air. Designers can of course use 14 deg C and 17 deg C water flow and return temperatures for fan coil systems if they wish to further enhance a very flexible and efficient system.
- The running cost calculations have included the AHU supplying air temperature of 18 deg C for the chilled beam systems, but 14 deg C for the fan coil system again compromising the fairness of the system comparison.
- Using the same water flow and return temperatures for all systems will obviously result in the same chiller COPs being achieved.
- The specific fan power of an AHU supplying fresh air to a chilled beam will be higher than supplying to a fan coil System, given the chilled beam systems higher pressure drops.
- A typical specific fan power level for a fan coil at design conditions is 0.2 W/l/s, which means on average the specific fan power will be 0.15 W/l/s.
- The cost of 13p/kWh is typical for domestic properties, but the average cost for medium size commercial premises is 8.6p/kWh*, which significantly diminishes the difference in running cost between systems.
- While the report assumes the same fixed specific fan power factor an AHU supplying a chilled beam system would not, in fact, have a fixed SFP.
The report does not take into account the option to vary fresh air and ventilation rates based on occupancy demand which is available with fan coil systems, but not with chilled beam systems.
The Fan Coil Group Unit said it believed there were several other inconsistencies within the report, which it will discuss with the CBCA in the coming weeks.
The group added that it intends to issue its own paper defining all of the advantages that a fan coil system can offer the whole supply chain within an office building / refurbishment programme.