Barton Associates recently retro-commissioned an air handler for a hospital client. The retro-commissioning process typically starts with a review of the Building Automation System (BAS). A graphic screen shot from the subject air handler is shown above. There is a lot of information on this screen, but focus on the basic information circled in red and diagrammed below:
The heating and cooling coil loads were calculated using the documented air flow and entering and leaving air temperatures recorded at each coil. Now review the information on heating and cooling coil valves circled in green above. The heating valve is reportedly fully open – which seems accurate since the screen shot indicates a 26 degree temperature rise across the heating coil. The cooling coil is reportedly closed. Not just 0% open, but -10% open! Even so, the cooling coil sensible load is calculated to be 1,430 MBH, (over 119 tons!) with a 38.6 degree supply air temperature drop.
Upon further investigation, we discovered a mechanical failure of the chilled water control valve actuator; the valve was fully open when commanded closed. In other similar instances, we’ve found that the valve action was actually the reverse of what the BAS system was programmed to control. Either way, the end result is excessive use of heating and cooling energy, plus higher pumping energy to circulate more heated and chilled water than required.