We have recently had CIBSE gains Version 25.00.51 calculations for the offices to five mixed use warehouse / office buildings produced by a consultant that we have used for years, however, the consulting engineer on the project has raised a query that they have not been able to answer to his satisfaction. This is becoming a serious headache so if anyone can point us towards some questions that we can ask, that might help make sense of the situation, it would be greatly appreciated.
The calculations are for the selection of equipment for a VRV DX system that will heat and cool the building. There is mechanical ventilation via HRVUs which the manufacturer's calcs show will deliver air to the rooms at 24.2C at the specified design conditions, during the cooling cycle (project engineer has asked us to use 26C to allow for performance degradation between maintenance visits due to dirty filters and heat recovery cores).
The issue is that the calculations are showing negative convective loads on the heat gains, which the project engineer says he can't understand as the outside air is warmer than the desired room temperature. As an example, we are looking at around -400w to -750w in an 8 person meeting room and -1700w to -2500w in a relatively small open plan office, throughout the day on the 15th July. At no point during the day do the convective gains go positive, even though the outside air is significantly warmer than the room temp.
As I see it, given convective loads are the sensible load from infiltration and any fresh air or conditioned air introduced locally to the room, infiltration would therefore be positive as ambient temp is higher than room temp, fresh air is conditioned but is being delivered warmer than the room temp so would be positive.
I'm therefore hearing the project consultant's concerns and am at a loss to understand whats going on, really could do with some help in working out what's going wrong.
I would recommend that the first thing to do would be to update the software - the version mentioned is over 8 years out of date at this stage and now unsupported.
After that, without seeing the project itself it will be very difficult to give a specific clarification on the reasons for your results, but it may be due to your Fresh Air Loads settings. I would suggest setting to No Additional Fresh Air and re-running the calc as a test for this.
Shane Regan | Principle Building Performance Support Engineer