Design air flow rate

Francesco Passerini

engineer

LEEDuser Basic Member

65 thumbs up

July 27, 2012

I don’t understand the meaning of paragraph G3.1.2.8 of ASHRAE 90.1, in particular: “System design supply airflow rates for the baseline buildings design shall be based on a supply-air-to-room-air temperature difference of 11°C”. In the proposed building there is a heat recovery system for the supply air (heat exchange between the exhaust air and the supply air, but there is no other supply air handling). Therefore the supply air temperature depends on the external temperature. There is no other mechanical ventilation system. During winter season the zones that are mechanically ventilated are maintained at 18°C. Shall the baseline model have a heating system for the supply air in order to maintain the supply air at 7°C (= 18°C – 11°C)? Best Regards

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Luke Knapp

P.E.

Midwest Engineering Professionals

LEEDuser Basic Member

2 thumbs up

Tue, 07/31/2012 - 18:20

I believe you're confusing the requirements of Appendix G with the ASHRAE requirements for your proposed building. Appendix G deals soley with your baseline building. Your baseline building will have some "Baseline HVAC system" from table G3.1.1A. This baseline system is what paragraph G3.1.2.8 is giving you guidance on. If this paragraph were absent, one could, theoretically, put that temperature difference at 1 degree and then your system would have to move a LOT more air to compensate, which in turn would drastically drive up your baseline energy cost. That would inflate your energy savings.

Francesco Passerini

engineer

LEEDuser Basic Member

65 thumbs up

Wed, 08/01/2012 - 14:00

Thank you, Luke. You are right: my ideas are confused :-) According to Table G3.1.1A the HVAC system of my baseline model has to be “System 5 – Packaged VAV with Reheat”. In such a system heating energy and cooling energy are provided by the supply air. Should I interpret paragraph G3.1.2.8 of ASHRAE 90.1 in the following way: if the internal set-point temperature during the winter season is Twin then system design supply airflow rates have to be calculated considering a design supply airflow temperature of Twin + 11°C and if the internal set-point temperature during the summer season is Tsum then system design supply airflow rates have to be calculated considering a design supply airflow temperature of Tsum - 11°C? Does the procedure of paragraph G3.1.2.8 of ASHRAE 90.1 regard only the sizing run or even the simulation with the test reference year? Are the ventilation flows of the “proposed building model” completely independent from the statements of paragraph G3.1.2.8 (i.e. shall the supply air flow rates of the "proposed building model" correspond to the supply air flow rates of the actual building?)? Best Regards