I am working for a client who has a variable refrigerant HVAC system with a separate fully condition ventilator supplying fresh air. They are over budget and the contractor is proposing ducting the air into the back of the units in lieu of separate ducts for the ventilation system. We are worried that this would lead to them not meeting the LEED IEQ prerequisite. Would they not meet the prerequisite because it is already fully conditioned and not just a heat exchanger?
I am trying to understand how outside air loses effectiveness and would require more. In an effort to maximize the potential energy savings and minimize first costs (reduced unit size and main ductwork sizes as part of the shell package) the outdoor airflow quantity was based on a ventilation effectiveness of 1.0.
In this case that meant space delivered neutral air. If the air were ducted to the back of each unit it would be heated to a temperature that reduces the effectiveness to 0.8, thus requiring more total outdoor air.
Would this change of effectiveness hurt our chances of obtaining the LEED IEQ prerequisite related to ASHRAE 62.1?
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Ilona Johnson, PE, CEM, LEED AP
AssociateLilker EMO Energy Solutions
8 thumbs up
October 20, 2014 - 12:54 pm
Dustin, I believe you are correct. If the air is heated, the effectiveness would be reduced to 0.8. The outside air design requirement would need to be based on an Ez value of 0.8 instead of 1.0. You will need to update your calculations with 0.8 and see how much outdoor air is required with the new configuration.
Dustin Balton
Project EngineerHolstein White, Inc.
October 20, 2014 - 12:59 pm
Thank you Ilona. Would the change in effectiveness to 0.8 from 1.0 result in the loss of the LEED IEQ prerequisite? Would some sort of system change alleviate this issue and still maintain compliance with the prerequisite?
Ilona Johnson, PE, CEM, LEED AP
AssociateLilker EMO Energy Solutions
8 thumbs up
October 20, 2014 - 1:11 pm
Dustin, I think you would have to run the numbers to confirm, but yes, if your outside air system was designed with an Ez value of 1 and you now switch to an Ez value of 0.8, you may not have enough outside air. This would hurt your chances of meeting the prerequisite. Your options are to a) ask the contractor to stick to the initial project design which requires direct delivery of outside air b), increase the volume of outside air provided by the AHU. If you are hunting for other options, I suppose you could try to change the supply air configuration to reduce temperature stratification. But at that point, you might as well just ask the contractor to install the system the way it was originally designed!
Dustin Balton
Project EngineerHolstein White, Inc.
October 20, 2014 - 1:21 pm
That's what I was afraid you would say. Thanks for your help.