I am doing an ASHRAE 90.1-2007 PRM analysis. We have a DOAS CV laboratory design that has a requirement between 10-20ACH in lab zones and a room set point of 20degC.
The baseline system is System 5 – Packaged VAV with Reheat
Section G3.1.2.8 states airflow rates shall be sized off supply-air-to-room-air difference of 11degC, or the required ventilation air/makeup air, whichever is greater. In our case the required ventilation rate is greater than the flow rate using an 11degC temperature difference (supplying at 9degC).
Design outdoor air requirement =12,400l/s
Supply flow rate at 11degC temperature difference = 7,500l/s
My question is, for the baseline building do we still supply our outdoor 12,400l/s air at 9degC + 2.3degC supply reset (G3.1.2.12) from the AHU? Doing this leads to the baseline overcooling to 11.3degC (9+2.3) and then re-heating to 17-18degC to achieve the space set-point of 20degC. This creates an inefficient baseline and therefore a positive effect for the energy cost saving.
The same problem seems to have been posted here, with the answer suggesting that this is acceptable to overcool and then reheat:
http://energy-models.com/forum/discourse/general-discussion/vav-minimum-...
Any help would be much appreciated.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
March 9, 2016 - 3:39 pm
It appears as if G3.1.1 exception d applies, so the 50% reduction applies during unoccupied periods. The rest of what you included above sounds right.
Jordan Kirrane
AssociateHurley Palmer Flatt
6 thumbs up
March 24, 2016 - 7:56 am
Marcus, thanks for your response. This proposed Laboratory system uses 2 AHU’s per floor, one AHU will be conditioning a wing to 25-35% RH 20degC during all occupied hours, the 2nd will be conditioning a wing to a more standard 40-70%RH 20degC. The baseline is system type 5, with one per floor.
To achieve the low 25% to 35% RH and 20degC the proposed system is using chemical desiccant dehumidification. To achieve 25%-35% RH and 20degC in the baseline the cooling coil off coil temperature of System type 5 would need to be 2degC (using a psychrometric chart).
Firstly, is it acceptable to change the baseline System type 5 cooling coil off coil temperature to 2degC, and therefore no longer be supplying at the 11degC delta T required by G3.1.2.8 (it would now be 18degC delta T).
Secondly, because there is only one system per floor in the baseline, this would then mean both wings would have to be conditioned at 25-35%RH (rather than only one wing) as the baseline has no re-humidification method for the wing with 40%-70% RH requirement. Does this then mean the low RH% wing needs to be modelled as System Type-3 but still with the 2degC cooling coil off coil temperature?
Thanks for your help.
Jordan Kirrane
AssociateHurley Palmer Flatt
6 thumbs up
March 24, 2016 - 8:09 am
Marcus, thanks for your response. This proposed Laboratory system uses 2 AHU’s per floor, one AHU will be conditioning a wing to 25-35% RH 20degC during all occupied hours, the 2nd will be conditioning a wing to a more standard 40-70%RH 20degC. The baseline is system type 5, with one per floor.
To achieve the low 25% to 35% RH and 20degC the proposed system is using chemical desiccant dehumidification. To achieve 25%-35% RH and 20degC in the baseline the cooling coil off coil temperature of System type 5 would need to be 2degC (using a psychrometric chart).
Firstly, is it acceptable to change the baseline System type 5 cooling coil off coil temperature to 2degC, and therefore no longer be supplying at the 11degC delta T required by G3.1.2.8 (it would now be 18degC delta T).
Secondly, because there is only one system per floor in the baseline, this would then mean both wings would have to be conditioned at 25-35%RH (rather than only one wing) as the baseline has no re-humidification method for the wing with 40%-70% RH requirement. Does this then mean the low RH% wing needs to be modelled as System Type-3 but still with the 2degC cooling coil off coil temperature?
Thanks for your help.
Jordan Kirrane
AssociateHurley Palmer Flatt
6 thumbs up
March 24, 2016 - 10:13 am
Marcus, thanks for your response. This proposed Laboratory system uses 2 AHU’s per floor, one AHU will be conditioning a wing to 25-35% RH 20degC during all occupied hours, the 2nd will be conditioning a wing to a more standard 40-70%RH 20degC. The baseline is system type 5, with one per floor.
To achieve the low 25% to 35% RH and 20degC the proposed system is using chemical desiccant dehumidification. To achieve 25%-35% RH and 20degC in the baseline the cooling coil off coil temperature of System type 5 would need to be 2degC (using a psychrometric chart).
Firstly, is it acceptable to change the baseline System type 5 cooling coil off coil temperature to 2degC, and therefore no longer be supplying at the 11degC delta T required by G3.1.2.8 (it would now be 18degC delta T).
Secondly, because there is only one system per floor in the baseline, this would then mean both wings would have to be conditioned at 25-35%RH (rather than only one wing) as the baseline has no re-humidification method for the wing with 40%-70% RH requirement. Does this then mean the low RH% wing needs to be modelled as System Type-3 but still with the 2degC cooling coil off coil temperature?
Thanks for your help.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
March 25, 2016 - 11:04 am
1. No you can't change that parameter.
2. Check the exceptions to G3.1.1 to see if any apply. If they do you must implement them in the baseline. The humidity setting must be identical in both models. Any humidification system is considered a process load and must be modeled identically.
Jordan Kirrane
AssociateHurley Palmer Flatt
6 thumbs up
March 30, 2016 - 6:03 am
Thanks Marcus.
To clarify, from reading your second point dehumidification should also be modelled identically? Therefore the baseline will need to be modelled with a desiccant dehumidifier identical to the proposed rather than reducing the baseline cooling off-coil temperature.
I don’t see any other method for achieving the low RH requirement.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
March 30, 2016 - 10:59 am
Dehumidification beyond what is required for the comfort of people would be considered a process load and should also be modeled identically.