Dear All,
What is the methodology for LEED projects to claim credits (if
possible?) for natural ventilation in dwelling units? In the building I
am working on, only openable windows are used for natural ventilation,
there is no engineered system. Cooling / heating will be provided by VRF
units. Can one make an assumption that occupied spaces will be
ventilated by natural means during the times they are occupied in the
proposed case, while for the baseline case mechanical ventilation will
apply?
Thank you,
Omer Moltay
Omer -
A few questions -
What do the mechanical codes require at the project site?
What is common industry practice for this style building?
Based on the climate and loads, would natural ventilation over mechanical ventilation be considered an occupant amenity or a downgrade in indoor air quality?
Does natural ventilation meet the requirements of EQc2 - (eg climate is favorable for nat vent year-round, spaces served do not have internal loads that exceed the OA conditioning capacity)?
In the Mid-Atlantic, natural ventilation for dwelling units is a common, if not predominant case, and the mechanical codes require either natural ventilation or mechanical ventilation systems to be provided in buildings. Since it's required by code, both the baseline and the design would be required to have a ventilation plan, whether natural or mechanical. And as LEED requires the ventilation quantities to be the same in the baseline and the design, I would expect the ventilation strategy to match between the two cases. In addition, since our climate isn't favorable for natural ventilation at winter and summer peaks, the indoor air quality may suffer (shut windows!) compared to a mechanical system that treats the outside air fully.
There may be some exemptions or CIRs I'm not aware of, but my intuition tells me that Multifamily buildings (at least in the Midatlantic USA) would have a hard time classifying natural ventilation as an ECM.
Any contrary experience/perspectives out there?
Aaron
Aaron Dahlstrom , PE, LEED? AP
Omer,
First, your method for ventilation needs to be the same in the baseline and
proposed cases. You will need to provide documentation & calculations that
shows each space within the dwellings complies with Natural Ventilation
requirements. In Equest, the best way to account for NV is to adjust your
infiltration. Change the internal loads to account for 0.35 Air Changes per
hour for each NV space, in the baseline and proposed. LEED reviewers are
particular that natural ventilation is accounted for 24/7. If your HVAC fan
runs continuously this isn't a problem, but most residential systems are
intermittent and modeling natural ventilation as infiltration is the
solution.
Thanks,
Joshua W. Chapman, P.E., LEED AP
Dear Joshua,
Thanks for the info. Does this mean that it is not possible to claim
credits for natural ventilation?
When you say natural ventilation should be accounted for 24/7, are you
suggesting that both the baseline and proposed cases need to have
continous infiltration that corresponds to the natural ventilation
(although this is not realistic since residential units are never
ventilated 24/7 by operable windows). What about the cooling effects of
this ventilation? Can eQuest model heat loss and gain due to natural
ventilation? Will this be taken into account during simulation so that
mechanical cooling is reduced in both baseline and proposed cases
because there is natural ventilation (and also I presume there will be a
substantial increase in heating energy if the spaces are naturally
ventilated 24/7 in winter)?
I would appreciate some more guidance on this.
Thank you very much,
Omer Moltay
Classification: UNCLASSIFIED
Caveats: NONE
The Advanced Energy Modeling For LEED Technical Manual discusses natural
ventilation in Appendix D. I have never done this, so do not have any
experience on how well it works.