We have a narrow plan 3 storey naturally ventilated building with radiators (baseboard) in temperate Ireland. Some windows are user controlled and some controlled by the BMS, with night purge.
We have reviewed CIR 1734 and others which refer to 1734, but these interpretations seem only practical for small buildings: we have about 80 zones.
We have problems with ASHRAE 90.1-2007 G3.1.2.2 "unmet loads hours" and have bolted on VAV (to meet Table G3.1 10.d) which destroys our energy efficiency and seems pointless. The building has been occupied over a year and just went through a "hot" summer without difficulty.
VAV would not fit in the building without dropping in false ceilings, loosing the thermal inertia in the slab and radiant cooling therefrom, lowering ceiling heights so reducing beneficial stratification (although we haven't modelled stratification).
We are using EnergyPlus with DesignBuilder front end. We are using calculated natural ventilation. Each single year long simulation is taking about a week to run! We do short period runs too.
Any suggestions or others experience would be most welcome.
We hear on the grapevine that LEED may be struggling itself with how to deal with naturally ventilated buildings.
Thanks.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
December 11, 2013 - 11:55 am
Hard to say anything about your unmet load hour issue without having much more information. Is it solely due to the natural ventilation? Did it go away when adding the cooling system? What are your temperature set points?
Regarding G3.1.10(d) - I think it is silly to add a cooling system since the easy work around is to increase the cooling set point until it does not run. So you could just leave it out and tell the reviewer that you will add it if necessary but increase the cooling set points rendering it a moot point. Also keep in mind that you are not adding it in reality, just in your Proposed model, so no need to add drop ceilings, etc.
In our experience your models should not take a week to run. We have modeled much larger and more complex projects with DB and the runs take a few hours at most. Something is wrong there.
Yes there are a group of folks working on better ways for LEED projects to deal with natural ventilation, especially for international projects. I have not seen any results yet. Right now the best guidance is in the Advanced Energy Modeling Guide for LEED in one of the appendices.
Paul Overy
Overy+AssociatesDecember 11, 2013 - 12:21 pm
What are your temperature set points? 22/24/25 heating/nat vent/cooling
DB is telling us, for one zone e.g. loads not met htg=70hrs, clg=1049, occupied htg=7, occupied clg=1007, yet ASHRAE comfort summer or winter clo=1.5 hrs/annum. Maybe we are not understanding the output. I look at the E+ output and it looks like cooling is never coming on in that example zone.
"Regarding G3.1.10(d) - I think it is silly to add a cooling system since the easy work around is to increase the cooling set point until it does not run.":
ASHRAE G Table3.1-4 which tells me to use setpoints "approved by the rating authority" for proposed and baseline. So if proposed summer setpoint=40C, baseline =40C. Local ratings authority says use 24C for cooling in offices.
"Also keep in mind that you are not adding it in reality, just in your Proposed model, so no need to add drop ceilings, etc." Understood. My understanding is LEED are trying to anticipate systems which would be retrofitted because the building is too hot. This particular building, if retrofitted, would get VRV.
"Something is wrong there." Calculated nat vent? 4 time steps per hour (for nat vent modelling). Or we just need a new PC, but E+ only runs in a single core so I dont see much gain. Or maybe our daylighting is too sophisticated. We have self-shading etc.
"Right now the best guidance is in the Advanced Energy Modeling Guide for LEED in one of the appendices." That refers to CIR 1734 which appears to be for a very simple building.
Thanks.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
December 11, 2013 - 12:53 pm
If cooling is not coming on then you can't have any unmet load hours. As I recall EP reports the total unmet load hours in a table within the main output reports.
The rating authority referred to in this case is USGBC and you are allowed to use whatever set points you wish since Appendix G does not specify what they should be.
Not sure why your model is taking so long to run. We recently modeled a 30 story office tower with a double facade and VRF systems in EP/DB and each run took only an hour or so.
That is the only guidance currently available unless you wish to propose your own methodology and submit an Interpretation for approval.
Paul Overy
Overy+AssociatesDecember 11, 2013 - 1:12 pm
We've posted a question re DB reports direct to DB re unmet loads.
OK, USGBC is the rating agency. Doesn't the Proposed and Baseline building have to have the same cooling setpoints, per appendix G? Maybe we should submit a CIR confirming this need not be the case for nat vent.
We are just installing a 64 bit version. See how that goes.
If our client would wait, maybe we should wait for the USGBC to come out with more guidance on nat vent buildings, though I doubt the client will wait. We have been at this particular LEED application a long time already.
Thanks for help.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
December 11, 2013 - 1:16 pm
Yes that is the only requirement for temperature set points, they must be identical in baseline and proposed.
Good luck.