I have a 11000 sqm warehouse (3 units) 24/7 in operation with natural ventilation, no cooling designed or required, heated to 15°C. The OA requirement is met by the level of infiltration of 0.1ACH, that gives over 0.3 L/s/m2, so the fresh air demand in both Baseline and Proposed is set to 0. The proposed system is similar to system-1 to mimic the operation of decentralised flued gas convectors, and there is also DX coil modelled, but the cooling set point was adjusted to 70°C so that the cooling system never comes on in both proposed and baseline models.
Baseline selected as system-5 as per G3.1.1A
The minimum VAV flowrate was set to 2.15L/s/m2 as per G3.1.3.13 (ASHRAE 90.1 2007) that is actually higher than the heating flow rate required to maintain 15°C set point.
Consequently the baseline system operates at constant flow rate through winter and summer that gives me a high energy usage, ie obtaining 40% saving in the favour of the proposed system that only operates in heating mode when there is a heating demand.
Is everything OK with this modelling scenario and accept this level of saving that my client is happy to hear? At the same time I want to be sure that the submission is not going to be rejected as I don't have a long LEED modelling experience.
Your assistance is highly appreciated if there is anything wrong with my scenario.
Thanks
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
March 8, 2018 - 10:02 am
We would recommend that you use a system #9 according to Addendum dn. This is by far the more accurate way to model this project and does not require you to use any sort of workaround to disable the non-existent cooling system.
That said you are not required to use this addendum and are allowed to do what you are proposing. We are a bit surprised at the level of savings you are showing as a result. The fans should be cycling based on the temperature settings and not operating continuously since they do not supply any outside air. If you cycle the fans the saving should drop significantly.
Zsolt Bako-Biro
GT Advanced Ltd.March 12, 2018 - 3:18 am
Hi Marcus, thanks for advice.
Re System-5, (in case not using Addendum dn) if I cycle the fans on/off is it not contradictory to G3.1.3.13, which tells me to maintain 2.15 L/s or the minimum ventilation rate, "whichever is larger", i.e. regardless of the heating/cooling demand or the OA requirement?
Also when I import the prototype system-5 (using IES VE) there is a default minimum flow setting for the system of 10%. Should I override this setting to 0%? If I turn off a VAV system for any reason that is not going to reflect a normal VAV system operation in real buildings, right?
Thanks
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
March 12, 2018 - 10:57 am
This is not contradictory. The minimum flow set point only applies when the systems are on, not when they cycle off.
Not sure how IES VE does this but I would guess that you should change that setting to 0%. Regarding real buildings the first thing I would say is that the 90.1 baseline systems don't operate in real buildings, only in your computer, so don't get caught trying to design the baseline system. In buildings with minimum outside air the VAVs would be at a minimum setting when cycled but that does not appear to be the situation in your case.
Again just model a System 9 and avoid all this. The reviewer is going to recommend that you do so anyway.
Zsolt Bako-Biro
GT Advanced Ltd.March 16, 2018 - 7:52 am
Thanks Marcus, I'll use System 9 anyway.
While I'm at this I'd like to ask your opinion about the baseline system of a smaller storage place with hazardous materials isolated from my above warehouse units with an airlock (pressurised room). The hazardous space has its own ventilation unit and DX cooling. Initially I modelled this with System-3, using G3.1.1 exception b). for having cooling loads more than 31.2 W/m2. The airlock was also modelled with System-3 in the baseline according to G3.1.1 Exception c).
However I'm wondering to treat the hazardous space as a technological area and consider the related energy use as process energy, ie to model the baseline identically as the proposed system according to Table G3.1#12. Can I do that? This is because a special air handling unit for ventilating/conditioning hazardous materials cannot compete with a normal baseline system fit for comfort requirements.
Thanks
Zsolt
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
March 16, 2018 - 12:53 pm
You could probably make that case especially if the conditioning is more for the materials stored than it is for any people occupying the space. I assume that people go in and out and not one is in there for long periods at a time. You could probably also justify it as a conservative approach.
Zsolt Bako-Biro
GT Advanced Ltd.March 16, 2018 - 2:26 pm
Thanks, I'll do that. The space is indeed not for prolonged human occupation.