I have a school which will be served by ground source heat pumps.
Outside air will be provided to the classrooms with a Dedicated Outside
Air Unit/ ERV with a VAV fan which operates based on CO2 sensors within
the spaces. The heat pumps won't have any outside air provided to them.
My question is what's the best way to model this? Since you can't apply
more than one HVAC system to a particular space, do I create a fake
space and apply the outside air unit/ERV to that? If so, how do I model
the fake space such that there is no heat loss and only people to
require outside air?
I tried to just model the dedicated outside air system by way of the
heat pumps, but I run into problems when trying to specify when the
dedicated outside air unit would operate. I would want it to operate
independently from just when the heat pump is operating and I can't find
a way to do this.
Has anyone had any luck modeling something similar to this?
Thanks.
David B. Everhart, P.E. LEED AP
Hi David,
I've been looking for a solution to this for a while. Modeling a DOAS
using the dummy zone (set all walls to adiabatic, or with a u-value of
0.00001) is one thing, and can be done pretty easily. All your zones can
get their OA from that zone. There's plenty of posts in the archive
for this (search for dummy zones), so I'll leave it there.
The trouble is, unless I'm missing something, you can't route the
exhaust air back to the DOAS, so your ERV won't model correctly. I've
resorted to creating an hourly report and calculating the additional ERV
savings, minus additional fan and motor losses. It's certainly not
ideal, and I haven't tried it on a LEED project yet, so I can't speak to
the acceptability to the USGBC.
I had taken an eQuest training class, maybe 1.5 years ago, and the
instructor had mentioned the developers were currently working on
developing a DOAS system for eQuest and they hoped to release it within
a few months, but nothing yet.
If anyone has found a way to get this working properly, or has any
updates on the development in eQuest (is it in the forthcoming 3.64?),
that would be great to know. It's frustrating, because it is certainly
only going to be more prevalent in the future.
Eric
We had this same situation on our projects, and one in particular that used chilled water FCU's, but with an ERU DOAS doing the outside air.
We modeled the ERU DOAS separately in a detailed model/custom spreadsheet workbook we created (with help of VBA code) that could run hourly calcs on a full set of TMY3 weather data.
We will be submitting this soon as exceptional calc as part of LEED submission.
As far as exhaust air conditions, I just assumed exhaust air conditions typically found in facilities (i.e. 75 F, 50% RH in summer, 70 F, 40% RH in winter). I think that is a very reasonable assumption.
I think this is the approach we will have to take until the DOAS ERU model arrives in eQuest, which I hope is soon.
The spreadsheet developed will serve us well because I'm also using it to quantify electrical demand savings for a local utility energy efficiency incentive program (i.e. peak demand reduction during summer months).
My challenge is modifying the spreadsheet to accommodate more complex ERU arrangements such as the Trane CDQ unit or the Semco Pinnacle unit, both with low temp dessicants that deliver low dewpoint area to building. Currently scratching my head over how to model these, which wouldn't be a problem, except that they are showing up on our projects ... :)
Thanks!
Regards,
James A. Hess, PE, CEM