Hello, we have this residential project were the HVAC system is as follows:
Baseline: System 2 - PTHP
Proposed: Mini Split System
The thing here is that, as known, conventional mini split systems do not enter OA directly, so the project supplies OA through independent fans not interlocked with the HVAC system, they rather work based upon occupancy (switch on, switch off), usually we enter the supply OA in system level but account for the fan power as a direct load with an schedule. In this particular case, proposed system includes not only OA but exhaust in order to maintain room pressure constant. How should we address the exhaust?
If we input the exhaust cfm rate in system level, the system requieres fan power (kw/cfm), if we enter the fan power of the independent fan, HVAC system fan power will increase, and baseline fan power won't be the expected allowed fan power for baseline (since in system 2 supply fan power shall always be 0.0003 kw/cfm).
I'm thinking two different approaches:
1.- Set exhaust fan power to 0 (or at least 0.00001 to make eQuest run). Include Exhaust power the same way as supply OA, modeled as a direct load with occupancy schedule.
2.- Model the actual exhaust fan power in system level, and somehow wrestle with supply fan power (decrase the 0.0003 kw/cfm) to meet te required fan power allowance.
I believe the first approach to be the more accurate one, since supply OA and exhaust may run even when HVAC is off, and second scenario requieres the HVAC to be running to account for the exhaust consumption.
David Eldridge
Energy Efficiency NinjaGrumman/Butkus Associates
67 thumbs up
November 14, 2018 - 4:27 pm
I see you are using eQUEST for your energy modeling program, you may be interested in this list external to Building Green where modelers congregate and ask many similar questions:
http://lists.onebuilding.org/listinfo.cgi/equest-users-onebuilding.org
Let me see if I'm understanding the system - what if there was a mini-split system running normally, and then you entered the OA supply and exhaust fans in the power consumption for the exhaust fan, and put in a schedule for the exhaust fan according to the projected occupancy?
If I'm understanding the system the mini-split runs for the zone loads, and there is an intake fan and exhaust fan in the unit that doesn't have any heating, cooling, etc. so that outside air would be coming into the unit unconditioned?
Jorge Lopez de Obeso
Architect / Environmental advisereosis Consulting
20 thumbs up
November 15, 2018 - 10:01 am
Yes, OA enters the room at exterior temperature through an independent fan (kinda like a f&c and OA Intakes), and leaves the room through another independent fan, however, in the system level I need to configure the OA and exhaust air, the thing here is that the system is asking me for the exhaust fan power, wich, is not part of the system per se. Thats why i do believe case 1 scenario is more helpfull.
btw, I've tried onebuilding but pretty much no one responds, energymodels as well.
David Eldridge
Energy Efficiency NinjaGrumman/Butkus Associates
67 thumbs up
November 15, 2018 - 5:21 pm
I think this would be best modeled at the zone level supply and exhaust - perhaps as infiltration - that way the air is brought directly into the space where if you model it with the air system it will be conditioned first and then supplied to the zone. What will actually happen is the fresh air is a load upon the zone rather than a direct load to the split systems.
If you model the exhaust fan within the air system, you'll be bringing the air in through the system's equipment and distribution rather than direct to the space.
I forget if it is possible to model the infiltration exactly that way to match the occupancy schedule, and then model the two fans as a direct electrical load on the same schedule.
Then let the split system do its thing responding to the loads to maintain temperature.
I hadn't seen your message(s) come through on either bldg-sim or equest-users - did you see the message come through yourself?
Jorge Lopez de Obeso
Architect / Environmental advisereosis Consulting
20 thumbs up
November 15, 2018 - 5:49 pm
Infiltration makes sense but I've had really hard times explaining the reviewers why the OA and exhaust does not appear in SV-A reports, so I just model it at system level. Besides, I have approx 300 conditioned thermal zones with different areas and system capacities and efficiencies and it would be really time consuming to calculate the infiltration rate for each zone.
The message was through a different user name, not mine.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
November 20, 2018 - 2:14 pm
We agree with David that zone level infiltration and exhaust would be the best way to model this configuration. To satisfy the LEED Reviewer you should provide a thorough narrative explanation including screen captures showing the infiltration inputs and the calculations performed to determine those values. The zone level exhaust inputs should show up in the SV-A reports. To figure this out for all those spaces we should use the eQUEST spreadsheet view, copy and paste the values you need to use and do the rest in Excel.
Jorge Lopez de Obeso
Architect / Environmental advisereosis Consulting
20 thumbs up
November 20, 2018 - 7:14 pm
Zone level exhaust inputs would not show up in the SV-A reports, since SV-A only reports system inputs, and exhaust is being made through independent fans not interlocked with HVAC.
We already made the trial for both scenarios and they basically have the same results. 1% variation in systems cooling capacities (which means same system EIR). Is it really worth it to follow the infiltration methodology rather than make a simple, quicker, equally effective approach?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
November 21, 2018 - 2:43 pm
We will often see zone level exhaust fans in the zone portion (lower part) of the SV-A report. They are not included with the system level fans but are often listed separately on those reports.
Our concern as reviewers with his approaches is that it is not clear how you would be accounting for the de-coupled operation of the mini split fans (cycling with load) and the OA/exhaust fans (cycling with occupancy). As far as we know you can’t model a continuous OA supply simultaneously with mini split supply fans which cycle on a call for heat/cooling unless you model the OA (through infiltration) and exhaust at the zone level.
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