[Hap-users] Proposed case outside airflow rates

[Aaron Smith via Hap-users]

Hi,

You shouldn't model double the amount of outdoor air.  You have two options:

1. When modeling a VRF System, you can choose "Common Ventilation System"
under the General Tab.  This will allow you to model your AHU under the
"Vent System Components" tab.  The one catch here is that it won't allow you
to choose chilled water cooling so if you have a chilled water cooling coil
in the AHU you'll need to model that as a separate system.

2. If you have chilled water cooling coil, I would also Choose "Common
Ventilation System" in the General Tab of your VRF system.  I would then
turn off the outdoor airflow in the Vent System Components tab by selecting
"Scheduled" Airflow Control and Creating a Schedule whereby the Ventilation
air is zero for all hours.  I would then create a separate system for the
AHU.  I would select "Chilled Water Air Handling Units" under Equipment Type
and HAP v5 now allows you to select "CAV - Make-up Air / DOAS" for the Air
System Type.  You can build the System and select the proper spaces which
should add the proper amount of outdoor air to the system.  If you don't
have HAP v5, select a "CAV - Single Zone" and under the Sizing Data tab, I'd
select "User-Defined" Sizing Data which will allow you to enter the actual
amount of ventilation (outdoor) air and you can enter the same number for
Supply Airflow in Zone Sizing.  This should create a 100% outdoor air unit.

Hope that helps,
Aaron

Aaron Smith, P.Eng
M&R Engineering Ltd.

-----Original Message-----
Date: Tue, 11 Jul 2017 23:35:12 +0300
From: ?a?r?l C?mert - Smart Eco Design via	Hap-users
	<hap-users at lists.onebuilding.org>
To: hap-users at lists.onebuilding.org
Subject: [Hap-users] Proposed case outside airflow rates
Message-ID: <755031f6299216c4ce1e53fa5962271a at smartecodesign.com>
Content-Type: text/plain; charset=US-ASCII; format=flowed

Hi,

Our project is an office building which pursues LEED v2009 certification.
This building is ventilated with a constant volume AHU and conditioned by
VRF system. AHU supplies 100% fresh air to the spaces, while VRF system
provides heating and cooling. AHU has heating and cooling coils, however the
system is not designed to control the room temperature, it only supplies
fresh air with constant temperature.

Because of HAP's limited ability, we had to model two separate systems for
the same zone; one system for AHU and another for VRF. And we entered the
same amount of outdoor airflow rate for both of these separate systems.
Therefore, total outdoor airflow rate is actually double-counted in the
proposed model.

Recently, we received this LEED review comment: "It is unclear whether the
minimum outside air rates (in CFM) were modeled identically in the Baseline
and Proposed Case for all zones not having Demand Control Ventilation in the
Proposed Case. Confirm that the minimum outside airflow (in units of cfm)
was modeled identically in the Baseline and Proposed case using the proposed
case rates." This comment refers to ASHRAE 90.1-2007 Section G3.1.2.8. How
should we approach this? Should we model the VRF system with zero outside
air flow, since it works separately from the ventilation system? Any help
would be appreciated.

Thanks in advance.



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