[Equest-users] Help - Modeling VRV-VRF System with eQuest

[Dahlstrom, Aaron]

Gents -

Quick Q - why not just define one PVVT for each thermostat?

Aaron

Aaron Dahlstrom , PE, LEED(r) AP
In Posse - A subsidiary of AKF| 1500 Walnut Street, Suite 1414, Philadelphia, PA 19102
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From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Bruce Easterbrook
Sent: Thursday, January 12, 2012 8:30 PM
To: Neil Bulger
Cc: equest-users at onebuilding.org; Loi Ng
Subject: Re: [Equest-users] Help - Modeling VRV-VRF System with eQuest

I haven't done a VRF system so my remarks apply to trying to get multiple zones under control using a single thermostat.  The biggest problem is you probably do not have enough airflow to begin with to carry the heat required.  The eQuest default is 0.5 chm/SF.  Once you get into heating dominated climates this is not nearly enough air to accomplish your heating with the air handler.  eQuest will size your airflow to handle the cooling demand which it has done in your case but leaves you with some pretty extreme under heated hours.  Start bringing up your cfm/SF up in 5% increments starting at 1.0 cfm/SF.  Monitor your heating capacity of your air handler(s) on the SV-A page.  The SS-J heating peaks will also start to increase.  Once you aren't getting any change in your heating capacity of the system air handler you have gone far enough and may need to backtrack a bit.  You can adjust your throttling range a bit say to 4 deg instead of 2.  You can adjust your control zone, or the location of your T-stat to see if you can find a better location.  Your main tool is the cfm/SF.  In my area, much colder than Oregon I get into the 1.2 to 1.25 cfm/SF range.  On one I just did the under heated hours per zone went from the 900-1000 hour range down to 40 to 180 hours for some of the trouble zones with a majority in the 0 to 10 hour range.  Then I added in the baseboards and all the under heat hours vanished.  You don't want to use the baseboards for control too fast as you want to have your AHU carry the majority of the load because your AHU will typically use gas to heat and your baseboards will use electricity.  I think this will work with either work around for VRF.  You have a heat coil and a cooling coil each at a specific temperature.  The only other variable you have to heat or remove heat from your zone is your airflow.  So eQuest has provided your maximum airflow required for cooling at around 0.5 cfm/SF, it will adjust the flows to different zones, your increased airflow for heating will give you the approximate maximum flows required for heating.  So you have basically sized your terminal unit flow.  With your maximum heating and cooling effects all on the AHU you will get a better picture of what your work around system will have to handle.  Excessive heating with the baseboards could give you a false picture if you are not careful, you just want them for trimming to get your errors to disappear and hopefully most of your warnings.  They do provide the heating differentials from the control zone.  The SV-A will also give you a good picture of what is happening in each zone.  Other reports will help you track down trouble zones, what is happening and hopefully why.
Bruce Easterbrook P.Eng.
Abode Engineering

On 12/01/2012 04:21 PM, Neil Bulger wrote:
Hi Loi,

I have run across this as well with trying to model VRV in eQuest. PVVT is a difficult system to work with because it requires specifying 1 zone as the thermostat that drives the whole system. Therefore, 1 zone will drive what supply air temperature the central AHU delivers to each zone. This can make hours out of range on cooling difficult but, if heating is the issue you may have more flexibility with zone-level heating options (reheat or baseboards). However, that can start to look strange when in reality, these features do not exist. But, the hours out of range typically stem from this single-zone-thermostat issue with PVVT.

Another approach I have not perfected yet that I thought of was to try using the eQuest Water Heat Pump system. The Water Heat Pump ties to a WLHP water-loop (it appears yellow in the Water-Side tab). If you were to place a dry-cooling tower on this loop, effectively, the dry cooling tower would be (in my mind) the same as an air-source condenser, like we would see in reality with a VRV system (typically).

This may be good or bad on the airside in eQuest (I am hoping good) in that it provides another set of options and heat flow control at each system. Again, I have not dug into how this would go together best and I would imagine some post processing would be needed to understand the results.

Would love to hear any other creative ideas out there for VRV work-arounds.

Neil Bulger
Project Engineer
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From: equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org> [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Loi Ng
Sent: Thursday, January 12, 2012 9:38 AM
To: equest-users at onebuilding.org<mailto:equest-users at onebuilding.org>
Subject: [Equest-users] Help - Modeling VRV-VRF System with eQuest

Hi,

I am modeling VRV system based on a "Energy Trust of Oregon, Inc." guidance document (attached) posted by Mathew Larson of Colorado dated 12/12/2011.  The document recommends using a PVVT system to simulate a VRV system.  I used an air cooled PVVT system to model a VRV system.  However, the unmet hours on the heating is very high.  The cooling part is okay.  Does anyone has experience on bringing down the unmet hours?

Thanks!

Loi Ng
Mechanical Engineer, LEED(r) AP
Diversified Engineering, Inc.
914 Silver Spring Avenue, Suite 208
Silver Spring, MD 20190
Tel: 301-565-2000 x 234
Fax 301-495-1563





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