[Equest-users] Excessive Unmet Heating Load Hours withan Auto-Sized HVAC System

[Robert Des Rosiers]

Dakota:

There is definitely diversity in the zones and I have used the glass library for all window types.  Also, I agree that not having reheat is inappropriate for this building use due to the programmatic and occupancy diversity within each zone (there are two AHU's feeding multiple zones in the proposed design and I have maintained those thermal blocks per ASHRAE appendix G).  Unfortunately the baseline system I have described is what is required per the "Required Treatment of District Thermal Energy" document for LEED compliance.  Assuming the system is functioning correctly (or as best as possible considering the design) and the heating loads are not met would LEED allow the addition of zone level baseboards for purposes of the Optimized Energy credit?

 

Note: I have included the suggestions listed below (except for local baseboard heating) and the improvement in unmet hours is marginal.

 

Thanks -

 

From: Dakota Kelley [mailto:dakotak at teliospc.com] 
Sent: Friday, June 11, 2010 9:40 AM
To: desrosiers at studioma.com
Cc: equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Excessive Unmet Heating Load Hours withan Auto-Sized HVAC System

 

Robert,

 

50,000 hours is much more than I would anticipate for a capacity issue.  You are essentially describing a hydronic VAVT system, which is begging for unmet heating hours if there is a lot of diversity in the zones’ demands and no reheat capabilities.  My experience has been that eQUEST over-sizes hot water systems, and this is exacerbated if the “simple” method is used to define the windows.  It can be as much as 250%-300%.   That many hours sounds like the system is not functioning as intended – almost as if the heating loop is not assigned to the zones/system, or something similar.  Or, not having reheat could be inappropriate for the building use or climate.  Regardless, adding zone reheat as Bruce and Pasha indicated should make a huge difference.

 

Thanks,

 






  
                                

DAKOTA KELLEY 

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From: Bruce Easterbrook [mailto:bruce5 at bellnet.ca] 
Sent: Friday, June 11, 2010 6:33 AM
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Excessive Unmet Heating Load Hours withan Auto-Sized HVAC System

 

In your air side HVAC section go into the zone level and check that your sizing option is set to "adjust loads".  If it is from loads go to the summary tab and change all your zones.  Also beside that is throttling range, you can try adjusting that upwards say to 4 deg.  You can also change your control zone to see it this helps.  If your AHU is off at night you can also adjust it to come on before the occupancy.  You SS-O report will show this if the hours are lumped near the start up time.  Then add more heat as Pasha said.
Bruce Easterbrook P.Eng.
Abode Engineering

On 10/06/2010 10:55 PM, Pasha Korber-Gonzalez wrote: 

I have found with my own projects that even sometimes when the HVAC designer has specified a specific capacity for htg water equipment that sometime in my model file I still have unmet loads for htg.  I suggest at this point specify additional htg capacity by either increasing the reheat delta-T from say 30F to 50F and/or adding zone level baseboards.  By adding the baseboards you are adding a component of the system that will supplement the skin losses being experienced by the simulation model.

 

by adding baseboards for the additional requrired heat then you should be able to meet your heating loads.

 

hope this helps,

pasha

On Thu, Jun 10, 2010 at 7:38 PM, Robert Des Rosiers <desrosiers at studioma.com> wrote:

All:

I have completed an energy model for a baseline scenario from which to compare my proposed design.  The baseline HVAC system is a constant volume single zone AHU w/ no reheat; the AHU is served by HW and CHW loop from a central plant.  I have set this up in eQuest as a "Variable Air Volume" system type with the AHU fan control set to "Constant Volume"; my thought here is that each zone fed by the AHU will control temperature using a VAV box w/ no local reheat or cooling.  Flow and temperature from the AHU will be delivered based on the coolest or warmest zone it serves.

 

All seems fine with the design flows, ventilation flows and cooling capacities.  My problem is with the heating capacity as I have excessive unmet heating hours (50,000).  I have allowed eQuest to size the heating capacity of the preheat and heating coil as well as the minimum flow ratios.  Does anyone have any thoughts as to what is causing these unmet heating load hours?  Is the way I have set-up the desired baseline HVAC system incorrect?

 

Any guidance is greatly appreciated.  Thanks -

 

Robert Des Rosiers, LEED-AP

STUDIO MA

130 N Central Avenue, No.300 Phoenix, Arizona 85004

Telephone 602 251 3800

Fax 602 251 3100

 


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