Excessive Unmet Heating Load Hours with an Auto-Sized HVAC System

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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

Robert Des Rosiers's picture
Joined: 2011-09-30
Reputation: 0

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

Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 400

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.

Bruce Easterbrook's picture
Joined: 2011-09-30
Reputation: 0

Pasha,

Can I use a baseboard as radiant tube heat from the ceiling (gas hookup with
Flue)? My thought is it's a computer how does it know whether the heat is
coming from the ceiling or the floor as long as its heating the perimeter.
Would I be correct or should I just go sit in the corner now and be quiet?

PS: I'm too lazy to draw perimeter zones just for a radiant heater and its
Friday. (Just joking)

Thanks,

PETER HILLERMANN

Peter Hillermann's picture
Joined: 2011-09-30
Reputation: 0

Peter--I can help you with this, you're on the right track, but Yes-being
Friday I too am feeling lazy for the weekend, not wanting to type you a
whole page of info. Give me a call and I'll talk you through it. I'm
available now.

Pasha

Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 400

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

From: Bruce Easterbrook
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.

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

Dakota Kelley's picture
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Joined: 2011-09-30
Reputation: 1

Due to popular demand I am teaching a two day beginners
course on whole building energy performance simulations
using eQUEST. The course will take place
Tuesday/Wednesday,
June 29th/30th, in Chapel
Hill, North Carolina. You can
see the
course outline at http://naturalintelligence.us/training.

The cost is $499 per person for the two days. The price
includes continental breakfast and lunch. I have arranged for
special rates at two hotels within walking distance of the
classroom: the Siena Hotel and the
Days Inn.

Students are required to furnish their own personal computers
loaded with eQUEST
version 3.63B.

To reserve your seat you can register
here or notify me via
email (off-list) or telephone.

Thank you.

Sincerely,

- Christian Stalberg

Account Deleted's picture
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Joined: 2011-09-30
Reputation: 0

Pahsa,

Will you revise your design at this time?

Thanks!

Lan Li, PE

Li, Lan's picture
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Do the baseboards, once you click yes it is all automatic. If you have
a lot of zones use the summary tab and go down the list. The baseboard
heat will be broken out in your sim so you know what the contribution is
in each zone. I had a zone this week with 13000 hrs, the adjust loads
adjustment was missed, this brought the problem down to 1000 hrs,
changing the zone command got me to 300 hrs. Even with baseboards that
was the best I could do with a constant volume system on that zone. I
was running 4 deg throttling range.
Bruce

Bruce Easterbrook's picture
Joined: 2011-09-30
Reputation: 0

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 -

Robert Des Rosiers's picture
Joined: 2011-09-30
Reputation: 0

Robert,

In following this discussion, several issues come to mind:

First, you are trying to model the "baseline HVAC system is a constant volume single zone AHU w/ no reheat" using a VAV system in eQuest for a LEED project. This setup, even with your modifications, will be flagged by the reviewer as inappropriate. I am assuming that without the DES, the baseline system should have been a system #3. With the changes to the baseline required by the DES methodology, the single zone systems should be modeled as individual fan-coil units, not VAV. (Without DES, system #3 should always be modeled as individual PSZs in eQuest.)

Second, with the excessive unmet load hours in a VAV unit, you most likely have not activated the AHU heating coil. This is done on the system level heating tab by entering a "Hot deck max leaving temp". Doing a right-click on that data entry space gives valuable help on how and when to activate and when not to. In eQuest, VAV systems that have no "Hot deck max leaving temp" or reheat have no source of heating to supply to spaces served by that VAV unit.

Also, a Baseline building for a LEED submission should not have baseboards unless they serve a semi-heated space (in which case they must be modeled identically in both buildings).

Good luck!

Sheila Sagerer

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You are in trouble shooting mode now, do a save as to get a play file.
Dakota seems right 50,000 hrs is a problem. Add in the baseboards, they
might indicate a few problems zones and give you a clue to the high
hours. Sheila has some good suggestions too. Once you figure out the
problem just go back to the original file and tune it up.
Bruce

Bruce Easterbrook's picture
Joined: 2011-09-30
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