Difference between "Building -Loads" and "Building -HAVC"

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Dear equest users,
I generated hourly report blocks looking for the loads.
I saw that there are two options:
1. "Building Loads" which as far I understood contains losses, gains etc....
2. "Building - HVAC" - Coils - losses&gains at te beginning I believed
that it cantrained : The Loads from the fresh air + "Building Loads". So
in order to test that I made zero the fresh air at my AHUs. I was expecting
that now the "Building - HVAC" would be equal with "Building Loads". But
still there is a difference. "Building - HVAC" is higher almost 50% than
"Building Loads".

Any Ideas or Comments about what "Building - HVAC" contains more than
"Building Loads"?

Thank you for your help

Konstantina Kalliakoudi

Mechanical Engineer,M.Sc., LEED AP

KHM

Filolaou 28, Ilioupoli, Athens, Greece

Tel. 0030 210 99 69 680

www.khmoe.gr

Konstantina Kalliakoudi's picture
Joined: 2019-03-25
Reputation: 0

Dear Konstantina

These are loadsoccurring outside zones in the system. They include items such as fan heatgains, the ventilation load and plenum loads.

Kindly check theattached file.

?

Regards,

Mohamed Abdelrahim Abdelaal

LEED AP BD+C, CFPS,HFDP & HBDP Certified.

Member ASHRAE, NFPA& USGBC.

Senior MechanicalEngineer.

On Wednesday, March 11, 2020, 10:49:23 AM GMT+3, Konstantina Kalliakoudi via Equest-users wrote:

Dear equest users,I? generated? hourly report blocks looking?for the loads.I saw that there are two options:1. "Building Loads" which as far I understood contains losses, gains etc....2. "Building - HVAC" - Coils - losses&gains?at te beginning?I believed that it cantrained : The Loads from the fresh air +? "Building Loads". So in order to test that I made zero the fresh air at my AHUs. I was expecting that now the?"Building - HVAC" would be equal with?"Building Loads". But still there is a difference.?"Building - HVAC" is higher almost 50% than?"Building Loads".
Any Ideas or Comments about what?"Building - HVAC" contains more than?"Building Loads"?
Thank you for your help

Konstantina Kalliakoudi

Mechanical Engineer,M.Sc., LEED AP

KHM

Filolaou 28, Ilioupoli, Athens, Greece

Tel. 0030 210 99 69 680

www.khmoe.gr

?
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Mohamed Abdelrahim's picture
Joined: 2019-04-22
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This is a great question that has puzzled me in the past.

The difference between the hourly reports BUILDING-LOADS and BUILDING-HVAC is the same as the difference between the heating and cooling reported in the LS- reports (LS-C for peak and LS-F for total) and the heating and cooling reported in SS-D.

LS- reports and BUILDING-LOADS are determined after the LOADS module is run. SS- reports and BUILDING-HVAC are determined after the SYSTEM/PLANT module is run.

You are correct that the biggest difference is generally caused by ventilation. There are a number of other factors that move the SYSTEMS heating and cooling energy.

Reheat ? reheat is not accounted for in LOADS but comes into play in SYSTEMS. With a VAV system and a low supply air temperature you will see an increase in heating energy between your LOADS and SYSTEMS reports even without ventilation.
Humidity control ? while LOADS reports latent loads, SYSTEMS reports the system response including reduced supply air temperature and reheat.
Thermostat setpoints ? SYSTEMS uses the zone thermostats whereas in LOADS the space temperature estimate is used to determine loads for all hours. This can have a significant impact on unconditioned spaces where the default SPACE temperature is 70F for LOADS but may vary considerably from this in the SYSTEMS module.

Mohamed provided a much more comprehensive list of the differences between the LOADS and SYSTEM runs ? I am very curious about the source of that list?

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Brian Fountain
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This is an oft-repeated question, and an important one that I would like
to elaborate.

DOE-2, the simulation engine behind eQUEST, simulates the building in steps.

In the first LOADS step, it calculates the heat flows in and out of each
zone at a user-defined temperature (default is 70F). These heat flows
are what appears in the eQUEST output as "Building loads".

In the second SYSTEMS step, it simulates the HVAC system and controls to
derive the actual temperature of the zones, with the heating or cooling
input by the HVAC system reported in the eQUEST output as? the Building
- HVAC".

These two "loads" are very distinct.? In fact, I rarely look at the
building loads except for debugging an input file.? Several things to
remember:

1. The Building Load (BL) contains information for each building
component (walls, roofs, lighting, infiltration, etc.) because those are
all individual heat flow paths that are simply added together to get the
BL for the space or building.

2. The Building - HVAC (BH) does not contain any of that because at this
step DOE-2 is just using the total Space BL (SBL) in doing its
simulation (note that in DOE-2 terminology,? it's called a "space" in
Loads and a "zone" in Systems).? Furthermore, since DOE-2 derives the
actual zone temperature in Systems, the SBL has to be adjusted, making
it impossible to extract the component loads unless one does "reverse
engineering" on the temperature difference, but would always be
approximate due to thermal lag, thermostat deadband, etc.? (I can talk
for days about extracting component-level loads from DOE-2 which I had
to do for several DOE-2 projects back in the 1990s).

3. Natural or window ventilation is considered a Systems operation in
DOE-2, so any effects from that will not appear in the BL, but only in
the BH.

So, I hope that clarifies why the BL is always different from the BH!

Joe

Joe Huang

Joe Huang
White Box Technologies, Inc.
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Moraga CA 94556
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http://weather.whiteboxtechnologies.com for simulation-ready weather data
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