[Bldg-sim] thermal balance loads vs. system loads (PowerDOE)

Steve Gates stvgates at pacbell.net
Wed Aug 21 13:19:17 PDT 2002


The SPACE loads are calculated at a CONSTANT temperature all year, and should normally be specified as the average temperature for the year (such as 73F).  The ZONE loads take into account the actual thermostat setting, changes in the setting (night setbacks/setups, etc.), and deviations from setpoint.  

Changes in setpoint cause mass effects to come into play.  For example, if a zone is held at 55F all night, and is then raised to 70F the next morning, the HVAC system will have to warm the internal mass as well as offset the envelope losses.  As a result, the heating load after the temperature setback is terminated will be greater than it would be if the space were held at 70F all night.  (Note that mass effects last for multiple hours, and gradually taper off as the mass comes into equilibrium with the new temperature.)

Deviations in setpoint (and loads) occur because of the system response to the load.  For example, if the thermostat setpoint is 75F during cooling, but the HVAC system is too small to handle the load, then the actual zone temperature will float higher than 75F; the higher temperature then acts to reduce the load. The actual temperature will be the balance point between system capacity, envelop and internal loads of the space, and mass effects within the space.

Air handler loads are a function of the zone loads, latent cooling, duct heat gain/losses, fan heat, outside air introduced for ventilation and/or economizer savings, reheat energy in VAV systems, mixing losses in dual-duct systems, etc.

Loads on a boiler or chiller are a function of the coil loads in airhandlers, pump heat, and piping heat gains.

So the loads summarized in the LS-A, etc. reports are space loads that occur at a constant temperature. They exclude all of the mechanical effects of temperature swings, ventilation air, etc. that are accounted for in the SS-A, etc. reports.

Given that the LS-** reports do not actually report the true loads, are they useful?  YES!  You should always review these reports when creating a new model. In addition to reporting lighting, daylighting, and equipment energy, they are excellent reality checks on the origin of the space loads.  The reports give breakdowns on loads arising from lights, equipment, people, infiltration, walls, windows, roofs, etc. An abnormally large or small load in a given category is a clue that there is a mistake in your model.  

The SS-* reports summarize the response of the HVAC system to the space loads.  It is wise to acquaint yourself with all of these reports.  One of my favorite reality checks is the SS-F report which reports the maximum/minimum temperatures within a given zone.  Temperatures too far beyond the thermostat settings are a sure sign that your model is not working correctly (or if modeled correctly, a sure sign that the HVAC system was not designed properly!)

The BEPU report is another good reality check, as it breaks down gas and electricity consumption into the major end-use categories (lighting, misc equipment, heating, cooling, fans, etc.).  Again, once you have become practiced in knowing what buildings typically consume, abnormal numbers can jump out at you. 
  ----- Original Message ----- 
  From: Familia Ordenes 
  To: Bldg-sim at gard.com 
  Sent: Wednesday, August 21, 2002 12:21 PM
  Subject: [Bldg-sim] thermal balance loads vs. system loads (PowerDOE)


  Hi,

  When I compare the results from the Space Loads and the System Loads in PowerDOE, I can see a difference between these values (sometimes the Space Loads are bigger than the System Loads and sometimes  not). I do not understand how an HVAC system actually responds to building loads. How does the thermostat measure the space temperature ?  Is this hourly space temperature the same for the building loads calculation ? Can the HVAC systems affects the space loads ?  


  I understand these questions may not be easy to answer, but these topics are under construction in the PowerDOE Help and I would like to understand better the program. I would be very grateful if someone could send me some information to clear my doubts.

  Best regards.

  Martín Ordenes M. 
  Departamento de Ingeniería y Gestión de la Construcción.
  Pontificia Universidad Católica de Chile

==================
You received this e-mail because you are subscribed 
to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe 
from this mailing list send a blank message to 
BLDG-SIM-UNSUBSCRIBE at GARD.COM




===========================
You received this e-mail because you are subscribed 
to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe 
from this mailing list send a blank message to 
BLDG-SIM-UNSUBSCRIBE at GARD.COM
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20020821/9714d8ad/attachment-0002.htm>


More information about the Bldg-sim mailing list