Design Loads & Outside Air in the Space Use Tab Explained

At the top of the Space Use tab are two cards which offer control of "Design Loads" and "Ventilation and Outside Air".

When a project is set up, we make sensible decisions as to what these should be for you based on the building type you select (listed below). You can change them of course and create your own templates.

This section provides more detail on how these two inputs work and what they're for.

Summary:

Design loads inputs represent what the HVAC designer would input as internal design loads for sizing the HVAC system. 

Ventilation and Outside Air inputs establish how much fresh or outside air is being supplied as part of the HVAC system. They also give you control over recirculated ventilation (for buildings like laboratories).

Design Loads

When the HVAC engineer sizes the HVAC system for a building, they need a "worst case" value for internal space loads to form the basis for sizing equipment. This is where this information is entered. Key notes

  • For sizing runs with summer days, these loads are fully applied to the space  for all "on" HVAC hours.
  • For sizing runs with winter days, these loads are considered to be zero, regardless of what they are.
  • For annual runs these loads are modified hourly by the schedule for "on" days and are set at 5% for "off" days (eg if you have everything on 5-days per week, then there are 2 "off" days).

Note: these values are later modified by the diversity schedule at the bottom of the space use tab for the energy run.

Sefaira breaks the design loads into 3 components:

  • Occupant Density - This sets heat gain from occupants (which is partially latent and partially sensible) and also informs ventilation requirements.
  • Equipment Power Density - This sets heat gain for all non-lighting equipment (which is all sensible and has almost no radiant component).
  • Lighting Power Density - This sets heat gain for all interior lighting components (which is all sensible and has a small radiant component). 

You can enter a value into the box and if it's outside the range it will still be applied.

The values for the standard use templates are provided in the tables below. 

Ventilation and Outside Air

All HVAC systems require mechanical supply of outside air into the space. This is where Sefaira determines what is the correct amount of outside air. Note that different HVAC systems have different ways of integrating outside air into their system. For DOAS systems these values have the impact of sizing the DOAS air that goes into the space.

The outside air UI looks like this.

Screen_Shot_2017-04-25_at_11.07.02.png

Sefaira works out the largest of two variables and uses it for that space use outside air. This is defined as the largest of:

  • Outside Air Rate = Outside Air Rate/Person + (Outside Air Rate/Unit Area * Occupant Density)
  • Air Changes - OA value: measure of the air volume of OUTSIDE AIR added to or removed from a space per hour divided by the volume of the space. 

Total Air Changes Per Hour is the total amount of air that has to be supplied to the space, outside or otherwise and has the following additional impacts:

  • Total air changes will always be at least as high as air changes from Outside Air (OA)
  • If the HVAC system is 100% outside air, the Total Air Changes will also be Outside Air regardless of the OA value because the Air Handling unit is 100% Outside Air. 
  • If the Total Air Changes is higher than what the Design Airflow is (eg for a VAV system) then the Total Air Changes will drive the system size too.

Values in our default templates for the People Outdoor Air Rate and Area Outdoor Air Rate are provided in the table below.

In metric units:

  Occupant Density Lighting Power Density Equipment Power Density People Outdoor Air Rate Area Outdoor Air Rate Air Changes per Hour
  m2/person W/m2 W/m2 L/s*person L/s.m2 OA / Total
Office (plugin defaults) 10 10 25 15 0 0
Residential (plugin defaults 50 10 5 10 0 0
School (plugin defaults 4 15 10 10 0 0
Healthcare (plugin defaults) 10 20 15 15 0 6/6
Laboratory (plugin defaults) 30 20 25 15 0 4/4
Retail (plugin defaults) 20 20 15 10 0 0

In IP units:

Note these are direct conversions from metric, which is why the values are not rounded.

  Occupant Density Lighting Power Density Equipment Power Density People Outdoor Air Rate Area Outdoor Air Rate Air Changes per Hour
  m2/person W/m2 W/m2 L/s*person L/s.m2 OA / Total
Office (plugin defaults) 107.6 0.9 2.3 31.8 0 0
Residential (plugin defaults 538 .9 .5 21.2 0 0
School (plugin defaults 43 1.4 .9 21.2 0 0
Healthcare (plugin defaults) 107.6 1.9 1.4 31.8 0 6/6
Laboratory (plugin defaults) 322 1.9 2.3 31.8 0 4/4
Retail (plugin defaults) 215.2 1.9 1.4 21.2 0 0

 

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Comments

  • Avatar
    Alfred Uzokwe

    Is there a way to change the OA requirements as a percentage of total airflow required?

  • Avatar
    Simon Schneider

    looking at results: does that mean that a residential building with 100% factor for space use at night is consuming 100% of its lighting power density during that time?!
    Does that also mean, that a building's lighting demand is in no way correlated to its daylighting situation?

  • Avatar
    Simon Schneider

    Also, what is the default occupant density for residential buildings?

  • Avatar
    Andrew Corney

    Thanks for the questions - we had not been notified because an old staff member owned the article. Sorry for the delay.
    I've updated this and hopefully this answers some questions.
    Alfred - the only way to do this is if you have Systems, look at the Plant Sizing tab and review outside air as a fraction of total supply air (I assume you want to do this for a VAV system)