6.7 Ventilation/Rooms – Above Green

6.7.1 Utilization Pattern

The maximum design airflow is determined by the largest of 2 factors:

Supply air requirement (based on occupancy)
Exhaust air requirement (based on kitchens, baths, etc.)

The capacity of the ventilation system installed needs to exceed whichever rate is greatest (see Section 3.5).

*For pre-certification, it is recommended (but not required) that residential projects model with a ventilation rate of 0.3 ACH or higher.

The average air flow rate (cfm) and average air change rate (1/hr) is calculated as a reduction from the “design air flow rate”. It is determined by the daily operation schedule and “fraction of design” airflow at the scheduled hours.

For non-residential projects, multiple utilization patterns may be set up to reflect varying levels of ventilation rates for different days (i.e. weekday vs weekend). Alternatively, one utilization pattern may be used and the ‘daily operation schedule’ can be revised to reflect the appropriate ratios of design airflow rates during occupied and unoccupied hours.

6.7.2 Rooms Ventilation

In WUFI Passive, it’s best practice to enter both exhaust rooms and supply rooms, though only exhaust room entries are required if the model is setup with ‘Balanced Ventilation’ (under PH Case>Additional Data).

Entering all rooms ensures the ventilation design is balanced.
“Supply rooms” must be entered if there is to be more than one H/ERV device entered.
When adding exhaust rooms, input 0 cfm supply. When adding supply rooms, input 0 cfm exhaust.

For multi-unit buildings with repeated exhaust rooms, the room may be input once and the quantity adjusted appropriately. For example, a 50-unit MF building with 50 kitchens may have a single line entry for the kitchen, quantity 50, if the exhaust flow rate is planned to be the same for all kitchens.

PHIUS recommends starting with the following exhaust rates as rules of thumb:

Kitchen: 35cfm

Bathroom: 24 cfm

Half Bath: 12 cfm

6.7.3 Summer Ventilation

Summer ventilation may be notionally either natural, via windows, or mechanical automatically controlled (e.g. whole house exhaust fan.)

Summer Bypass

Summer/HRV Humidity Recovery: Most heat recovery ventilation units (Both ERVs and HRVs) can bypass heat recovery in the summer condition, when outdoor temperatures are more favorable than indoor. Also referred to as ‘economizer mode’. Typically, this is temperature-controlled bypass.

None: If there is no heat/moisture recovery ventilation during the summer.
Temperature-controlled bypass: If the bypass mode is activated based temperature.
Enthalpy-controlled bypass: If the bypass mode is activated based on both temperature and humidity/moisture.
Always: If there is no bypass mode and recovery mode is always active.

Additional Summer Mechanical Ventilation

Use these inputs when implementing a mechanical exhaust system in addition to the ERV. Ex: whole house fan. Use [Mechanical automatic controlled ventilation] if the system is tied to sensors for either temperature or humidity difference and define the control system in WUFI Passive.

ACH via mechanical ventilation exhaust air
Mechanical automatic controlled ventilation
Specific Power Consumption

Summer Natural Ventilation (day)

ACH via natural ventilation (day):

If a cooling system is planned, no natural ventilation may be included in the model. An entry of ‘0’ is required for PHIUS+ 2018 certification.
If no cooling is planned, up to 0.3 ACH may be used.

Summer Mechanical Ventilation by the HRV/ERV

ACH via mechanical ventilation: This value should match the year round average ACH from the ERV/HRV. In WUFI Passive, if you leave this cell empty, it will use your typical year round operation rate of the ERV.

If the climate allows for summer ventilation to be taken care of through windows and other passive features, and the HRV/ERV will be shut off, input 0 here.

Note: Occupants must provide other means of ventilation if turning off continuous ventilation system.

Additional Summer Ventilation (night)

ACH via natural ventilation (night):

If a cooling system is planned, no natural ventilation may be included in the model. An entry of ‘0’ is required for PHIUS+ 2018 certification.
If no cooling system is planned:
- Be cautious and realistic with the amount of natural ventilation being counted on for cooling. There are two main concerns with the amount of cooling.
  - 1. Humidity - look for dew point temperature in the Climate Data. In many climates in the United States, there are high levels of humidity in the cooling situations.
  - 2. The built-in summer ventilation calculator can generate quite high air change rates, but during the night usually interior doors are closed so that cross ventilation is cut off.
- For PHIUS+ Certification, either:
  - 1. Account for no higher than 0.3 ACH for nighttime ventilation; or
  - 2. Follow Building America House Simulation Protocol, which states: 33% of the open-able windows are opened. This can happen 3 days a week (M, W, F), which would translate, to a fraction of opening duration of 43%, Or 10.3 hrs/day.
    - a. Use the additional calculator found under Summer Ventilation in WUFI Passive Model each window in one column. A second window group is allowed for stack effect and cross ventilation only for day, not night (unless there are no intervening doors.)

6.7.4 Exhaust Ventilation

Exhaust Air Appliances:

In WUFI Passive, account for the exhaust appliances on the Ventilation/Rooms > Exhaust Appliances tab.
- There are pre-defined devices for dryers and kitchen cooktops, which are tied to the annual usage patterns of the dryer and cooktop.
Bathrooms with direct exhaust fans: There are two acceptable methods to account for this:
- Assume 60 min/bathroom/day per BAHSP protocol
  - Exhaust volume flow rate [cfm]: Sum total bathroom exhaust fans in the building and enter that here
  - Run time per year [min]: Use (365 days/yr * 60 min/day) = 21,900 minutes/yr
- Alternate: Assume 30 min/person/day
  - Exhaust volume flow rate [cfm]: Average exhaust flow rate of all bathroom exhaust fans in the building. Enter that exhaust rate for the average bathroom here.
  - Run time per year [min]: (30 min/occ/day * Occupants * 365 days/yr) = minutes/yr

Tips:

Consider moving an exhaust dryer to the mudroom or outside the thermal envelope.
Make-up air for a directly vented range hood is acceptable as long as the total meets the source energy requirements. If a make-up air system is planned then it can be tied directly to a vented dryer as well, i.e. it comes on when the dryer is running and venting.

Multiple Exhaust Dryer Protocol:

If a community building or similar is serving multiple buildings with a laundry room, multiple exhaust dryers with high usage patterns put a lot of load on the space conditioning system. If the makeup air is either fully ducted to the dryer case, or even near / behind it, then the makeup air may be regarded as not mixing with the room air and therefore does not increase the load/demand on the space heating/cooling system, because the room thermostat will not ever feel it. The extra energy must be accounted for, but it is considered to be moved from the space-conditioning category to the source energy category. That accounting may be done as follows:

Duplicate the existing case, create a new ‘test’ case.
Add the dryers to the Exhaust Appliances tab in WUFI Passive .
Take the difference between the added heat demand and the added cooling demand. (On the theory that cold air makes the dryer work harder, warm air makes it work easier, than it does under its rated conditions.)
Convert that to a source energy using the COP and SE factor of the dryer. Divide by 2.8 (source energy factor for electricity) and enter the result as an auxiliary electric load outside the thermal envelope.
Furthermore, it is recommended to install an electric heater in the makeup air duct (near the point of entry) capable of raising the air temperature to at least 49 F (dew point of 68 F air at 50% RH.)
If the dryer is not electric resistance, the SE factor in the above energy calculation should be adjusted with a seasonal weighting based on the percentage of the year that the makeup air heater would be expected to operate. This would prevent the makeup air from causing a frost or condensation problem.

Notice that, if the space heating system is efficient or has a low source energy factor, a heated makeup air duct may actually use more energy overall in a site or source energy sense. Therefore this is not necessarily the best strategy in all cases. It is offered as an option that may help some smaller buildings meet the certification criteria for space conditioning.