What is the Mechanical Ventilation Unit?

The Mechanical Ventilation Unit is the part of the HVAC system that provides ventilation to a space. Examples of the Mechanical Ventilation Unit might be:

• A centralised MVHR (Mechanical Ventilation Heat Recovery) unit in a school
• A rooftop AHU (Air Handling Unit) providing conditioned air in an office
• A through-the-wall ventilation unit in an apartment 

In some applications, the Ventilation Unit does more than just provide ventilation. It might cool and dehumidify the air in an office building, it might have heat recovery in a residential building to avoid losing heat, etc.

In the Systems view, users can control many aspects of the mechanical ventilation unit in the air-side tab. An image of these controls for a fan coil unit is shown below. There is more information about these controls here.

Why should I care?

Changing when the mechanical ventilation comes on (or whether it's even included) will have an impact on the comfort, fresh air and energy use of any naturally ventilated project. 

How does Mechanical Ventilation work with Natural Ventilation?

Natural ventilation only works in Sefaira with DOAS HVAC systems. These are systems where the ventilation part of the HVAC system is separate to the heating and cooling unit. 

Mechanical ventilation can be added to any natural ventilation control strategy and operates independent of how the zone heating and cooling works. The mechanical ventilation can be included, partially controlled or not included as part of the design. The image of the drop-down below shows these controls.

There are 3 ways that mechanical ventilation can be added:

1. On during occupied hours
2. Off if windows are open
3. Always off

These are described in more detail below.

1. Mechanical Ventilation Unit is "On during occupied hours"

There has been a strong trend in recent years to insist on mechanical ventilation for many naturally ventilated buildings, especially in colder climates where buildings are being sealed better and may not get much ventilation in winter.

As a result, this is the default mode.

In this mode, the Mechanical Ventilation will be on at all times during the HVAC operating hours. HVAC operating hours are determined in the space use settings for each zone.

In this mode, mechanical ventilation will operate even if windows are open and even if you have selected the “Natural Ventilation Only” strategy for zone heating and cooling.

When should I use this?

This is appropriate for all new build projects in climates where winters are cold and where codes tend to be strict when it comes to ventilation. 

2. Mechanical Ventilation Unit is "Off if windows are open"

Because the mechanical ventilation unit is probably superfluous when windows are open, it may be of interest to see what kind of energy savings could be achieved by switching it off when the windows are open.

This mode simulates a Mechanical Ventilation Unit control that knows when windows are open and switches off the Mechanical Ventilation Unit for that zone at that time.

Note the Mechanical Ventilation Unit is always on for centre zones and perimeter zones without operable windows.

When should I use this?

This sort of solution could potentially be expensive to implement. it might not be realistic if the project has lots of centre zones that cannot be ventilated by windows.

It might also not be realistic if your project expects to have a central air handling unit for ventilation unless that unit has been budgeted to have variable flow controls integrated into it.

It's recommended talking with your mechanical engineer about whether this is a realistic solution for the project before assuming it is feasible.

3. Mechanical Ventilation Unit is "Always off"

The project you're working on may not expect to have any mechanical ventilation. Many older buildings that are naturally ventilated do not have a backup mechanical ventilation system. New buildings in mild to warm climates with no real centre zones may not require mechanical ventilation.

In this option, the Mechanical Ventilation Unit is always off for perimeter zones with operable windows. 

Note the Mechanical Ventilation Unit is always on for centre zones and perimeter zones without operable windows.

When should I use this?

If you project does not have any Mechanical Ventilation Unit proposed either because it's older or because the windows can be expected to provide ventilation all year round.

In colder climates you may also want to simulate this with trickle vents for residential projects. You can simulate a trickle vent by increasing the "crack-flow" infiltration rate in the envelope tab.