When and how windows are controlled to open can have a big impact on energy use, comfort and also capital cost of a project. By offering some simplified controls over window controls, you can test different strategies to see what kind of impact they will make.

Why do I care?

Strategies like automated night purge enable windows to open at night to cool a building down in warmer weather. Sometimes these sorts of approaches can be the difference in making a natural ventilation project feasible.

Similarly, whether openings are designed to work in adverse conditions can affect how regularly they can be used as part of a natural ventilation system. This could affect their feasibility and performance a lot in some locations.

How it works

There is a card for window controls in the natural ventilation tab (shown below). In the card you can control two aspects of the windows:

1. What happens if the building is unoccupied
2. What happens if it's windy outside

If the building is unoccupied...

A zone is considered unoccupied if the HVAC system is off at that time. Each window is associated with a zone so how it interacts is driven by the zone itself and when that zone is occupied (the building can have some zones on and some off and still work for the zones that are on.

There are 2 options available:

All openings closed

Most people close the windows before leaving a building for many reasons - security, weather protection, etc. This option reflects that kind of behaviour.

In other words, when the zone HVAC is off, its assumed that the zone is unoccupied and the windows are always closed, regardless of how hot it gets inside.

All openings open

Sometimes, particularly in warmer climates or when buildings have high internal loads, its good for windows to be able to open even if the space is unoccupied. This could enable the space to be kept cool by natural ventilation when otherwise it would heat up, making it harder to cool down once occupants arrive.

In most climates, leaving the windows open all night would probably add heating energy in the morning (if there is heating) and could be counter-productive. So this feature assumes automatic openings of windows after hours. 

The space thermostat closes the windows again once the space reaches its heating setpoint so that the chances of adding heating energy are reduced.

For example:

The setpoint range is 21-24C (70-75 F). At the end of the day, the zone is at 23.5C (74.1F). 

In this mode, the windows remain open, so long as the temperature outside is cooler than inside (but no more than 10C (18F) cooler). 

The space gradually cools down and eventually gets to 21C (70F). At this point the windows close.

If internal loads cause the space to warm up again, the windows will keep opening and closing to keep the space at 21C.

You might still see heating energy increase if you use this control option, particularly in colder climates because closing the windows at 21C may not prevent the zone from getting colder later in the evening that it would have if there was no night ventilation.

If it's Windy outside...

Do occupants close windows if it's windy to prevent disturbances to their work environment? Perhaps you want to see how designing the windows to minimise this impact might help users.

This provides the user the ability to decide what happens if it is particularly windy. Any hour in the .epw weather file with a wind speed of 5m/s (11mph - moderate breeze on Beaufort scale) or more is considered "windy".

There are 2 options available

All openings closed

Natural ventilation openings are assumed to be fully closed whenever it is windy. This is the most conservative approach and so it the default option.

All openings open

Regardless of the wind speed, all window openings work to their fullest extent.