Why Change Heating and Cooling Controls with Natural Ventilation?

If you're looking to save energy on a project, one solution might be for natural ventilation to supplement partial or full operation of the heating and cooling systems that are in each perimeter zone. In other words, when it is mild outside, windows open (either automatically or manually) and HVAC systems turn off, saving energy.

If you're looking to test how comfortable a natural ventilation system that does not have either heating or cooling will be, you need to make sure that mechanical heating or cooling is not stepping in when the occupied space starts to get uncomfortable.

In both cases above, you need a way to make sure the units in the zone are switching on and off the way you expect them to.

Here are some examples of heating and/or cooling units that might be expected to operate in tandem with natural ventilation:

• Radiators
• Heated floor
• Package Terminal Air Conditioners (aka window-rattlers)
• Package Terminal Heat Pumps (aka split systems)
• Fan Coil Units (of all varieties)

This section of the knowledge base describes how these controls work.

A couple of things to remember before we start!!!

1. The temperatures you're trying to control the space to are set in the Space Use settings. These should not be set outside the temperature range you want Natural Ventilation to achieve. Find out more here.
2. The heating and cooling unit does NOT include the DOAS ventilation unit. There might still be heating or cooling going on here that is not controlled by these settings. Find out more here.
3. Perimeter zones with operable windows are the only zones affected by these controls. Centre zones and perimeter zones without windows will always be conditioned.

How zone heating and cooling controls work

In the Natural Ventilation inputs there is a dropdown menu that lets users choose when the heating and/or cooling units inside the zone will be shut down in lieu of natural ventilation. How you control these units can substantially impact how effective natural ventilation is and reducing energy. It can also impact the comfort of the building.

 

There are four HVAC Integration controls options in Sefaira that let you decide how you would like the HVAC system to interact with the natural ventilation system:

• Natural Ventilation + Heating (Default - there is assumed to be no cooling in the zone)
• Natural Ventilation + Cooling (there is assumed to be no heating in the zone)
• Natural Ventilation + Heating & Cooling (there is heating and cooling still available - natural ventilation only works when it can maintain comfort)
• Natural Ventilation only (there is no heating or cooling unit in the zone)

Default Setting - Natural Ventilation Plus Heating

If you're doing a residential or school project in any mild to cold climate then this is probably the system you'll have. It assumes the heating parts of your zone unit are on and work normally, but the cooling parts are always off.

How it works

There are a few key elements that control natural ventilation operation in each zone that are checked every hour:

 

• What is the temperature in the zone?
• What is the heating setpoint (defined by you in the space use section - see below)?
• What is the cooling setpoint (defined by you in the space use section - see below)?
• What is the outside air temperature?

 

Based on these values, the natural ventilation system will operate in one of 4 modes (these are not communicated in the application):

1. Heating Mode
2. Heating off with Windows Closed
3. Natural Ventilation Mode with Windows Open
4. Natural Ventilation Mode with Windows Closed

1. Heating mode

What's going on?

The temperature in the zone is below the heating setpoint for the zone (most likely to happen at the start of the day).

In this mode the zone heating is on and trying bring the space temperature up to the heating setpoint.

Note - this can happen if you have very large free areas in a cold climate straight after windows open.

2. Heating Off with Windows Closed

What's going on?

The zone is warmer than the heating setpoint, so the unit heating in the zone is off.

However...

The zone temperature is less than 1C (1.8F) warmer than the heating setpoint, so it's not yet time to open windows (opening them will probably just force us to turn heating back on).

 

Example:

I have the heating thermostat at home set to 18C (65F). 

Inside it's 18.5C (65.9F). 

I keep the windows closed, but I don't need the heating on.

3. Natural Ventilation Mode with Windows Open

What's going on?

The space has warmed up so it's now at least 1C (1.8F) warmer than my heating setpoint.

The temperature outside is no more than 10C (18F) colder than it is inside but also cooler than it is inside.

I've opened the window now to keep the space cool.

Example:

I have the heating thermostat at home set to 18C (65F). 

Inside it's now 21C (70F). Outside it's 18C (65F)

My windows are open, keeping the space cool.

How much are my windows open?

It depends on difference between the temperature outside and inside.

I crack the window open as long as it's no more than 10C (18F) colder outside than inside.

I proportionally increase the window opening proportion as the difference goes from 10C (18F) to 5C (9F).

If it's less than 5C (9F) cooler outside than inside, the window's provides the full free area available.

4. Natural Ventilation Mode with Windows Closed

What's going on?

It's warm inside (at least 1C (1.8F) above the heating setpoint) but the air outside is hotter than it is inside.

I keep the windows closed until it's cooler outside than it is inside (no point trying to cool the building with air that's warmer than my space!)

Example

It's 24C (75F) inside. I'm feeling warm and normally I'd open the windows.

It's 26C (78.6F) outside.

I keep the windows closed because it'll just get hotter if I open them. (I'm better off stopping heat gains by closing blinds/shutters, etc).

Natural Ventilation Plus Cooling

If you're doing a residential, school or office project in any temperate to hot climate that doesn't really have a heating season then this is probably the system you'll have. It assumes the cooling parts of your zone unit are on and work normally, but the heating parts are always off.

How it works

There are a few key elements that control natural ventilation operation in each zone that are checked every hour:

• What is the temperature in the zone?
• What is the heating setpoint (defined by you in the space use section - see below)?
• What is the cooling setpoint (defined by you in the space use section - see below)?
• What is the outside air temperature?

 

Based on these values, the natural ventilation system will operate in one of 3 modes (these are not communicated in the application):

1. Passive Heating Mode with Windows Closed
2. Natural Ventilation Mode with Windows Open
3. Cooling Mode with Windows Closed

1. Passive Heating Mode with Windows Closed

What's going on?

The temperature in the zone is below the heating setpoint plus 1C (1.8F) for the zone.

In this mode the windows are closed to minimise heat loss from the zone.

There is no heating so if the zone is colder there's nothing the system can do to make the zone warmer other than keeping windows closed.

Example: My heating setpoint is 21C / 70F. It is 21.5C (70.9F) in the zone. The windows are closed. 

2. Natural Ventilation Mode with Windows Open

What's going on?

The space has warmed up so it's now at least 1C (1.8F) warmer than my heating setpoint.

The temperature outside is no more than 10C (18F) colder than it is inside but also cooler than it is inside.

I've opened the window now to keep the space cool.

Example:

I have the heating setpoint set to 21C (70F). 

Inside it's now 24C (75F). Outside it's 18C (65F)

My windows are open, keeping the space cool.

How much are my windows open?

It depends on difference between the temperature outside and inside.

I crack the window open as long as it's no more than 10C (18F) colder outside than inside.

I proportionally increase the window opening proportion as the difference goes from 10C (18F) to 5C (9F).

If it's less than 5C (9F) cooler outside than inside, the window's provides the full free area available.

3. Cooling Mode with Windows Closed

What's going on?

It's warmer than my cooling setpoint inside. It's warmer outside than inside.

I close the windows and switch on cooling.

Example

It's 24.1C (75.2F) inside. My cooling setpoint is 24C (75F).

It's 26C (78.6F) outside.

The windows are closed and the cooling system is on.

Natural Ventilation Plus Heating and Cooling

If you're doing a traditional “Mixed Mode” or "Hybrid" system where the natural ventilation supplements a full HVAC system then this is the correct option. In this mode, it is assumed that the building has both a heating and cooling system (per the HVAC dropdown). 

How it works

There are a few key elements that control natural ventilation operation in each zone that are checked every hour:

• What is the temperature in the zone?
• What is the heating setpoint (defined by you in the space use section - see below)?
• What is the cooling setpoint (defined by you in the space use section - see below)?
• What is the outside air temperature? 

Based on these values, the natural ventilation system will operate in one of 4 modes (these are not communicated in the application):

1. Heating Mode
2. Heating off with Windows Closed
3. Natural Ventilation Mode with Windows Open
4. Natural Ventilation Mode with Windows Closed

1. Heating mode

What's going on?

The temperature in the zone is below the heating setpoint for the zone (most likely to happen at the start of the day).

In this mode the zone heating is on and trying bring the space temperature up to the heating setpoint.

Note - this can happen if you have very large free areas in a cold climate straight after windows open.

2. Heating Off with Windows Closed

What's going on?

The zone is warmer than the heating setpoint, so the unit heating in the zone is off.

However...

The zone temperature is less than 1C (1.8F) warmer than the heating setpoint, so it's not yet time to open windows (opening them will probably just force us to turn heating back on).

Example:

I have the heating thermostat at home set to 18C (65F). 

Inside it's 18.5C (65.9F). 

I keep the windows closed, but I don't need the heating on.

3. Natural Ventilation Mode with Windows Open

What's going on?

The space has warmed up so it's now at least 1C (1.8F) warmer than my heating setpoint.

The temperature outside is no more than 10C (18F) colder than it is inside but also cooler than it is inside.

I've opened the window now to keep the space cool.

Example:

I have the heating thermostat at home set to 18C (65F). 

Inside it's now 21C (70F). Outside it's 18C (65F)

My windows are open, keeping the space cool.

How much are my windows open?

It depends on difference between the temperature outside and inside.

I crack the window open as long as it's no more than 10C (18F) colder outside than inside.

I proportionally increase the window opening proportion as the difference goes from 10C (18F) to 5C (9F).

If it's less than 5C (9F) cooler outside than inside, the window's provides the full free area available.

4. Cooling Mode with Windows Closed

What's going on?

It's warmer than my cooling setpoint inside. It's warmer outside than inside.

I close the windows and switch on cooling.

Example

It's 24.1C (75.2F) inside. My cooling setpoint is 24C (75F).

It's 26C (78.6F) outside.

The windows are closed and the cooling system is on.

 

Natural Ventilation Only

If your building is in a mild to warm climate and you don't have any heating or cooling systems installed in the zone then this is the right system to select. Lots of users might select this option if they want to study a house to see how hot or cold it gets without any heating or cooling. Similar applications might be buildings that may not typically be conditioned, like warehouses, bus terminals, open exhibition halls, etc.

How it works

Even though there is no heating or cooling, it's still very important to use the setpoints in the space use tab to set the temperatures users of the space would like it to be. These are used to control the opening of windows and doors and they're also used to test whether your space is comfortable or not (when assessed for dry bulb temperatures).

There are a few key elements that control natural ventilation operation in each zone that are checked every hour:

• What is the temperature in the zone?
• What is the heating setpoint (defined by you in the space use section - see below)?
• What is the cooling setpoint (defined by you in the space use section - see below)?
• What is the outside air temperature? 

Based on these values, the natural ventilation system will operate in one of 4 modes (these are not communicated in the application):

1. Passive Heating Mode with windows closed
2. Natural Ventilation Mode with Windows Open
3. Natural Ventilation Mode with Windows Closed

1. Passive Heating Mode with Windows Closed

What's going on?

The temperature in the zone is below the heating setpoint plus 1C (1.8F) for the zone.

In this mode the windows are closed to minimise heat loss from the zone.

There is no heating so if the zone is colder there's nothing the system can do to make the zone warmer other than keeping windows closed.

Example: My heating setpoint is 21C / 70F. It is 21.5C (70.9F) in the zone. The windows are closed. 

2. Natural Ventilation Mode with Windows Open

What's going on?

The space has warmed up so it's now at least 1C (1.8F) warmer than my heating setpoint.

The temperature outside is no more than 10C (18F) colder than it is inside but also cooler than it is inside.

I've opened the window now to keep the space cool.

Example:

I have the heating setpoint set to 21C (70F). 

Inside it's now 24C (75F). Outside it's 18C (65F)

My windows are open, keeping the space cool.

How much are my windows open?

It depends on difference between the temperature outside and inside.

I crack the window open as long as it's no more than 10C (18F) colder outside than inside.

I proportionally increase the window opening proportion as the difference goes from 10C (18F) to 5C (9F).

If it's less than 5C (9F) cooler outside than inside, the window's provides the full free area available.

3. Natural Ventilation Mode with Windows Closed

What's going on?

It's warm inside (at least 1C (1.8F) above the heating setpoint) but the air outside is hotter than it is inside.

I keep the windows closed until it's cooler outside than it is inside (no point trying to cool the building with air that's warmer than my space!)

Example

It's 24C (75F) inside. I'm feeling warm and normally I'd open the windows.

It's 26C (78.6F) outside.

I keep the windows closed because it'll just get hotter if I open them. (I'm better off stopping heat gains by closing blinds/shutters, etc).