9. Air
In this tutorial we are going to look at the air sheet in the Zebra model. This is where we define the thermal losses that occur because of air moving in and out of the building, and taking heat with it.
In the air sheet we have a number of inputs, starting with an assumed infiltration rate. This defines the uncontrolled movement of air through the fabric of the building, which may be through the building fabric material itself, and also through any cracks or holes there may be in that fabric. This infiltration rate is based on a pressure of 50Pa, which is typically the pressure that infiltration is tested at, and it will depend on both the form of construction that you choose, and also the experience of the construction team with constructing low-energy buildings. The objective with low-energy buildings is to get this infiltration rate as low as possible, but you also have to be realistic about what can be achieved in your area. Beneath the infiltration rate input cell we require that you give a justification for your chosen infiltration rate. We don't validate that value that you put in, but we do force you to input a value to make sure that you have given proper consideration to this property.
After that you need to define the internal volume of the building. And then the average number of occupants. This will dictate how much we need to ventilate the building, because we would typically ventilate the building a certain amount per occupant of the building. Some typical occupation rates for different types of building are provided in the notes.
And once we've defined all of these inputs, the spreadsheet will calculate our space heating demand, and our space cooling demand. The will be broken down into infiltration and ventilation, and also broken down for each month of the year. Now one thing that is a little confusing about this sheet as opposed to others: In general the calculations move forward through the spreadsheet tabs, but on this sheet there are actually some inputs that come from subsequent tabs. If we go into the space conditioning tab there are some values relating to heat recovery of ventilated air, and relating to whether we have ventilation or not. These inputs will affect the values in the air sheet, and it’s important to be aware of this.
If we go into a higher complexity level we will find there are a number of additional inputs into the air sheet. There's also a more detailed break-down of how these values have been calculated. The additional inputs are a ventilation rate, which is assumed at 30m³/h/p if you don't go into a higher complexity level. We can also define a shelter factor, which is how sheltered the building is from the wind. The more wind and the more infiltration we will have through the fabric. The default value is 0.07, but if you have a particularly exposed or less exposed building you can change this value.