When we talk about supply air (also commonly known as make up air), we’re referring to the new air that is delivered into a home. Supply air is really important to consider when you’re looking at ventilation systems, as no system/strategy is complete without it.

Without a steady supply of new air entering the home, your extraction fans can create negative pressure. When negative pressure builds up, the home results in drawing air in through gaps and cracks, or just not at all. This can cause health issues including headaches and nausea, irritation of the eyes, nose, and throat, dry and itchy skin, and respiratory problems such as coughing and wheezing. Alongside this, negative pressure can cause damage inside the home to things like wooden furniture, fixtures and fittings.

As homes become more airtight to meet energy efficiency targets, ensuring a proper supply of new air is more important than ever. Without it, we risk trapping moisture and pollutants inside, leading to condensation, discomfort, and health concerns.

How is supply air delivered?

There are 2 main ways to provide supply air into a dwelling:

• Continuous supply fans (Such as an MVHR)
• Background ventilation (Trickle vents or wall grilles)

For this newsletter, we’ll be focussing on mechanical means of supply air, but it’s important to remember that if you are using intermittent, dMEV, or MEV as extract fans, you need to have window trickle vents or wall grilles installed to ensure that you make up the supply air. Table 1.7 in the 2021 Approved Document Part F will give you the specifics on this.

The magic numbers…

When designing or installing a ventilation system/strategy, it’s important to make sure the whole dwelling supply rate is where it should be. There are 2 calculations you can use to measure this:

1. Internal floor area x 0.3 l/s
2. Number of bedrooms

Whichever figure is the highest is the one that you use. Let’s look at an example…

A 100m² 3 bedroom house, based on the floor area alone, would have a supply rate requirement of 30 l/s. However, based on the number of bedrooms (please see part F table extract below), the supply rate required is 31 l/s. Therefore, 31 l/s is the supply ventilation required in this example as it is the highest figure.

You also must compare this against the required extract rate (see previous newsletter), but we won’t over complicate things for now…

The supply rate for the number of bedrooms is as below:

NOTES:

1. If the dwelling only has one habitable room, a minimum ventilation rate of 13 l/s should be used

2. For each additional bedroom, add 6 l/s to the values in the table above.

More things to think about…

Calculating the supply rate using the calculations above is easy enough. But there is another crucial detail to consider: Internal doors.

As detailed in Part F of the building regs, internal doors need to allow air to flow through the property. Doors need to be undercut by 10mm above the finished floor surface (or 20mm if the floor isn’t finished yet).

This key detail allows air to move freely between rooms and keeps your system balanced and compliant.

In summary…

All homes need air both in and out. Which sounds simple, but the moment one side of the equation is off, things stop working…

If you’re an installer, a wholesaler, or a landlord, it’s important to ensure that you have both sides covered.

• Are you calculating the supply rate properly?
• Do the doors have enough gap to allow free movement of air?
• Are trickle vents in place if using extract only fans?

We’re always here if you need help figuring things out.