How do you size an FTX unit? Example for 100–200 m²

The size of the FTX unit you need depends primarily on the air flow required by the property and the resistance created by the ductwork. The floor area provides a good initial estimate, but the number of occupants, the duct layout, the floor plan and the unit’s performance also need to be taken into account.

As a rough estimate, you can assume 0.35 l/s of outdoor air per square metre of floor area. For 150 m², this corresponds to approximately 52.5 l/s or 189 m³/h. However, it is not sufficient to choose a unit whose maximum air flow rate just reaches that level. The unit must also be capable of handling the air flow at the pressure required by the finished installation.

What determines the size of the FTX unit you need?

It is common for FTX unit are categorised according to the size of home they are suitable for. This is a practical initial selection, but two houses with the same floor area may still require different solutions.

Above all, you need to bear in mind:

• Floor area. Provides the basis for estimating the air flow in the dwelling.
• Number of people. The air flow must also be sufficient for the number of people the rooms are designed to accommodate.
• Floor plan. The number of rooms, the floor level and wet rooms all affect how the air should be distributed.
• Ducting. Length, dimensions, bends and branches affect the resistance in the system.
• Pressure drop. Ducts, silencers, dampers and air diffusers create resistance that the fans have to overcome.
• Noise and energy consumption. It must be possible to deliver the correct air flow without unnecessarily high fan power or disturbing noise.

If you’d like to find out more about how the system works, we’ll go through the basics in our guide on FTX ventilation and heat recovery.

How to calculate an approximate air flow rate

For residential properties, the ventilation system must be designed to provide at least 0.35 l/s of outdoor air per m² of floor area. For rooms in residential properties, the following also applies: at least 4.0 l/s per person. You can read more about the ventilation requirements set by the Swedish National Board of Housing, Building and Planning.

For an initial calculation, use:

Floor area × 0.35 l/s per m² = air flow in l/s

To convert l/s to m³/h, multiply by 3.6.

For a detached house with a floor area of 150 m², this works out as:

150 × 0.35 = 52.5 l/s

52.5 × 3.6 = 189 m³/h

This gives you the base flow based on the floor area. You then need to check the occupancy load, how the air is to be distributed between the rooms, and what pressure the duct system requires.

Don’t forget the air flow per person

The area calculation does not provide the full picture. For example, a bedroom for two people needs to be sized to accommodate the number of occupants, even if the dwelling’s total air flow has already been calculated on the basis of the floor area.

It is therefore not enough simply to ensure the correct total volume of air. The air also needs to be distributed correctly between the rooms.

Examples of dimensioning

These examples provide an initial selection. The final choice of unit must be based on air flow, duct pressure and other conditions in the installation.

FTX unit for 100 m²

For 100 m², the base flow is approximately 35 l/s or 126 m³/h.

Our range includes EvoAir A60T G2 a model to evaluate. It has a flow range of 15–70 l/s and is designed for homes of up to 120 m².

How well the model fits also depends on the duct layout. A simple system with short, well-dimensioned ducts requires less fan pressure than a system with long duct runs and numerous bends.

FTX unit for 150 m²

For 150 m², the base flow is approximately 52.5 l/s or 189 m³/h.

Here, for example, EvoAir A100S G3 or EvoAir A110T G4 be suitable. The A100S G3 has a flow rate range of 20–120 l/s and the A110T G4 of 20–132 l/s. Both are designed for homes up to 180 m².

The A100S has side-mounted connections, whilst the A110T has top-mounted connections. This affects the positioning and how the cables can be routed.

FTX unit for 200 m²

For 200 m², the base flow is approximately 70 l/s or 252 m³/h.

Here you can EvoAir A170T G2 or EvoAir A200S G2 are models to be evaluated. Both have a flow range of 50–190 l/s and are intended for homes of up to 400 m².

The A170T G2 has a top connection and the A200S G2 has a side connection. The fact that these models can deliver significantly more than 70 l/s does not mean that the property is automatically over-ventilated. The air flows are set at commissioning.

The living space specified for a unit is therefore an initial selection criterion. The actual choice is based on the air flow and pressure required by the installation.

Air flow and pressure determine the choice of unit

Two detached houses with the same floor area may require different units. A single-storey house with short, straight ducts may have a significantly lower pressure requirement than a two-storey house with long duct runs, numerous bends and several branch ducts.

That is why the combination of air flow and external pressure which is important when selecting the unit.

The combination of air flow and pressure at which the unit operates is known as operating point.

If, for example, the installation requires 55 l/s and the duct pressure is 100 Pa, it is the unit’s performance at approximately 55 l/s and 100 Pa that is relevant. The maximum flow rate stated in the product information does not, on its own, indicate how well the model is suited to the application.

Filters, heat exchangers and other components inside the unit also create resistance, but this occurs internally within the unit and should be distinguished from the external pressure drop in the duct network.

If you’d like to check how our EvoAir units perform at different operating points, you can run a data analysis in EvoCalc.

This is particularly important during refurbishments and in older houses, where the possible routes are often more limited.

The right size and spare capacity

If the unit is too small

An air handling unit that is too small may struggle to deliver the correct air flow when the resistance in the ductwork increases.

Among other things, this can result in:

• Higher fan speed and energy consumption.
• Increased risk of disruptive noise.
• The margin is smaller as the filters become dirtier.
• There is a risk that the designed airflows cannot be maintained.

You should therefore avoid choosing a unit that is only just capable of operating at the planned operating point.

If the unit is too large

A larger unit is not automatically a worse choice. A larger model can sometimes operate efficiently at an air flow rate well below its maximum capacity.

At the same time, a larger model may entail higher purchase costs, require more installation space and need larger duct connections. The aim is therefore to choose a model that performs well at the operating point that the house actually requires.

How much spare capacity is needed?

There is no single percentage that applies to all properties. For example, always adding 20 % does not take into account ductwork, pressure drops or the unit’s operating range.

Homes should be able to achieve increased air exchange when necessary, but this does not mean that the FTX unit must always be oversized by a certain percentage. How the increased air exchange is to be achieved depends on the specific installation.

However, a certain margin is beneficial to allow for, for example, changes in filter resistance, adjustment and differences between calculated and actual duct pressure.

Other factors to consider when choosing FTX units

Air flow and pressure are the key factors, but energy consumption, noise and heat recovery also influence which unit is best suited.

• SFPv. Shows how much electrical fan power is required in relation to the air flow. Under comparable conditions, a lower SFPv means lower fan energy consumption. Read more in our guide on SFP value.
• Ducting. Small duct dimensions, long runs, numerous bends, dampers and silencers increase the pressure drop.
• Sound. High air velocities, hard-working fans and inadequate sound insulation can cause disturbing noise, particularly in bedrooms.
• Heat recovery. Efficiency is influenced by factors such as air flow, temperature and operating point. A maximum catalogue value therefore does not tell the whole story.

The unit and the ductwork system need to be viewed as a whole. A more powerful unit does not fully compensate for ductwork that requires more pressure than necessary.

When is more precise dimensioning required?

A rough estimate based on the floor area is sufficient for an initial selection. When planning the system in more detail, it is also necessary to determine the air flow rates for each room, the duct dimensions and the pressure drops.

This is particularly important in the following situations:

• Multi-storey buildings or long canal sections.
• Many bends, branches and rooms.
• Older houses and refurbishments where there is limited space for ductwork.
• High occupancy rates or strict requirements for low noise levels.
• Installations where the operating point is close to the unit’s capacity limit.

After installation, the system needs to be adjusted to ensure that the correct volume of air reaches each room.

Select the correct size for your FTX unit

Start by estimating the air flow based on the floor area and the number of occupants. Then check which units can handle the air flow at the duct pressure the installation is expected to experience.

With our tool for selecting air handling units You can make an initial selection of EvoAir models based on your desired air flow.

You can then run a data analysis in EvoCalc and check how the unit performs at the desired air flow and duct pressure.

So do not choose an FTX unit based solely on maximum capacity or the size of your home. Airflow, pressure, energy consumption, noise, heat recovery and installation all need to work together.

If you’d like help with sizing and installation, please fill in the form below with your details. One of our partners will then be able to provide a quote based on the specific circumstances of your home.