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 pressure drop created by the ventilation system. The size of the house provides a good initial estimate, but the number of occupants, the floor plan, the ductwork and the unit’s performance at the actual operating point 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. This corresponds to 1.26 m³/h per m². For 150 m², this amounts to approximately 52.5 l/s or 189 m³/h. This is a good initial figure to work from, but it is not enough simply to choose a unit whose maximum air flow rate just reaches that level. The unit must be capable of maintaining the air flow rate at the system’s actual pressure drop and have a suitable operating margin.

The table shows a simplified estimate in which the surface area in the examples is assumed to correspond to the floor area used in the design calculations. The final choice of unit must also take into account factors such as occupancy load, air distribution and pressure drop.

What determines the size of the FTX unit you need?

It is common for FTX unit are described in terms of the size of home they are suitable for. This is useful when making an initial selection, but two houses of, say, 150 m² may still require different solutions.

Above all, you need to bear in mind:

• Floor area. Provides a basis for calculating the outdoor air flow capacity required for the ventilation system.
• 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, wet rooms and other areas all affect how the air should be distributed.
• Ducting. Length, dimensions, bends and branches affect the resistance in the system.
• Pressure drop. Ducts, filters, silencers, dampers and diffusers create resistance that the fans have to overcome.
• Increased air exchange. The system must be able to cope with periods when the demand for ventilation is higher than normal.
• 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 itself works, we’ll go through the basics in our guide on FTX ventilation and heat recovery.

How to calculate an approximate air flow rate

At On the Boverket website, you can read about the requirements for the design of ventilation systems. For residential properties, the ventilation system must be designed for an outdoor air flow of at least 0.35 l/s per m² of floor area. For rooms in residential properties, the system must also be designed to provide at least 4.0 l/s per person.

For an initial rough estimate based on the floor area, you can 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:

0.35 × 3.6 = 1.26 m³/h per m².

Let’s take a detached house where the design floor area is 150 m². The calculation is then as follows:

150 × 0.35 = 52.5 l/s
52.5 × 3.6 = 189 m³/h

In this simplified example, the ventilation system must therefore be designed to handle at least 52.5 l/s based on the floor area. Subsequently, factors such as the number of occupants, the distribution between rooms and the system’s pressure drop must be checked.

Don’t forget the air flow per person

The calculation based on floor area does not provide the full picture. The Boverket website also states that ventilation systems for rooms in residential properties must be designed to provide at least 4.0 l/s of outdoor air per person.

This becomes relevant when airflows need to be distributed between different rooms. For example, a bedroom intended for two people needs to be sized to accommodate the number of occupants, even if the house’s total airflow has already been calculated on the basis of its floor area.

It is therefore not possible to check that the design is correct simply by adding up the square metres of the dwelling. The air flow also needs to be directed to the right places.

Example: FTX unit for 100 m²

Imagine a small detached house where the design floor area is 100 m². As a first step, the air flow is calculated based on the floor area:

100 × 0.35 = 35 l/s
35 × 3.6 = 126 m³/h

This means that the unit needs to be able to deliver around 35 l/s at the pressure drop caused by the completed installation. If the building has relatively short and well-dimensioned ducts, the operating point may be relatively favourable. Long or more complex duct layouts, on the other hand, may require a higher fan pressure to achieve the same air flow.

Our range includes EvoAir A60T G2 An example of a model that may be worth evaluating in this size category. It has a flow range of 15–70 l/s and is designed for homes up to 120 m².

This does not mean that the model is automatically suitable for every detached house measuring 100 m². The final choice must be based on the actual air flow and pressure drop of the installation.

Example: FTX unit for 150 m²

Let’s take a standard-sized detached house of 150 m² as our next example. If the floor area corresponds to the design floor area, the air flow will be:

150 × 0.35 = 52.5 l/s
52.5 × 3.6 = 189 m³/h

Here, 52.5 l/s is therefore the starting point for the area calculation. In a detached house with a simple duct system, this may be a relatively easy operating point to achieve, whilst a two-storey house with longer duct runs, multiple branches and more silencers may require a significantly higher pressure.

Our range includes, for example, EvoAir A100S G3 or EvoAir A110T G4 are worth evaluating. 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 model best suited to the installation depends on more than just maximum capacity. The A100S is side-connected, whilst the A110T is top-connected, which affects placement and how the ducts can be routed.

Example: FTX unit for 200 m²

In a larger detached house of 200 m², the corresponding rough estimate would be:

200 × 0.35 = 70 l/s
70 × 3.6 = 252 m³/h

The calculated air flow rate of 70 l/s must then be considered in relation to the pressure drop in the ducts. This is particularly relevant in larger buildings, where duct runs are often longer and the system may need to supply several rooms or several storeys.

For this type of installation, for example, EvoAir A170T G2 or EvoAir A200S G2 There are models in our range that are suitable for evaluation. Both have a flow rate range of 50–190 l/s and are designed for homes up to 400 m².

The A170T G2 has a top connection, whilst the A200S G2 has a side connection. This offers different options depending on where the unit is to be positioned and how the ductwork is intended to be laid out.

The fact that the models can deliver significantly more than the 70 l/s used in the example does not mean that the house is automatically over-ventilated. The airflows are set at commissioning. What is interesting is how the unit performs at the operating point actually required by the installation.

Why shouldn’t FTX be sized solely on the basis of floor area?

The surface area gives an initial rough estimate of the air flow rate required, but it says nothing about the amount of resistance the air encounters as it passes through the system.

Compare two detached houses, each measuring 150 m². In one, the unit is situated centrally in a single-storey house with short and relatively straight ducts. The other house has two storeys, longer duct runs, more bends and more branches.

Both may start with the same calculated air flow, but the second system may require a higher fan pressure to move the same volume of air. That is why the combination of air flow and external pressure is crucial when selecting the unit.

This is particularly important during refurbishments and older houses where the routes are often more limited. In our guide to Install an FTX unit in a detached house we will go through further aspects that need to be taken into account when planning the entire system.

The operating point is more important than maximum capacity

A fan does not deliver the same airflow regardless of the resistance it is working against. As the resistance in the duct system increases, the conditions for the fan change, as does the power required to move the air.

The combination of air flow and pressure at which the unit is to operate is usually referred to as operating point.

Suppose, for example, that an installation requires 55 l/s and that the external duct pressure is calculated at 100 Pa. In that case, it is the unit’s performance at around 55 l/s and 100 Pa that is relevant. A high maximum air flow rate stated in the product information does not, on its own, indicate how well the unit is suited to that installation.

If you’d like to check how our EvoAir units perform under different conditions, you can run your own data analysis in EvoCalc. There, you can enter the air flow and duct pressure to obtain more relevant data for the design.

What happens if the FTX aggregate is too small?

A unit with insufficient capacity at the current operating point may struggle to deliver the design air flow. If, at the same time, the fans need to operate close to the limits of their operating range even during normal operation, the margins become smaller.

Possible consequences are:

• Higher fan speed to achieve the desired air flow.
• Higher energy consumption for the fans.
• Increased risk of disruptive noise.
• A smaller margin when the pressure drop across the filters changes.
• Reduced ability to increase air exchange when the need is greater.
• There is a risk that the designed airflows cannot be maintained at the system’s actual pressure drop.

It is therefore advisable to avoid a unit that only just meets the planned operating point under favourable conditions.

What happens if the FTX aggregate is too large?

A larger FTX unit is not automatically a poorer choice. In some installations, a larger model can operate efficiently even at an airflow rate that is well below its maximum capacity.

However, there is rarely any reason to go up several sizes without first checking what this means for the installation. A larger model may, for example, result in:

• Higher purchase cost.
• Greater space requirements for installation.
• Larger duct connections or other requirements relating to the duct installation.

The aim is therefore not to choose the smallest or largest possible unit. The aim is to find a model that performs well at the air flow rate and pressure that the house actually requires.

How much reserve capacity should an FTX unit have?

There is no standard percentage for reserve capacity that applies to all dwellings. A rule stipulating, for example, that 20% should always be added does not take into account differences in ductwork, pressure drops or the need for increased air exchange.

The Boverket website states that homes should be able to provide increased air exchange unless this is unnecessary. The capacity required for this temporary increase in air exchange must be determined during the design phase.

It is therefore better to check the unit’s operating range at the intended operating point than to apply an arbitrary percentage.

A reasonable working margin may be required, for example, for:

• Increased air exchange when the occupancy in the dwelling temporarily increases.
• Changes in filter resistance over time.
• Adjustment of airflows between different rooms.
• Differences between the calculated and actual pressure drop in the completed installation.

SFP indicates how much fan energy is required

Capacity is only one aspect of selecting a unit. It is also important to consider how much electrical power the fans require to move the air.

SFP stands for Specific Fan Power, specific fan power, and describes the relationship between the fans’ electrical power and the air flow. The unit is kW/(m³/s).

The designation SFPv is often used in connection with ventilation units. A lower value essentially means that less fan power is required to move a given volume of air. However, for a comparison between two units to be meaningful, they must be compared at equivalent air flow rates and pressures.

In our guide, we go through both the calculations and practical examples of what the SFP value is and how it affects the ventilation system.

The route of the duct affects both the SFP and the dimensioning

The fans need to overcome the resistance in the ventilation system. The greater the pressure drop created by the system, the harder the fans have to work to move a given volume of air.

The pressure drop is influenced by, amongst other things:

• The dimensions of the channels.
• The length of the channels.
• The number of bends and branches.
• Air diffusers and adjustment dampers.
• Silencer.
• The filter and the unit’s internal resistance.

It is therefore not possible to fully compensate for a duct layout that places unnecessary demands on the pressure by simply selecting a more powerful unit. The unit and the duct system need to be designed as a single integrated system.

The noise level is affected by more than just the unit itself

If the fans need to work harder, this may affect the noise level, but the unit is not the only possible source of noise. Noise can also occur when air velocity is high in ducts and diffusers, or when sound insulation is inadequate.

The sizing of the ducts, the positioning of the air outlets, the silencers and the calibration therefore all affect how quiet the system actually is. This is particularly relevant in bedrooms and other areas where a low noise level is important.

Heat recovery and efficiency are also important

The purpose of the FTX system is both to ventilate the home and to recover heat from the exhaust air. The performance of the heat exchanger is therefore a key factor when comparing different units.

However, a maximum catalogue efficiency value does not tell the whole story about how the unit will perform in the specific installation. The result is influenced by factors such as air flows, temperature conditions and the operating point.

When actually selecting an air handling unit, it is therefore better to assess air flow, pressure, SFPv, heat recovery and noise levels together, rather than choosing a model based on a single value.

FTX units for 100, 150 and 200 m² – a comparison

The examples are intended as an initial guide and assume that the floor area corresponds to that used in the rough calculation. The model suggestions are based on the specified flow ranges of the units and the recommended dwelling sizes. The final choice must be made on the basis of the installation’s air flow, pressure drop and other conditions.

When is more detailed design required?

For an initial estimate, it is often sufficient to calculate the floor area and identify a reasonable capacity range. When the duct system is to be installed, the design calculations need to be more detailed.

This is particularly justified in the following cases:

• Multi-storey building.
• Long canal routes.
• Many bends and branches.
• Older houses where the possible routes for the ducts are limited.
• Refits where parts of an existing ventilation system are to be retained.
• A large number of rooms or a higher number of people.
• High standards for low noise levels.
• Higher air flow rates.
• Installations where the planned operating point is close to the unit’s capacity limit.

During the design phase, it is necessary to determine, amongst other things, the air flow rates for each room, duct dimensions, pressure drops and the appropriate operating point. Following installation, the system must also be calibrated to ensure that the correct volume of air actually reaches each room.

Select the correct size for your FTX unit

The first step is to estimate the air flow required for the property. You then need to check which units can handle that air flow at the duct pressure the installation is expected to experience.

You can start by our tool for selecting air handling units. There, you can specify your desired air flow and see which EvoAir models fall within the relevant flow range.

Once you have found one or more suitable models, you can go on to run a data analysis in EvoCalc. There, you can enter the air flow and duct pressure and check how the unit performs at a more realistic operating point.

Don’t just look at maximum capacity. Air flow, pressure drop, SFPv, heat recovery, noise levels, installation space and duct connections all need to work together. This will give you a better basis for choosing the right FTX unit than if you base your decision solely on the living area.

If you would like help with both sizing and installation, please fill in the form below with your details. One of our certified partners will then provide you with a quote for the installation of FTX ventilation, based on the conditions in your home.