AirMaster for Passivhaus

Benefits of decentralised MVHR in Passivhaus buildings:

Lower energy consumption due to low specific fan power
Lower operating cost due to reduced electricity demand
Limited project planning and dimensioning are required as positioning does not require excessive planning
Ventilation becomes a component of the building instead of a governing design factor
No ducting, diffuser grilles and suspended ceiling required due to the use of the Coanda Effect
Low pressure drops through limited ductwork
Faster building process in that installation can be done room by room as soon as the room is ready, instead of waiting for building phases to be completed
Fire dampers are not required as units do not exceed the room fire cell
Demand control on a room-by-room basis, by PIR sensors, CO₂ sensors, or TVOC + CO₂ sensors
Operational issues only affect one room, not a large group of rooms
Lower cost of ventilation units including a great reduction in the cost of both the building and installation
Units can easily be relocated if room function changes

The UK government is aiming to reach net-zero greenhouse gas emissions (GHG) by 2050, preceded by the goal of cutting emissions by 78% by 2035 in comparison to 1990 levels.

These goals are extremely ambitious. In achieving them, we will have to both scrutinize our methods of energy generation and pour resources into limiting energy consumption.

The Scottish government has brought their target forwards to 2045, with the City of Edinburgh Council (CEC) having set their net-zero goal for 2030. CEC have identified certified Passivhaus design as a method for slashing carbon emissions. To that end, all new schools built in Edinburgh will be certified Passivhaus.

The aim of Passivhaus is to minimise building energy consumption by using a fabric first approach, ensuring minimal requirements for heating or cooling. One result of this process is that buildings are required to be virtually airtight. Consequently, mechanical ventilation with heat recovery (MVHR) forms a major component of any Passivhaus design.

SAV Systems has a long history of supplying energy saving products to CEC, and to much of Scotland. We have been involved in numerous school projects, including the new low-carbon Queensferry High School, located within reach of the Firth of Forth bridges.

Certification

With plans to continue the relationship with CEC, SAV Systems undertook certification of one of the AirMaster SMVs in the range. In July 2021 the AirMaster AM 1000 was awarded Passivhaus Component certification under the category of “Decentralised ventilation system (school room).” This makes it possible to use the AM 1000 in Passivhaus school classrooms, as well as other Passivhaus certified buildings.

AirMaster, however, provides a different method of ventilation to typical MVHR systems used in schools. Instead of ducting through the building, AirMasters are dedicated room-by-room ventilation, providing demand-controlled ventilation directly into the required space.

Energy Consumption

With the certification of the AirMaster AM 1000, new possibilities are opened to Passivhaus designers. AirMasters are decentralised MVHR. Due to both the decentralised and duct free air distribution design, the SFPs for AirMasters are very low. The AM 1000 has been certified with the following characteristics:

AM 1000 Characteristics
Airflow range	264 - 962 m3/h
Heat recovery rate	ηhr = 75%
Specific electric power	Pel,spec = 0.24 Wh/m3

A centralised system able to deliver similar conditions typically has a certified SFP of approximately 0.45 Wh/m³. Below, we compare the energy demands of a centralized system vs. AirMaster AM 1000.

	Centralized	AM 1000	Unit of measurement
Airflow	922	922	m3/h
Heat recovery rate	85	75	%
Specific electric power	0.45	0.24	Wh/m3
Electrical consumption	0.42	0.22	kWh
Heating load per room (15 kWh/(m²yr) @ 60 m2)	0.90	0.90	kWh
Heat recovered	0.77	0.66	kWh
Total energy consumption	0.55	0.46	kWh

The above example also assumes that both units run at a constant flow rate. AirMasters are typically demand controlled based on room CO₂ level, such that they will turndown to 30% when rooms are unoccupied. This results in much lower energy consumption for unoccupied rooms, which the above calculations could not account for. Room-based demand-controlled ventilation is very difficult to achieve with centralised systems.

Challenges

In departing from the norm, AirMasters presents designers with some small challenges. However, there are solutions available which should be employed for optimal AirMaster installations in Passivhaus buildings.

With Passivhaus having such a strong focus on air tightness, any penetrations through the building fabric must be properly managed. A typical AirMaster installation requires that both the intake and exhaust ducts penetrate the external wall. Normally, this would reduce the air tightness of a room. However, Passivhaus rooms are sealed with a membrane and so solutions exist for sealing ductwork penetrations in such rooms.

We recommend that installers use the DAFA universal pipe collar on these penetrations. When correctly employed, these pipe collars have an air tightness of 0.07 l/s (= 0.25 m³/h) per duct at 50 Pa, contributing virtually nothing to the acceptable air change rate under Passivhaus standards (0.6 ACH-1 at 50 Pa).

As Passivhaus buildings must be free of thermal bridges, the metal spiral duct normally used with ventilation units is not permissible. We recommend the use of rigid polyurethane duct as this has insulative properties, where metal duct would be conductive. Polyurethane, for example, has a conductivity of approximately 0.022-0.028 Km/W, which is lower even that brick at approximately 0.49-0.87 Km/W. Duct connections to outside should also be insulated to prevent condensate forming within the ductwork.

Embodied Carbon

Decentralised MVHR is not only low energy at source but has a substantially lower level of embodied carbon due to the considerable reduction in the quantity of equipment required per installation.

In a case story based on a Danish school of 2,630 m³ containing 31 classrooms, AirMaster estimates that they have been able to reduce the weight of equipment by 50%. The comparison was made using the AirMaster installation compared with a fictitious centralised ventilation system inclusive of ducts, dampers, and diffuser grilles delivering the same total quantity of air.

It should be noted that the case story is a generalised example and therefore specific information was not available. However, a dramatic reduction in the weight, and therefore the quantity of equipment, should lead to a demonstrable reduction in embodied carbon.

The same case story was able to demonstrate a 52% reduction in CO₂ emissions from the ventilation equipment when compared to the same hypothetical building.