Ventilation in an airtight building
Controlled ventilation is essential in highly airtight buildings, such as Passive Houses. In contrast to all other forms of ventilation, mechanical ventilation offers the possibility to recover the heat. This is called Mechanical Ventilation with Heat Recovery (MVHR or HRV). Compared to natural ventilation with trickle vents, a highly efficient MVHR (88% efficiency) reduces the heating demand by 57% (based on Passivhaus standards). Therefore, recovering the heat from the stale, used air and transfering it to the fresh, but cold incoming air is a major factor for energy and emission saving.
This is how it works:
Air is extracted from rooms that typically produce heat, moisture and unwanted smells such as kitchens, utility rooms and bathrooms. Before this air is expelled to the outside it passes through a heat exchanger where the heat is transferred to the incoming fresh air, thereby eliminating the need to completely heat the fresh air as it enters the building. It is important to highlight that the stale exhaust air and clean fresh air do not mix in the heat exchanger. The stale and the clean air are channelled through closely spaced, but separate, narrow sleeves in the core of the heat exchanger.
In order for the heat recovery to work properly, it is vital that the building is airtight. Therefore, a maximum air permeability of n50= or less than 0.6/h is allowed with a Passive House. This means, that 60% of the heated volume of the building is allowed to enter or escape at 50 pascal pressure difference inside the building (both ways get tested: under and over pressure). Some window suppliers offer a free pressure test in Passive Houses after the window installation to adress any leakages before the plasterboard and internal finishes are installed.
Apart from the heat recovery, are there any other benefits of a MVHR?
- Elimination of bad odours
- Preservation of the building fabric through steady ventilation, elimination of mould etc.
- Continuous supply of fresh, filtered air (great advantage for people with allergies) which provides a comfortable indoor climate
- With ground heat exchanger, you save even more energy as it pre-heats the intake air in winter and pre-cools it in summer (natural air conditioning)
- Less noise inside (windows can remain closed); undisturbed sleep
- Depending on the design of the Passive House, there might be no need for a (central) heating system as the MVHR is able to distribute the heat throughout the entire house
Recommended Ventilation Rate
According to the Passivhaus Institute, the appropriate air change rate for dwellings is between 0.3 and 0.4 times the volume of the building per hour, with a general recommendation leaning toward the lower rate. This maintains high indoor air quality while ensuring a comfortable level of humidity and maximizing energy savings. The PHPP software also suggests that 30m3 per person per hour should be provided in dwellings to ensure good air quality. These two measurements can be used to choose an appropriately sized device for different dwelling designs. For example, an occupancy of 5 people would require 150m3 of fresh air delivered to the house per hour. In terms of extract, the PHPP software uses the following standard rates for different room types: Kitchen = 60m3/h, bathroom or shower = 40m3/h and WC = 20m3/h. Supply and extract air volumes should be balanced and the air flow volumes in each room can be accurately set by using a digital anemometer and adjusting the valves on the vents.
Adjustment of Fan Speed and Exchange Rate
Most MVHR systems have different settings for different circumstances. These are often referred to as a ‘party’ setting, where there are a lot of people in the house and additional fresh air is required, and ‘holiday’ setting, where the house is left vacant and the flow of air is reduced. The former of these settings will use more energy and also decrease the level of humidity whereas the latter will use less energy and perhaps lead to an increase in humidity. It is not advisable to constantly run the equipment on the lower setting just to save energy when the house is occupied. MVHRs use surprisingly little energy given the important role that they play in the Passive House. The maximum for a Passive House is 0.4Wh of electricity consumption for every m3 of transported air. When designing a Passive House, the specific fan power should be carefully considered as the electricity consumption of the fans has a direct impact in terms of the primary energy performance and the recently introduced Building Energy Rating (BER).
Winter and Summer Mode
There are generally two ventilation modes in a Passive House: Summer Mode and Winter Mode. In winter, the MVHR uses the heat in the exhausted air to warm the incoming fresh air. In summer, a bypass in the unit can be set so that the incoming fresh air is not heated. Alternatively in summer natural cross ventilation may be used and the MVHR system can be switched off. This of course depends on the design and layout of thePassive House as sometimes it is necessary to let the fans run in order to avoid overheating in summer.
Insulation and Positioning of Duct Work and Vents
It is very important to either keep the supply air ducting outside the thermal envelope of the building or adequately insulate it so that there is minimal temperature loss before reaching the heat exchanger (this ensures heat is only taken from the stale air). The recommended thickness of insulation used for installations is 140mm. It is also preferable to locate all other ducting (except for that of the exhaust air after the heat exchanger) within the thermal envelope and to keep pipe runs as short, and with as few bends, as possible by locating the MVHR unit in the best possible position. This helps to keep the fan power low and requires careful planning at a very early stage of the building design.
Vents are normally placed in the ceiling but can also be placed in the wall if necessary. The air inlets are typically designed to spread the air horizontally across the ceiling, minimizing downward drafts. There should be an 8-10mm gap either under or over the door of each room to enable the easy movement of air from one room to the next.
Noise
Fan and valve noises can be almost completely eliminated by sound control measures (e.g. vibration isolation mounts, low air speed and acoustic lining in ducts). The grilles on vents generally guide incoming air along the ceiling from where it uniformly diffuses throughout the room at velocities that are barely perceptible. If the ventilation equipment is operating on a high setting (‘Party Mode’) the noise of the equipment and the air flow may be more noticeable. MVHR machines are generally housed in a well insulated casing and noise should not be a critical issue. In order to avoid noise transmittance, silencers are fitted, e.g. before the bedrooms.
Maintaining Good Air Quality
It is important that attention is paid to regular replacement of air-filters for both incoming and exhaust air. Filters are used not only to provide clean air for the occupants but also to ensure that the heat exchanger does not get clogged up with dust and other matter. If the filters are not regularly replaced (for example every six to twelve months) and they themselves become clogged with dirt and the MVHR will have to work harder to provide the same volume of air to the house, thereby increasing the speed of the fans and, ultimately, using more energy. In countries where this system is relatively new, occupants may not be aware of this maintenance need and indoor air quality may suffer as a consequence. Equipment differs with respect to the types of filters used; some have to be replaced while others can be washed and reused. Sometimes the extractor hood in the kitchen is connected to the MVHR system to extract kitchen smells and to use the waste heat from cooking to warm the incoming fresh air. In such instances, it is very important that the hood is fitted with a high quality filter that can easily be cleaned or replaced in order to prevent the built up of grease in the ducting system which could be a fire hazard. The kitchen, or any other fan, does not to extract the stale air directly to the outside as this would prohibit the MVHR running properly.
What happens in the event of a power failure?
If there is a loss of electricity (and the dwelling has no back-up system), the ventilation system will stop working and the supply of fresh air will be greatly reduced. If power is lost for a short time (for example a few hours), then it is unlikely that there will be a noticeable difference in indoor air quality. If the loss of power is prolonged, the simple solution is to open the windows and to create natural cross flow ventilation through the building.
