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  • Writer's pictureSoba Faris

Passive house design with mechanical ventilation and heat recovery (MVHR)

Planning for the ventilation of a building is crucial to ensure the comfort, health, and safety of a building's occupants as well as the longevity of the building itself. It is also a principle componant of the "fabric first" approach of passive house design where the energy (warm air) within the house is controlled and conserved to ensure it's energy efficiency.

Large sliding doors together with an airtight construction can give light, comforable conditions throughout the year.

What are the types of ventilation within a building?

Ventilation is the process of extracting stale air, from active rooms such as bathrooms, utility and living spaces to the outside. All buildings need both 'background ventilation' (a continuous trickle of clean air) and 'purge ventilation' which can respond to changing conditions (eg cooking or showers) and extract a large volume of air quickly. As well as this, clients may wish to choose between 'natural' and 'mechanical' ventilation options. Natural is the most common form of ventilation for a typical house as the materials and the construction strategy allow for a building to be ventilated by natural air movement. This can involve trickle vents positioned in doors and windows in order for fresh air to enter the interior as well as openable windows which can purge the space.

Traditionally, opening and closing a combination of windows and doors allows fresh air into a building. This in collaboration with the building fabric allows the building to breath, although in most cases not adequately.

This may sound like the sustainable approach, but in fact natural ventilation can be somewhat difficult to control, either by occupants forgetting to open and close windows, or especially in older houses, where the venilation relies upon gaps within the building fabric which are difficult to seal. This can lead to uncomfortable drafts and energy loss on cold windy days, but also the builds up of moisture which can cause condensation and mould growth. This is often combated with the use of a dehumidifier. Better still, mechanically ventilating the home can solve these issues whilst providing a host of other benefits.

What is mechanical ventilation and why is it being used?

The other form of ventilation is mechanical which is more commonly used in modern buildings that are well sealed & airtight. Mechanical ventilation is the process by which fresh air is brought into a building interior by way of a machine that can be controlled and adjusted as needed. This machine essentially consists of a fan that draws fresh air inward whilst another fan exhausts stale air outward. This is known as an air exchange.

The volume of fresh air required for an efficient level of ventilation of a space is determined by factoring in the size, the number of inhabitants and the use of a space. The quantity of air supplied to a space, expressed in terms of the number of times the total volume of air in the space is replaced in a specific period of time, is typically one hour. Issues of condensation are also resolved with the implementation of a Mechanical Ventilation with Heat Recovery (MVHR) unit as the outgoing stale air, in which water vapour sits, is extracted from the interior spaces whilst also passively heating the incoming fresh air. As the stale air is cooled upon extraction, a small reservoir captures any condensation and drains it away; the same is also true for cool incoming air being passively heated. Smart controls could also be implemented along with sensors to determine when a space is in need of more air changes either due to a temperature rise, an increase in occupants or other factors.

As we develop ever more efficient and airtight buildings, the requirement of mechanical ventilation becomes a sensible consideration in order to control air quality, ventilation rate and heat removal from our interior inhabited spaces. The mechanical ventilation system of choice today is one with heat recovery which further improves the comfort to the inhabitants as well as boosting the efficiency credentials of a building. This ventilation strategy is being used to counteract heat loss in highly insulated homes via the ventilation system and is far more efficient than earlier variations of mechanical ventilation as heat is recovered from stale outgoing air without ever mixing with the fresh supply. Over time, this technology has further developed to include filters on the incoming air supply to remove pollen and other elements from the incoming air which has been proven to create a safer habitat for those with respiratory issues.

The sketch below details how a Mechanical Ventilation with Heat Recovery system (MVHR) operates.

The process of a Mechanical Ventilation Heat Recovery System

  1. Cool, fresh air from outside is drawn inside via a fan and filtered to remove dust and other fine particles

  2. Incoming cool air is heated passively in the heat exchanger and distributed throughout the building

  3. Warm, stale air is drawn from interior spaces

  4. Outgoing stale air is cooled passively in the heat exchanger and exhausted from the building. Cooling the outgoing stale air removes the moisture which is drained away

  5. Filtering the incoming air helps people with respiratory issues such as asthma.

Project Focus

In a previous article on airtightness, we discussed how Wellington Lane in Bristol was constructed to airtight standards. As a result, there are no unintentional drafts or air leaks and so a mechanical form of ventilation was integrated into the build to provide a continuous and constant supply of fresh air to the building interior. Simultaneously, the stale interior air is exhausted outward and the heat from this air is exchanged to the incoming air supply. This benefits the inhabitants as they are receiving fresh air that is at a temperature that feels comfortable and so creates a comfortable interior environment throughout the day. Without this heat exchange, the incoming fresh air would feel cool and affect the comfort of the interior environment.

The continuous exchange of air is also beneficial to the building structure as moisture is wicked away from the interior spaces and eliminates the chance of rot and mould growth. This is especially true in high moisture environments such as the bathroom and kitchen.

The cross section of Wellington Lane below highlights the location of the MVHR unit and the supply of fresh air to the interior spaces. The exchanger is highlighted in purple which is where the heat recovery occurs from the stale outgoing air (not shown) and the incoming supply of fresh air (highlighted in blue). We can also see how the ducting is hidden in the floor/ceiling assembly and out of sight in the finished construction.

A cross section of Wellington Lane, Bristol showing how an MVHR was integrated into the design and final build

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