Passive House

Mechanical Ventilation With Heat Recovery – MVHR

The health and comfort of the inhabitants is the most important objectives of a Passive House design. Excellent air quality is vital; this can only be achieved in an airtight house if used “free” exhaust air is exchanged with fresh outdoor air at regular intervals.

Passive House’s only work if a highly efficient heat recovery system is also present. This recovers the heat from the exhaust air and using a heat exchanger, transfers it back into the supply air without mixing the air flows. Today, modern ventilation technology allows a heat recovery rate of between 75 and 90 %. This is possible due to counter-flow heat exchangers and special energy-efficient fans

Ventilation will only work correctly if the extract air is removed constantly from the kitchen and bathrooms. Fresh filtered air is supplied to all the living rooms, bedrooms and function rooms, to substitute the extracted air. There is no recirculated air, thus providing a hygienic, filtered, fresh air quality.

Heat recovery captures waste energy and reuses it by returning it to systems or processes; this can include heating for both space and

Due to this principle of directed air flow, the fresh air is optimally utilised: it provides high quality air in the living areas, removes any bad air from the transferred air zones (e.g. odours from clothes), and finally dehumidifies the humid areas.

Airtight Construction

Building envelopes under the Passive House standard must be extremely airtight compared to conventional construction. Air barriers, careful sealing of every construction joint in the building envelope and sealing of all service penetrations through it are all utilised to achieve the standard.

Air tightness minimises the amount of warm or cool air that can pass through the structure, enabling the mechanical ventilation with heat recovery system to recover the heat before discharging the air externally.

To reach Passive House standard, air leakage through unsealed joints must be less than 0.60 times the house volume per hour, this is around 10 times better than a typical new-build house. Passing the blower door test is a necessary prerequisite for the functioning passive building.

This airtightness layer will be designed at an early stage as a seamless layer, enveloping the entire building. Additional detailing is required at the external windows, doors and foundations.

Thermal bridge free design

A thermal bridge is a component, or assembly of components, in a building envelope through which heat is transferred at a substantially higher rate than through the surrounding envelope area.

Passive House buildings employ ‘super’ insulation to significantly reduce heat transfer through the floors, walls and roof compared to conventional buildings, with special attention given to eliminate thermal bridges.

Heat loss from buildings is a continuous process and can increase during the life of the building. Energy in the form of heat or cool is fed into to the building to maintain the required temperature; the two main routes for heat loss are ventilation or fabric (conduction) heat loss.

Energy balance in new buildings – simplified:

On the left hand are the heat losses from the building:

• Infiltration, and
• Transmission heat losses

On the right hand the gains to the inside of the house:

• Through the power of the sun entering through windows,
• Through the temperature of people, equipment, computers, the ground, and
• external energy input.

Outside of the building there are the so called ‘neutral gains’ including solar, wind, air and geothermal.

We also consider Southern Orientation and shading considerations

Ground Source Heat Pump

An opportunity exists, although not essential with a Passive House, to increase the efficiency of the ventilation systems using a ground source heat exchanger. The ground during winter has a higher temperature than outside air and during the summer a lower temperature than outside air. Therefore, it is possible to preheat fresh air in winter and cool it in summer, by utilising a buried earth duct. This is commonly done with brine circulating in earth buried pipes providing geothermal energy and then either heating or cooling the air with a water to air heat exchanger.

A ground source heat pump can be a highly efficient form of space heater, particularly where deployed in conjunction with a low energy heating system such as a MVHR system

The length of the ground loop depends on the size of the home or building, and the amount of heat required. Longer loops can draw more heat from the ground, but need more space to be buried in. If space is limited, a vertical borehole can be drilled instead. Normally the loop is laid flat in wide trenches about two metres deep, but if space is limited, a vertical borehole can be drilled instead containing a vertical loop. Some companies will suggest a using narrow trench which requires less area – we would not recommend this option

Ground source heat pumps (also known as GSHPs) benefits include:

• could lower your fuel bills, especially if you replace conventional electric heating,
• could provide you with an income through the government’s Renewable Heat Incentive (RHI),
• could lower your home’s carbon emissions,
• can heat your home and provide hot water,
• needs little maintenance – they are called ‘fit and forget’ technology.

Heat from the ground is absorbed at low temperatures into a fluid inside a loop of pipe (a ground loop) buried underground. The fluid then passes through a compressor that raises it to a higher temperature, which can then heat water for the heating and hot water circuits of the house. The cooled ground-loop fluid passes back into the ground where it absorbs further energy from the ground in a continuous process as long as heating is required.

Air source heat pumps are usually easier to install than ground source as they do not need any trenches or drilling, but they can be less efficient than GSHPs. Water source heat pumps can be used to provide heating in homes near to rivers, streams and lakes.