Case Study - Knight’s Place in Exeter

Built to the highest level of energy efficient construction, with MVHR from Zehnder Group UK, Knight’s Place in Exeter has set new Passive House design standards within the social housing sector.


Knight’s Place Passive House Housing is a sustainable social housing development of 18 one and two-bedroom apartments, designed by Gale & Snowden Architects for Exeter City Council. Built with meticulous attention to detail, the two blocks have been designed to strict Passive House standards, which deliver high comfort levels for residents via a mechanical ventilation system with heat recovery (MVHR). This produces consistent and uniform internal temperatures and excellent air quality whilst minimising energy use for heating and cooling, resulting in significantly reduced energy bills. Knight’s Place is amongst the first multi-residential, certified Passive Houses in the UK.



Knight’s Place in Exeter

With MVHR an integral feature of all Passive House developments, it was essential that a highly efficient, quality MVHR system was specified for Knight’s Place, enabling the buildings to meet stringent energy performance criteria. Offering excellent rewards in Standard Assessment Procedure (SAP) through the Dwelling Emission Rates calculation, Zehnder’s ComfoAir 200 whole house heat recovery system was specified for each apartment. Guaranteeing the provision of optimum quality indoor air all year round, the CA200 recovers heat from the warm air extracted from kitchens and bathrooms and transfers it to the fresh incoming air supplied to living rooms and bedrooms. The CA200 system provides both the ventilation and heating needs at Knight’s Place - no other heating system is required. As the heat loss in each flat is so minimal, this is met during winter extremes via a small air heater in the supply air duct just after the heat exchanger.

Leading the way

The first certified Passive House buildings in the UK were completed in Machynlleth, Powys, Wales in 2009.

ComfoAir 200

Outstanding Energy Performance

Commenting on the specification of the products, Principal Mechanical and Renewable Energy Engineer at Gale & Snowden, Jason Fitzsimmons explains, “Generally the key issue for passive houses is controlling heat loss through the fabric and windows and achieving excellent airtightness levels. You can insulate well, but in naturally ventilated homes you still have fresh, cold air affecting the interior climate so the use of a MVHR system with a high heat exchange efficiency rate is very important. While the average unit on the market stands at between 75 and 80 per cent efficiency, Zehnder’s CA200 model offers a 92 per cent heat exchange efficiency rate, which is outstanding. It’s one of the reasons we selected the product because, for this type of dwelling, there’s no doubt that it’s the most efficient on the market.”

Passive House Certified

Passivhaus Accredited

Alongside its superior energy and heat exchange performance, the CA200 has been independently certified by the Passive House Institute as an accredited component, making it suitable for a wide range of highly insulated new build developments and a perfect fit for Knight’s Place. “Achieving Passive House certification is still relatively unusual in the UK, particularly for this type of social housing development, but it was a key objective for Knight’s Place,” explains Fitzsimmons. “Obviously it made sense for us to select a high quality, independently accredited MVHR system such as Zehnder’s CA200 to help us meet the strict requirements of certification. Knight’s Place is now one of the first multi-occupancy buildings in the country to be recognised as achieving true Passive House standards.”

Another key benefit of the CA200 is its filtered bypass, which circumvents the heat recovery mode during warmer months. Ventilation is provided continuously without warm and humid air entering unnecessarily which can assist in reducing summertime overheating. Each unit can be provided with grade F7 filters on the supply and extract air, with GU4 filters as standard. Year round filtration of incoming air is crucial to ensuring a good level of indoor air quality at all times and is particularly important in highly insulated dwellings like Knight’s Place.

Strong technical support

Commenting on the choice of MVHR system for Knight’s Place, Fitzsimmons adds, “Aside from the main energy benefits of the CA200, one of the deciding factors for specifying Zehnder’s unit was the expertise and excellent technical support available from the dedicated team. We were very happy with the service they provided and they even supported us through our two year energy efficiency monitoring programme, where we examined the energy and comfort performance of the flats. They provided the MVHR duct sensors which enabled us to test in detail the heat exchanger efficiency and air stream humidity and temperature.”

Two years on from completion, Knight’s Place apartments maintain a comfortable temperature of 21ºC year round for residents, with minimal heating required and low running costs.

According to the SAP Energy Performance Certificate, some of the dwellings at Knight’s Place can be heated for as little as £18 a year.

Key achievements

  • Passive House certified
  • Minimal heating requirements and greatly reduced carbon emissions
  • Designed to meet Code 4 of the Code for Sustainable Homes (CSH)
  • Fully compliant with Lifetime Homes Standards
  • Designed to meet best practice daylight levels in accordance with the requirements of the CSH
  • Compliant with Secured by Design
  • Independently assessed under the Building for Life Standard with a final score of 18.5 out of 20
  • Considerate Constructors Scheme rating of 37.5 out of 40

Design Elements and House plans

Knight’s Place Plan

1 Airtightness

Exceptionally high levels of airtightness <0.6 m3/m2.hr. Careful detailing throughout to ensure continuous airtight barrier.

2 Windows & glazing

High performance triple glazed windows and doors - maximum U-value 0.85Wm2K. Daylight design is maximised in all rooms where possible to reduce reliance on artificial light and utilise solar gain. Light coloured walls help to reflect daylight into the rooms.

3 MVHR

Mechanical ventilation with high efficiency heat recovery (MVHR) - minimises ventilation heat losses through controlled ventilation, ensuring optimum indoor air quality and reduced heating requirements by retaining heat from exhaust air. The MVHR systems provide both the ventilation and heating needs for the flats. No other form of heating is required. Warm, ‘dirty’ air is extracted from the kitchen and shower room and exhausted once it has exchanged its heat with the fresh incoming air supplied to the living room and bedrooms as ‘clean’, tempered air.

4 Running costs

According to the Standard Assessment Procedure (SAP) Energy Performance Certificate (EPC), the flats in Knights Place can be heated for as little at £18 per year. The EPC shows that another £24 and £86 might be required for lighting and hot water per year respectively. This shows that these flats are truly affordable for future tenants.

5 Wall construction

Externally insulated masonry walls achieve high insulation levels and a U-value of no greater than 0.13Wm2K. The inclusion of thermal mass within internal spaces reduces internal temperature fluctuations, stores winter solar gain and reduces the risk of overheating in summer.

Comfort in the Rockies

An extremely well-insulated solar passive house was built in Old Snowmass in Colorado, at an altitude of 2,164 metres. Tropical vegetation flourished in the winter garden and the stove was seldom used. In 2011, at the International Passive House conference, this building was awarded the “Passive House Pioneer Award”.

“While the average unit on the market stands at around 80 per cent efficiency, Zehnder’s CA200 model offers a 92 per cent MVHR efficiency rate, which is outstanding. This, alongside its compactness, is one of the reasons we selected the unit because, for this type of dwelling, there’s no doubt that it’s the most efficient on the market.”

Jason Fitzsimmons, Principal Mechanical and Renewable Energy Engineer at Gale & Snowden Architects.

Two Passive House Myths

1

“You can’t open the windows!”

Windows can be opened in Passive House buildings but, in practice, most occupants choose to keep them closed as continuous fresh air is provided by mechanical ventilation.

Comfort levels are very high and the air quality is excellent. In summer, opening windows at night will help keep the house cool but in winter, doing so may increase fuel costs for the resident as heat will be needed to warm the house back up again.

2

“They don’t have a heating system!”

It’s true, Passive House buildings don’t have a traditional central heating system. Instead, they have smaller, lower cost heating systems which can meet the reduced heat demand.

On the coldest days, a post-air heater in the ventilation duct is usually enough to keep the house warm and comfortable.

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Carbon Emissions

Carbon Dioxide emissions per household have fallen dramatically over the past four decades – from 10.4 tonnes in 1970 to 4.5 tonnes in 2011.