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Which retrofit ventilation system - passive or mechanical?

Posted by Helena Ripley on 16 September 2015 at 10:15 am

Standard ventilation, which is suitable for most houses, uses trickle vents, extractor fans, windows and a lack of airtightness. Extractor fans are located in the ‘wet’ rooms of the house, such as the bathroom and kitchen. Trickle vents in windows allow a small amount of air to move in and out of the house while the window is shut. These are kept open in order to bring air into the house to replace air removed by the extractor fans. 

Two alternatives to standard ventilation are 1) passive ventilation and 2) centralised mechanical ventilation. Centralised mechanical ventilation is necessary if your house has been thoroughly draught-proofed and has an air tightness of 5m3/m2/hr or less @ 50 Pa (The measure of airtightness refers to the volume of air that moves out of a house in an hour, divided by the area of the external walls, at a fixed pressure). New builds are expected to have an airtightness of 10m3/m2/hr or less @ 50 Pa, and at that level trickle vents and extractor fans should cover your needs. But what if they don’t?

1. Passive ventilation

There are two types of passive ventilation: stack ventilation and wind-driven. These can be used together, as is the case with wind cowls, in the picture above, or the Ventive system.

Stack ventilation is driven by buoyancy: as air in the building is heated it expands and rises, so colder air is pulled in to replace it. An open chimney provides uncontrolled stack ventilation. Solar chimneys can be used to increase this effect, warming the air in the chimney and making it rise faster.

Wind-driven ventilation harnesses the wind to provide fresh air in the house and to help draw used air out of the house.

Benefits

  • There aren’t any moving parts or filters so there is little maintenance or energy required, and it operates silently.
  • The rate of air exchange can be controlled by opening or closing vents.
  • Airtightness is not so important for passive stack ventilation because you need to replace the air that is removed from the house. Note: trickle vents alone can’t to do this due to the small air flow they allow.
  • If you have a passive ventilation system you can still open windows.
  • Some ventilation systems respond to moisture automatically: the vents will open or close to boost the passive stack effect.

Disadvantages

  • As passive ventilation doesn’t have any motors to help move the air around, the ducts that are required are very large.
  • To make the most of the stack effect the ducts also need to be near vertical, which can restrict the design of a house.
  • As stack and wind-driven ventilation are dependent on natural forces, the performance is variable. A cloudy, windless day will reduce the efficiency of passive ventilation systems.

2. Mechanical Ventilation

There are two types of whole-house mechanical ventilation: mechanical extract ventilation (MEV) and mechanical ventilation with heat recovery (MVHR). These systems extract air from the wet areas in the house but rather than the waste air going straight outside it is taken to a central unit. In MVHR systems, the heat of the extracted air is used to warm the incoming air in this central unit.

Centralised mechanical ventilation is a must in houses with a high degree of airtightness (such as those that meet the Passivhaus standard) as it’s needed to maintain a healthy environment.

Benefits

  • These systems work continuously and can be boosted when needed.
  • Good airtightness and a mechanical ventilation system in a new-build will help it to meet building regulations.
  • MVHR systems can also help save energy and money by retaining heat that would otherwise be lost.  
  • Open windows will have no impact on the effectiveness of mechanical ventilation.

Disadvantages

  • MEV only works well in houses with an airtightness of 5m3/m2/hr or less @ 50Pa and MVHR only works in a house with an airtightness of 3m3/m2/hr or less @ 50Pa.
  • The ducts are around 100 – 150mm in diameter so they are difficult to hide if retrofitting.
  • A badly designed and installed mechanical ventilation system could have problems associated with noise, pollution and poor air exchange.
  • Systems designed for colder climates have an electric heating element to pre-heat incoming air in the depths of winter - an extra cost if you don't disable it.

Points to note

  • Mechanical ventilation systems can be turned off but this can lead to mould developing in the system. Also, when they’re switched off, there must be another form of ventilation (such as opening windows) to maintain a healthy indoor air quality.
  • Houses with MVHR are at risk of overheating so it’s important that the system has a summer bypass which will allow any overwarm air to be drawn out of the house without returning its heat to the interior.

For more information on heat recovery, look out for our upcoming blog.

You can join free SuperHome Open Days in September 2015 to explore homes with mechanical ventilation with heat recovery and passive ventilation.

Photo: Lisa Kirby

More information about ventilation and draughts on YouGen

Find a ventilation and draught proofing installer.

Related blogs

Why does my home need ventilation and what are the options?

How can ventilation with heat recovery help your home?

Need help with any Jargon?

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

1 comments - read them below or add one

Andy in Hawick

Andy in HawickComment left on: 25 September 2015 at 9:57 pm

We have never had the airtightness of our house measured and although we have done all sorts of draught sealing in our mid-60s council-build end-of-terrace house, I doubt that we have managed to exceed modern building standards for air tightness.

Despite this, our mechanical ventilation with heat recovery is very effective and not only reduces the amount of heating that we need to maintain a comfortable temperature but the level of condensation on the windows is very much reduced.

We have no summer bypass option and on the two or three days a year that overheating is an issue, we open the doors and windows and cool the place down like that.

I am not convinced that the stated levels of airtightness are necessary to benefit from heat recovery ventilation although improving air tightness will clearly enhance the benefits.

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