Passive Air Ventilation Systems

Why is there a need when a bathroom fan or opening a window will suffice.

The above comment indicates that an information gap exists as to the benefits and limitations of heat recovery systems.

The old regulations never included air tightness as a requirement and the present regulation of 0.5 ach is sufficient to allow air movement of sufficient quantity to exist.

In many cases the use of extract fans from bathrooms was thought to create air imbalances to draw fresh air inside.

Older houses are woefully leaky and air changes are continuous.

This form of rationale is no longer sustainable as houses become much more air tight.

The main reason to ventilate and reduce air pollutants etc but why the upsurge in interest.

They are several reasons among them

The need for better air quality.

Improving air circulation in the house.

Removing odours and stale air

Reducing air intake through walls etc which reduces condensation and air borne pollutants.

Stabilising and elimination of draughts.

Preventing energy being carried away.

A rarely mentioned side affect of very airtight rooms is the build up of carbon dioxide.

It doesn’t get the same comments as carbon monoxide but is a serious issue with air tight houses and rooms.

Now as you know the production of carbon dioxide is a by product of breathing.

The amount in fresh air is about 370ppm (parts per million).

If a person sleeps in a room with a build up of carbon dioxide to say 4000ppm then they would wake up feeling tired or listless.

The permitted limit of 5000ppm over an eight hour period OHSA is the maximum permitted limit and exposure to higher limits of 10,000ppm to 20,000 will give rise to headaches mental capacity reductions anxiety and in the most severe exposure death.

Sleeping in an airtight room without adequate ventilation is a cause of sick building syndrome.

In Passive houses the need for a properly designed system is essential to ensure that the fresh air intake and stale air extraction is balanced and adequate to meet the household needs.

Firstly the system must be capable to service the need of the household interns of air intake and extract.

This will depend on the air tightness of the house.

The Passive standard of 0.6ach is a very air tight house.

The UK and Irish standard is much more lenient and the use of heat recovery systems in such houses is doable but will be very inefficient.

The size of the unit should be sufficient to meet the needs of the house and should be sized accordingly.

The use of over sized units is both inefficient and gives rise to poor balance within the house.

The design of the system is vital to ensure that it performs properly.

Poor ducting runs small ducting will result in noise whistling and poor performance.

The duct runs should be thought through at day one before any joists are placed.

Indeed the position of joists will be determined by the ducting runs.

Where ducting conflicts it is good practise to make sure the client is informed at the start and not present a fait acompli at the end as it leads to problems.

Once the runs are agreed the type and design of the ducting is important.

Cheaper ducting may fail in time with performance losses.

The insulation of the ducting is crucial to maintain heat recovery.

Does every room need extract and intake?

No the answer is that the extract areas are kitchens wet areas etc the intake or supply is bedrooms and living areas.

The balancing of the system will distribute the air evenly.

So what happens in Bathrooms and Kitchens?

The bathroom and kitchen air is extracted and moves through the air exchange where any hot airborne air heats the incoming fresh air.

Are air Filters included within the system?Yes the use of filters to reduce or eliminate dust or pollen is essential to improve the incoming air quality.

Filters can vary in performance but generally must meet industry standards.

Better filter can give industrial clean room standards.

Thirdly the position of the heat recovery unit should be accessible and in an area that can be attenuated.

The position of the unit must be located to ensure that the intake or extract are not compromised.

The location should in an area that is away from living or sleeping areas.

The noise levels of the unit should be no more than 25 decibels.

Further noise attenuation by seating the unit on rubber bearings etc should be used.