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Vicki Cowan

Hi Phil

I really recommend you read the research that’s been done by Otago University and Beacon on these systems.

External air doesn’t almost always have a low absolute humidity than internal air .  You’ll find Lisa French’s (now Burroughs) research on this here:  http://beaconpathway.co.nz/further-research/article/testing_ventilation_systems

Essentially because dew forms on the ground overnight – and then evaporates during the day, there’s a massive increase in moisture in external air in the morning – and this air then moves into the roof space.  As a result air the in the roof space during the day is often more humid than that in the house.  And of course it’s much colder in the roof space at night.

What Lisa’s research found was that in many cases positive pressure systems were pushing damp air (sometimes warmer) into the house during the day, and cold air during the evening/night time.  So the systems aren’t drying out the house.  It’s just the speed of the air movement that stops the condensation settling on the windows.  And I’ve been to houses where even this hasn’t been enough.

So there aren’t humidity benefits from positive pressure systems.  There might be condensation reductions because of airflow, and even then not in all cases.

I know Massey University did some research on the systems as well, and did find some benefits in relation to contaminants in air quality.  But they were looking at houses where, for example, the households heated with unflued gas heaters.  Surely I say, just removing the unflued gas heater and installing a decent heater would deliver less air quality contaminants (nitrogen oxides in particular), less moisture (unflued gas heaters deliver about a litre of water per hour into a house) AND a warmer house – and you could put in a reasonable heating system for the price of a positive pressure ventilation system.