Inside a Positive Input Ventilation (PIV) unit

This thing is a monster! It's basically a quiet centrifugal fan that pulls air in from your attic via filters, and then blows it down into your home to create a slight positive air pressure that drives existing stale air out through any path it can find. This can solve humidity buildup issues and mould/mold problems in some homes.

For flats and apartments with no attic above, there are wall mounting versions that bring in outside air from a wall.

Although the power of the unit is low (about 10W) it does have an optional air pre-heater for winter that nudges the air temperature up with a 500W heater. That can be programmed to come on below a chosen temperature, and will potentially add a chunk to your electricity bill in winter. But it's worth mentioning that the unit can actually help heat your home at other times of the year by bringing in warm air from the attic that has been heated by sunshine on the roof.

The unit normally operates in two modes. The normal trickle mode and a boost mode when it detects that the air in the attic is warm. It will also shut off if the air in the attic is too hot or if it gets to freezing temperatures.

As with every technology there are pros and cons.

Pros:-
Very low power in normal mode.
Displaces excessively humid air.
Pushes out odours and mold spores.
Can provide heat from sunlight on the roof.
Gives a very fresh "open window" feeling.

Cons:-
In very humid climates it can potentially bring in humid air.
It can increase your heating costs by displacing heated air.
Some claim that it can push humid air into wall cavities where it condenses.
The air preheater function can use power in winter.

I've installed this unit to see what effect it has, and so far I'm liking the freshness it creates. I had experimented with a smaller one I made from a 120mm pressure optimised computer fan and a 3D printed ceiling adaptor, and while it produced noticeable results, this unit definitely pushes more air with a greater pressure.

These things are apparently quite popular here, so I asked on a local forum to see what users thought of them. There were a few that didn't like the cold flow of air in winter, and the extra heating costs. But most seemed enthusiastic about them. I did get the feeling that they don't realise they can have their airflow adjusted on the unit. Possibly not helped by the units being in their attics and therefore not easy to access for changes. It's even possible that the installer has set it too high in the first place.


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