I bought a house, it's granite, detached, built around 1870. In the
attic there's a strange cube hanging from a rafter on bungee cords.
It's <0.5m cubed with a flexible tube about 200mm diameter going to it.
The other end of the tube connects to what I had thought was a light
fitting on the landing but turns out to be a vent. The cube is covered
on five sides with some sort of thick fabric. It's difficult to get to,
so my description is a bit approximate.

The cube is plugged into the mains.

I took a photo and found out that it was a PIV system. It sucks air in
through a filter and blows it out through a ceiling vent. Apparently
good for reducing condensation and purging Radon.

The question is, how hard should it blow? There's definitely some
airflow from the vent, but hardly a gale. Having never seen or heard of
such a thing before, I have no idea if it's working as it should.

Any clue?

--
SS

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Simon Simple <SS@nonononono.co.uk> wrote:

I bought a house, it's granite, detached, built around 1870. In the
attic there's a strange cube hanging from a rafter on bungee cords.
It's <0.5m cubed with a flexible tube about 200mm diameter going to it.
The other end of the tube connects to what I had thought was a light
fitting on the landing but turns out to be a vent. The cube is covered
on five sides with some sort of thick fabric. It's difficult to get to,
so my description is a bit approximate.

The cube is plugged into the mains.

I took a photo and found out that it was a PIV system. It sucks air in through a filter and blows it out through a ceiling vent. Apparently
good for reducing condensation and purging Radon.

The question is, how hard should it blow? There's definitely some
airflow from the vent, but hardly a gale. Having never seen or heard of
such a thing before, I have no idea if it's working as it should.

I think the idea is to create a small positive pressure in the house, so
that moisture, radon, smells etc that might otherwise build up is pushed out through any gaps and vents there might be, replaced by filtered, possibly warmed, fresh air.

As to 'how much' I don't know, but I don't think it needs to be a howling
gale - with a small constant pressure it'll create a certain number of air changes per hour (ACH). If you know the flow rate of the unit you can calculate the volume of the house and thus the ACH figure and compare with recommendations.

Theo

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On 20 Jul 2025 at 22:55:09 BST, Theo wrote:

Simon Simple <SS@nonononono.co.uk> wrote:

I bought a house, it's granite, detached, built around 1870. In the
attic there's a strange cube hanging from a rafter on bungee cords.
It's <0.5m cubed with a flexible tube about 200mm diameter going to it.> The >> other end of the tube connects to what I had thought was a light
fitting on the landing but turns out to be a vent. The cube is covered
on five sides with some sort of thick fabric. It's difficult to get to,
so my description is a bit approximate.

The cube is plugged into the mains.

I took a photo and found out that it was a PIV system. It sucks air in
through a filter and blows it out through a ceiling vent. Apparently
good for reducing condensation and purging Radon.

The question is, how hard should it blow? There's definitely some
airflow from the vent, but hardly a gale. Having never seen or heard of
such a thing before, I have no idea if it's working as it should.

I think the idea is to create a small positive pressure in the house, so
that moisture, radon, smells etc that might otherwise build up is pushed out through any gaps and vents there might be, replaced by filtered, possibly warmed, fresh air.

I'm not sure that sort of system would work with radon - wouldn't it make the chimney effect worse, and draw more 'bad' air through the house?

I can see how it might work with condensation, though, especially if flow is encouraged by, for example, venting openings and leaving gaps at the bottom of internal doors. Although IIUC it wouldn't be very efficient in winter as it's purging warm air along with the nasties . . .

--
Cheers, Rob, Sheffield UK

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RJH <patchmoney@gmx.com> wrote:

On 20 Jul 2025 at 22:55:09 BST, Theo wrote:

I think the idea is to create a small positive pressure in the house, so that moisture, radon, smells etc that might otherwise build up is pushed out
through any gaps and vents there might be, replaced by filtered, possibly warmed, fresh air.

I'm not sure that sort of system would work with radon - wouldn't it make
the chimney effect worse, and draw more 'bad' air through the house?

I think if you have radon problems you have installed vents in cellars etc where it might collect. The PIV is intended so there's some pressure pulling air into the loft and pushing it out of the vents. ie you're pushing through fresh air which will pick up some radon on its way out.

I can see how it might work with condensation, though, especially if flow
is encouraged by, for example, venting openings and leaving gaps at the bottom of internal doors. Although IIUC it wouldn't be very efficient in winter as it's purging warm air along with the nasties . . .

Yes, unlike MVHR there's no heat exchanger so it pulls in cold air (possibly preheats it) and then pushes out warm (moist) air. It's less energy
efficient than a sealed house with MVHR but in an 1870s house it's not air sealed anyway. But it's a bit like cracking a window in the winter - you
get a bit of heat loss but worth it to lose the fug.

Theo

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