Positive Input Ventilation

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

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 . . .

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.

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