Recomend sub floor ventilation fans

[jeffbo01]

Hi there
Can anyone recomend a good subfloor ventilation fan (with constraints below)

We have sandstone foundations extending about 1-1.5 metres above ground with double brick cavity walls above. We only have 1.5 brick course at subfloor level.

We have had Alita type mesh vents, 2 per room installed in the brick course which sits half in the sub floor and half above (behind skirting boards) but they are not working well enough as they are in the two side walls only plus we have no cross ventilation as each room in the house has solid sandstone footings and we cannot punch holes in them.

Therefore the need for fans to get the air moving. Plus the bathroom has a suspended concrete slab so we can't get any vents into the subfloor of this room at all.

The fan-assisted vents Ive seen seem to be too high, needing more than two courses of bricks at sub floor level. Plus do we need to put a fan in every room as this seems to be expensive at $300 per fan. Surely there has to be one mother of a fan that would suck air through the entire subfloor (or is this unrealistic).

I should also mention that at the front of the house (on a sloping block to the back) we are going to dig the outside and line with plastic to stop water getting in from the garden beds. We can't lay agline as we are surrounded by concrete and have nowhere to send the water collected by the agline. Hopefully the plastic will help reduce the amount of damp coming through the foundations in the first place).

Thanks