We have had a system installed a month ago. It is the Cleanaire system from Negawatt. My husband did a lot of very tedious research and decided that this was the best one. It doesn't use air from your roof cavity, but rather fresh air from outside. It takes in fresh air into a heat exchange box in you ceiling cavity, and takes exhaust heated air from the living room and bathroom, exchanges the heat in the heat exchanger, and then pumps the slightly warmed fresh air into the bedrooms and living room. It is designed for a house up to 300sqm. We have a flued gas fireplace in our living room and electric radiators under the windows in the bedrooms. From the moment we had the system installed we no longer have any condensation on our windows or walls (we had it bad too - in the mornings we would have a pool of water on some window sills so big it would start dripping onto the carpet), the house smells fresher, and the humidity is much lower (our baby monitor displays humidity), and we have our heaters on a lower temperature now. One of the reasons we wanted a fresh air system, apart from the fact that many of the companies who sell this sort of system are moving towards fresh air systems (DVS have one too now but it was too large for our ceiling cavity, and I think Moisture Master might have one too), is that there is a lot of literature about the crud that you have up in your ceiling - fibres from pink batts, mouse urine, dead spiders, etc etc and that the filters don't necessarily filter everything. Systems that take air from your roof space are also illegal in many countries because of fire regulations. And don't be fooled by the initials 'HRV'. Just because the company is called HRV, which I suppose is meant to be 'Heat Recovery Ventilation' they don't sell a heat recovery system.