mhrv is far better than trickle vents -- all major fan companies do boxes with fans in them for whole house systems.

not difficult to build in on a diy basis

also specialist larger more efficient units are available too.

Some people don't like the SAP ratings but they are the only objective measure of efficiency that I can find. They can be viewed here http://www.sap-appendixq.org.uk/search.jsp.

I have done quite a lot of investigation into MHRV boxes recently for my own house and I have come to the conclusion that it's hard to see why some of the more expensive boxes cost as much as they do. At the cheaper end, companies like Vortice Prometeo and Vent Axia seem to produce boxes that have the same (or better) efficiencies than some of the more well known MHRV names and they cost half the price (or less). I hope to start building next spring and the Prometeo box is top of my list. It costs about £1100. If you have a dig round the forum then you will find at least one person who has this box installed and seems happy with it. Another one that might be worth looking at is the Vent Axia HRE350 (about £1400).

Most of the companies that I have spoken to have told me that installation is a job that a competent DIYer can do.

By the time you allow for a fan unit of reasonable efficiency, outlets and duct work, you're unlikely to find anything below £1500.

The ductwork is bulky and requires big cut outs and allowances for bends, so allow at least five man-days to fit it, or ten if you've not done it before. It is a DIY job, but you need good access and to plan ahead a little for the various joints and links you'll need.

It will be very much easier if you allow for duct runs and so on at the design phase. Consider where ducts go between floors, direction of joists and the optimal location of the fan unit (they are big, and the inlet/outlets are never in the position you need them. The unit does hum and vibrate a little (think quiet air-con), so be careful where you locate it to minimise noise transmission. The outlets themselves don't seem to be noisy.

Some of the suppliers do a design service to work out the best layout for your house, but it's best if you approach it iteratively - get a clean layout, adjust your house design where it can make duct runs cleaner or simpler, and then update the duct layout to take advantage. It doesn't seem to be terribly scientific though - there are better and worse layouts, but not an absolutely 'right' layout for a given house.

But if I remember rightly, another £600+ for the fitting kit (I am sure I have seen that on SF site before but can't see the link just now).

James, the Ventaxia for £640 from Screwfix only has a 60% heat recovery efficiency. No summer bypass and no frost-protection (as far as I can see).