Firstly may I say how pleased I am to see a blog entry on HRV. I do think, though, that Tim is too pessimistic.

We have a 1959 house which we have recently extended with three extra rooms. We did not want to put various vents (e.g. trickle vents) in the new part so that has been well sealed. As a result we opted for an HRV system and decided to do both the new and old (less well sealed) parts of the house. The company we purchased it from had an excellent 3D drawing package and even though they used 150mm rigid steel pipes you would be hard put to find them. They go down through fitted wardrobes and along between the joists. One even goes under the bath to get to the downstairs cloak room. So retrofitting was not a real problem provided it is given enough thought.

We have tried to draught proof the older part of the house as well as we can, particularly the suspended floors. We can't do much about the working fireplace though, so in no way are we fully airtight. However it all seems to work very well. Subjectively the house feels airier (you can certainly tell when the HRV is switched off) and the house feels of a more even temperature. The system works well transferring excess heat from the kitchen and shower rooms into the other rooms of the house. This evenness of temperature is not something the sellers of these systems major on but it is certainly something we have noticed (and we have been living here for 30 years)

As to efficiency it is difficult to tell. Again subjectively, with it -4C outside we were seeing only a 3C drop between the extract and supply air temperatures. e.g house temp 20, inlet fresh air 17.2.

Of course Tim might argue we are spending too much on the heating because of the lack if airtightness. I wish I could be specific but we had a new boiler, solar hot water, a thermal store and wet underfloor heating in the new parts at the same time. All I can say is that our gas consumption for heating, averaged over a whole year is just 45% of what it was before, and we have a rather bigger house to heat, and last winter was very cold too!

I also agree with Kate de's comments too.

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While the reduction in heating bills may not be spectacular (will leave that calculation to the experts) it is worth noting that MVHR allows you to increase ventilation rates (to what they should be) without losing heat or suffering draughts.

There is a lot of underventilation in UK homes in winter, and this can have worrying health implications - even in well-insulated homes - because of high humidity levels. The ventilation unit can also filter out pollen & air pollution, and allows fresh air without outdoors noise. So the decision is not just about energy, also comfort and health.

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Interesting.

 What about the single room heat recovery replacements for extractors - like the Vent-Axia HR-25 type units?(I'm sure there are others like it...)

 Can a bathroom/wet room be well enough sealed to make using one worthwhile?

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