The shit misses the fan

Steve from N.Ireland writes:

I'm considering installing a positivive input ventilation system like Nuaire’s Drimaster 2000? Any feedback?


Mark reckons:

Of all the varieties of ventilation systems you can fix into a building, this one has to be the worst.

Despite the rumours, positive input ventilation (PIV) like Nuaire's Drimaster (diagram above) probably won't be overlooked by the revised Part F of the E&W building regs, if only because there are a number of cases where it has been shown to work well. Usually these are on social housing schemes with condensation problems.

However, a new(ish) well-insulated house really shouldn't suffer from condensation problems. At normal relative humidity levels, the dew point is down at around 12°C and condensation won't occur at temps above this. So in this respect, it's answering a non-existent problem.

The only place where the temperature may fall below 12°C is within the roof or the external walls, so it's a good building principle to not encourage internal, moist air into these spaces. Which is exactly what PIV does.

The other major quibble with these single outlet fans (and this goes for the negative pressure fans as well) is that with just one inlet/outlet point, there is ridiculously little control over just where the air is blown or drawn from. Although the diagrams show the air gently wafting all through the house, in reality it's going to take the path of least resistance, which may be straight to or from the nearest trickle vent or badly-fitted window. The larger the house, the worse this effect will be: expect 90% of the house to remain completely unventilated.

Just compare this crude device with the heater/cooler in your car where you frequently have up to six outlets, some of them directional. I think the standard set-up of trickle vents and the odd extractor fan is preferable: they give you more control and are much more adaptable.