Heat recovering extractor fans

[Matty F]

Auckland New Zealand. Av temperature 8 to 23 degrees:

[Jon Fairbairn]

£180 is somewhat past my pain threshold, but it's just about at the point where it's worthwhile doing, but since I want to install the thing in a bathroom I'm clobbered with Part P so have either to pay an electrician something like £200 to install it or £100+ for building control approval and still pay an electrician to test it :-(.

But as far as the price of the units is concerned it's just that so few people currently install them. They're not intrinsically expensive to make, but with only a handful of people thinking it's worth doing, the manufacturers have to charge a lot to make a profit (cover costs of tooling, stocking etc) and can charge an extra margin because the people who do want to do it have little choice. And of course, so long as they're so much more expensive than a simple extractor, the simple extractors are what people go for. So a DIY design would be a good thing for the people who can d-i-y (but not so good for people who can't and want the price of units to come down)

Given the building regs requirement for mechanical ventilation in bathrooms, I reckon the best way to kick them into high volume production would be to change the regs so they reqiure (some form of) HRV in bathrooms for new build.

The other thing that bugs me is having to have a socking great hole to the outside when there's a gas cooker in a room. Sure, it's really important that there's fresh air coming in for such things, but providing that through a heat exchanger would make a huge difference to the comfort levels of a flat in Scotland. AFAIK, that's not allowed, but maybe I should ask that question in a separate thread?

[Jon Fairbairn]

It would be useful to do some experiments. My worry with your design is that the frictional losses would mean that the fan would have to work very hard. I suspect that the commercial designs just have a single straightish path with plates on top of one another.

Here's a rather hurried drawing:

So you'd have a pile of flat rectangular plates separated by gaskets alternating between the red dashed and the blue solid pattern. The thing would have to be longer than the proportions implied by the drawing, I think. You could make it so the cross-section was the same size as computer case fans (or some other readily available size of fan with a rectangular mounting). Since it's nearly a straight through path, you could just stick the whole assembly (bar the fans!) in soapy water to wash it out.

I think that's true, if you consider the Vent-Axia HR25's power consumption.

I was thinking of rectangular cross-section aluminium rod with thin rubber sheet top and bottom to make the seal, but I don't know what the optimum distance between the heat-exchanger plates would be, so maybe the thickness of the rubber sheet would make enough gap (see

for possible sheet). Or maybe use ordinary epdm rubber draught strip (M-seal) to make the gaskets -- that would be an easy experiment, but would require the plates to be quite stiff.

[John Rumm]

If your council is playing part P by the rules then they should not expect you to pay for any testing. That is what the building notice fee is for!

[Jon Fairbairn]

Thanks for your response; that's interesting. To quote from their reply to my enquiry:

if the works cost below £2000 then the fixed fee will be £100 plus vat [...] you will need to add to this the cost of employing a qualified electrician to test when works completed

are they not playing by the rules, then?

[Tim S]

Jon Fairbairn coughed up some electrons that declared:

No

[John Rumm]

No, go quote chapter 1.26 at them and see what they say.

quote:

"1.26 The building control body may choose to carry out the inspection and testing itself, or to contract out some or all of the work to a specialist body which will then carry out the work on its behalf. Building control bodies will carry out the necessary inspection and testing at their expense, not at the householders' expense."

(no guarantee you will get a result - but some people have...)