Brilliant video! It was great to see just how simple a construction they can be, too :)

Neatly done, ST, and presumably not that expensive, either.

I wonder whether using thin aluminium plates would give better performance?

My local metal factor will guillotine sheet to size for a modest charge and a 2.5 m x 1.2 m sheet of 1 mm thick 1050 is around £22 +VAT. I believe they can get anodized sheet too, which might be slightly preferable. I'm guessing that your heat exchanger sheets may be around 1.25 m wide by 0.4 m or thereabouts, so that would mean getting about 6 elements per sheet, or around £100 or so for enough for an 24 sheet heat exchanger made up from 1.25 m x 0.416 m plates.

I've just checked an online supplier (Aluminium Warehouse) and they can supply sheets of 0.9 mm thick 1050, cut to size (1.25 m x 0.416 m) for £7.37 +VAT each, so a fair bit more expensive than my local place but still probably within the bounds of affordability when compared with the cost of a read-built MVHR unit, and available anywhere in the UK.

Well done ST, DIY rules!!!!! A quick question, If I build a passive house can I make my own MVHR or will building regs require some stamp of approval?

Nice work. What sort of an application is this in? You said it was an intermittent bathroom fan? Controlled by what? Do you have a target efficiency that you need to hit to make it worthwhile, or is it a case of needing the ventilation anyway, so any heat recovery is a bonus? Is it just the one fan in the system?

Well done ST, are you including the heat generated from the fans in your efficiency calculations? A lot of HRV companies do this to enhance their results!
So yours is a horizontal flat plate heat exchanger http://en.wikipedia.org/wiki/File:Heat_exchanger.jpg
You're probably aware that with two airstreams on both sides of a heat-conducting plate, a boundary layer will build up, with lower velocity. http://en.wikipedia.org/wiki/Boundary_layer
This means that the airflow close to the plate is much slower, and can transfer much less heat. Only the leading edge of the fin works well. That's why high-power heat exchangers (like the 100kW radiator in your car) are only a few centimeters thick, or have a water tube every 5 mm, or both. What you want is mixing the air every millimeter or so. You do this by pressing peaks and troughs in the heat exchanger plates, thereby increasing the pressure drop quite a bit. This increases the heat released by the fans, giving the heat exchanger an even better fake efficiency!

How long do i buttons stay working for? could I bury them into a wall?

Thank you all.

Now see if I can answer a few questions.

The 8 by 4 sheets of 1mm PVC cost cost £16 each, fans were £16 each as well, the sheet I used for the seals was that stuff that signwriters/modelmakers use. Has a plastic skin with foam in the middle, think it was £12, glue was about a fiver or less, polystyrene sheets (4 off) where £3 each. Bits a pieces will bring it up to £100ish.

As the fans are inline, any heat from them is retuned to the house, the outlet one directly and the inlet one indirectly, via the heat exchanger, I did not monitor the energy they used though. I intend to in time, needs a bit of rewiring. Next weekend I shall go and get a couple more fans, and another sheet of PVC to make a few more plates (till the box is filled up) and measure the performance a bit more accurately.

I was aware that there is a velocity difference as the air approaches the plate, but then there is also turbulence. Also any air that is running slower has more time to transfer energy, and I was not after ultimate performance, rather a cheap method.

The plates are .75m by .3m, so that is actually 4.05m^2, but only half the area is on the warm side, so 2m^2 of area. The fans can shift 85 litre a minute each, but suspect that they only shift half of this at best.

Two advantages of using plastic is that it is easy to cut/glue, can be washed in the bath when it is mucky, and does not rust or need painting. The very thinnest aluminium that is rigid enough would be better, but harder to DIY.

I sized the box to fit between the roof joists, where it can drain any condensate down the hot outlet pipe and act as some extra insulation as long as an airgap is left.

MKIII, if I ever get to make it, will be similar but with a better box, better fans (need to find larger ones that are cheap) as this is just using what I had kicking about or already fitted. I intend to make them so that they can nest together, therefore increasing surface area in a modular fashion. Not sure how much the Ventaxia one is, around £400-£600 before fitting, as a good third of my price was the 2 fans (bought retail), for a similar amount of money I should be able to make something with ten times the area that can do the whole house.
Should be able to calculate a decent exchanger area to total house surface area area in time. If it is 1:1, then I could do it with 4 of these with some more plates in each.

iButtons can be set to log every 255 minutes, or about a year. They do some with a larger memory, go to www.homechip.co.uk and look around.

Now a question for you lot.
At the moment it is just exiting into the loft, if I exit it to the top of the airing cupboard, an area that is warm anyway, put a vent at the bottom of the airing cupboard door, that should waft warm air onto my landing and into the bedrooms, easy to do as well, one 4 inch hole in ceiling in a place that is not seen. What do you think?

I have built a flat plate heat exchanger basically using 4mm polystyrene wall paper cut up into strips as spacers, baco-foil (painted black) as my plates. Its 1m long (why not? ;-) ) times 300mm wide and is 22 plates thick. I used spacers inside each channel to put in a bit of a labyrinth - leaving a drain channel along the bottom. the whole kit is set at at slight angle so its self draining onto a PVC sheet which terminates at the air outlet, so the whole thing should drip out of the wall. There are two computer fans pushing air into the H/Ex for max turbelance. trunking hums a bit so one day I'll put in some glass fibre to reduce this as well as uprating the computer fans.
In general I am concerned about the air distribution from the air ducts to ALL of the plates, this is where I think corrax et al may fall down.
One day when I find my electronic thermometer I'll do some tests, in the mean time I am after a 30 minute electronic timer to run it automatically after a shower, we do it manually at the moment.
Frank

Chuckey
I tried the ali foil approach, but being clumsiness, all i managed to do was make holes in it. I still think there is milage in it though. Just need to think up a way to make a rigid enough frame. I found the same when using computer fans, they do not seem powerful enough.
A bathroom fan has a timer in it and can be set for about half an hour. Mine are the Manrose ones, got them from Trago.

Joiner
Pump the heat back into the room it came from, heat is heat, always goes from hotter to colder, so you need to put it somewhere hot rather than cold and a kitchen in use tends to be warm.
Made a mistake earlier, the fans move 85 m^2 and hour, 23 litres per second, or in Roman Catholic, 100 kg/hour, 28grams/second

Joe90
I wondered that, but as there will be a bit of pressure difference, and very little resistance, I think it will waft, rather than gush out. Only one way to find out really, worse that happens is that I have a grill at the bottom of the door that wil look like it shoudl have always been there.

Just started to do another test on it with a fan heater in the bathroom and the unit running for an hour or so. In for some busy nights for rest of week so results may have to wait till Saturday (unless I get up in loft at 5:30am.

Posted By: SteamyTeaChuckey
I tried the ali foil approach, but being clumsiness, all i managed to do was make holes in it. I still think there is milage in it though. Just need to think up a way to make a rigid enough frame.

Something like YBS Foil-Tec is a lot tougher than "kitchen foil" - but I think the smallest size is a 50m x 1m wide roll, so unless you have other uses...

Posted By: joe90If I build a passive house can I make my own MVHR or will building regs require some stamp of approval?

Looks like in my case in Scotland, at least, a commercial MHRV will be needed.

Basically, i could set the design airtightness a lot lower (more leaky) so that MHRV wasn't required but that would mean a lot of hassle getting my SAP calculations re-done and might cause problems as, though I'm comfortably in as far as emissions are concerned, there's a certain amount of balancing going on between supposed heating requirements and the outputs of my various solar panels.

Note, it's the design airtightness that matters. I could say I want a design airtightness of 15 m³/(m²·h) (or less ??? - I think he might have meant 5 m/h at 50 Pa) so MHRV is not required. If I then “accidentally” actually built with a leakage of say 1 m/h that'd be OK - mechanical ventilation would still not be required so I could do my own.

The moral of this tale is think about your design leakage early on.

Have a 'secret hole' somewhere.
I am still convinced that for every rule that says something is compulsory, there is another that says it is illegal.

Posted By: DarylPIn England and Wales new Part L will allow you to design with no need for an air-tightness test, if you design for air permeability 15m³/m²h. This is very leaky!

Quite shocked by this, does it apply to houses or just commercial buildings? OTOH if air testing is as slap dash as I have frustatingly experienced - MVHR not sealed, door blower fan not sealed in door - then it is all questionable anyway.

But to get back to Ed's question

Note, it's the design airtightness that matters.

In England anyway it is the ventilation implementation that matters to building control e.g trickle vents on windows, extractor fans in bathrooms etc. If you don't do these then you have to use MVHR instead, but I don't remember any requirements for permeability other than the 10m³/m²h tested limit.

My BC was confused about who could test/commission my MVHR system. In the end the plumber, that installed the ducts, hired a certified air speed meter did the testing and filled out the downloadable form. No special qualifications, and I guess that as long as it was a business that did it (not self certified) and all the number were filled in BC are content.