Heat recovery ventilation. Interesting design.

[TwoHorsePower]

stephendv, this is very interesting, thanks.

how are you controlling the speed of your motors? are you implementing CO2 and/or humidity sensors? what power of fans are you experimenting with?

do tell us more!

[SpeedEvil]

Indeed it's interesting - 180e from where'd be my next question.

[stephendv]

I stuck 2 of these fans in it: http://www.xigmatek.com/product.php?productid=79 just because they happened to be the best that PC world had in stock.  Don't remember the power rating, think it's under 2W per fan.  I may upgrade to these instead: http://www.pccomponentes.com/noctua_nf_s12b_flx_1200_120x120_1200rpm.html  less noise and more airflow.

There's no control of speed and they're on constantly. I've built the house quite airtight, so the reason for the MVHR is to provide fresh air, humidity control is just a happy side effect

The heat exchanger was shipping from viking-house.ie in Ireland.  Getting the box to make a good and proper seal was not easy, the original idea was that I'd vent the composting toilet out through the MVHR too, but after doing some tests by blowing cigarette smoke into the extract duct  I could smell the smoke being blown in to the input part, even after sealing with silicon.  So I gave up and vented the toilet out through it's own fan + dedicated pipe, so will be losing that heat and now have an unbalanced system   For now, have turned off the MVHR extract fan and am only using the intake fan.  Next step is to either slow down the extract fan or stick another intake fan inline to increase pressure. 

I'm typing this while surrounded by packing boxes and the knowledge that our drainage system is yet to be done... so will update when we've finally moved in and the dust has settled 

[Loir]

The difficulty of stretching multiple layers of foil was the exact issue I faced with foil.
Yes, it can be done, but it's not trivial to get right.
With the plastic, as it has some rigidity, so you can get away with simple measures such as silicone sealant blobs and gaskets as spacers.
It's quite easy to make a 4.0mm high spacer simply by running a bead of silicone, and then passing over it with a knife and a couple of blocks.
It is possible to make a heat exchanger block in a couple of hours on the kitchen table that will work, and not require external support, and is a block ready to go into insulation.
Foil would be considerably more annoying.

I bought a home ventilation system from Wickes about 20 years ago. The heat exchange and fan box is galvanised steel. The actual heat exchanger is dual pass and uses the twin walled polypropylene sheet (black but I don't know colour makes much difference). The design is as the Science Design paper. The separator between the sheets appears to be the foam strip that is sold as draught excluder.

The heat exchanger block is about 25 x 25 x 50 cms. Cool inlet air travels end to end of the block in the holes in the twin walled polypropylene sheets. The warm outlet air travels across in the gaps between the sheets twice, once near the outlet and back again near the cold inlet.

I have had to replace the fans but otherwise it is working well.

Loir

[SimonHobson]

Interesting thread. And I used to know a source where I could have as much of that sheet as I wanted - a signmaker I used to have as a customer many years ago got loads of it as packing for his raw materials. But that's years ago.

On construction, a thought that occurs to me is that you could do something like this ...

For the flow inside the sheets. Seal across the ends between the sheets, and then just box the end of the whole stack. The seals between the sheets restrict the flow to only down the internal channels.
That leaves the inter-sheet spaces sealed. When building, stop the seals a few inches short of the end - one end at one side, the other end at the other. Now form a box around the whole stack on these sections. The cell structure of the sheets will restrict the flow to between the sheets, and venting at opposite corners will force the flow to cross the sheets rather than possibly just flow down one side of the stack.
I think there are many things you could make the seals from. Someone has already mentioned silicone sealant, and did someone mention the foam draught excluder strip ? That should be reasonably strong over the sort of area involved here. Bear in mind that at least some of the seals will need to be waterproof.

And then for the connections, at the sides, this sort of thing might make a useful starting point
http://www.naplesuk.com/index/view/productDetail/category/223/product/208/Duct6-Round-to-rectangular-adaptor_white.htm
especially if you size the stack to suit direct attachment.

[eabadger]

Could you use the sheet off old conservatories? cut to sections say 2x2 then stacked, I have some I will experiment.

Here in france our diy shop has whole house ventilation systems from €50 or as they describe a double flux (heat recovery) unit for €399, maybe the cheeper version and some old plastic multilayered roofing sheet will be ok?

steve

[SimonHobson]

I doubt the material is critical - if it's reasonably cheap then it's no problem to make it larger in order to get the heat transfer (assuming you have space in the attic for it).

[roderickw]

I'm very late to the thread but I had similar thoughts a while ago and the aspect I am interested in is how to get the airflows into the DIY HX material.  I put together a design and home 3d printed an adapter to go on the end of a number of layers of sheet material, where each sheet contains a number of parallel channels.  The design and discussions are at:

http://www.thingiverse.com/thing:14889

The design is parameterised so can be adjusted to different HX sheet material dimensions, also the adapter is able to interleave the airflows so that the flow and return can be in adjacent channels in the same sheet.  As I understand it, this is the approach used by some of the the PH certified HXs.

Since working on the design I talked to an MVHR distributor who has seen the production of German plastic heat exchanges and he said the method they use is to assemble a jig full of sheet material and then laser weld the edges.  I don't know if the typical laser cutter at a hackerspace would be able to do this.  After hearing this I thought of using sheet Polyprop (0.2mm thick PP, http://www.saa.co.uk/artsupplies/yupo-painting-paper-25-loose-sheets-a4-110gsm-296-x-210mm-980890.html?oscsid=cqci9t6k39saapdj11h4fc9sd5) and using a computer controlled blade to create score lines on the material so it would bend into regular channels which could be folded closed at the edges and held in position by the adapter.  Using A4 sheets would result in a short HX but would probably be good enough for maintaining heat while removing humidity in a greenhouse in winter. 

The calcs for HX efficiency using different materials is at:
http://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html
PP is 98% the efficiency of metal.

Thingiverse also contains other useful parts for an MVHR:

fan adapters/mounts: http://www.thingiverse.com/thing:8774
radial fan blades: http://www.thingiverse.com/thing:6113

I was looking at using a WiNode or Nanode for the fan control and temp sensor monitoring.

I haven't done anything on this recently but it seems the sort of thing that there should be a public domain design for and as 3d printers become mainstream its should be possible to replicate reliably.