PeterW Posted June 28, 2018 Share Posted June 28, 2018 Can’t have inlet and outlet in the same room - you would get no flow. 15mm copper wouldn’t work - too thick at 0.9mm wall You could always just get on with tiling .... ? Link to comment Share on other sites More sharing options...
jack Posted June 28, 2018 Share Posted June 28, 2018 47 minutes ago, PeterW said: So a structure of drinking straws inside I now get, but how you would support and seal that whilst still maintaining flow would be a challenge. I’ve built a couple of contraflow copper heat exchangers and they are hard enough as you can pressure test them to find leaks ..! There's not a huge pressure difference in this situation, and it's air/air, so I don't think perfect sealing is required. A small amount of leakage probably wouldn't hurt anyway, as there isn't a big temperature difference between any pair of points on the adjacent paths in a counterflow heat exchanger (for example, as I understand it, a 90% efficient exchanger when it's 0 deg C outside and 20 deg C inside will only have at most a 2 deg C temperature difference at any particular point. That's why you need so much surface area and reasonably good conduction for the tubes). You can seal the "straws" to the plates they pass through by pouring in a sealant (maybe use a temporary collar, or better yet surround it with a seal that will fit into the pipe you're using). 15mm pipe feels a little big and thick. Ideally you'd use a larger number of smaller, thinner-walled tubes. Aluminium might work (it's less conductive than copper). Plastic might work (you want it very thin, like drinking straw thin). All complete and utter speculation on my part, and again I'm definitely not saying this is something you should do! Link to comment Share on other sites More sharing options...
jack Posted June 28, 2018 Share Posted June 28, 2018 (edited) 18 minutes ago, PeterW said: Can’t have inlet and outlet in the same room - you would get no flow. There are commercial (domestic) MVHR extractors that do this. Some work on regeneration (single fan that changes direction periodically, with a heat absorbing labyrinth in the single path), but I believe there are others that just blow the incoming air in a direction away from the outgoing. Not ideal, but it'll work to an extent. Edited June 28, 2018 by jack clarified "commercial" didn't mean for commercial application Link to comment Share on other sites More sharing options...
PeterW Posted June 28, 2018 Share Posted June 28, 2018 Just now, jack said: There are commercial MVHR extractors that do this. Some work on regeneration (single fan that changes direction periodically, with a heat absorbing labyrinth in the single path), but I believe there are others that just blow the incoming air in a direction away from the outgoing. Not ideal, but it'll work to an extent. Yep probably should have said in the ceiling as the flow would be one to the other unless it was directed properly. I’d (theoretically) put the inlet on the outside of the bathroom door and extract above the shower and then the flow is near perfect for taking warm moist air away. 2 Link to comment Share on other sites More sharing options...
Onoff Posted June 28, 2018 Author Share Posted June 28, 2018 (edited) As a bit of a hybrid design could a long, thin stack of hit & miss Coroplast sheets replace the drinking straws then just seal around the rectangular hole where the stack passes thru the plates at the end? Edited June 28, 2018 by Onoff Link to comment Share on other sites More sharing options...
jack Posted June 28, 2018 Share Posted June 28, 2018 12 minutes ago, Onoff said: As a bit of a hybrid design could a long, thin stack of hit & miss Coroplast sheets replace the drinking straws then just seal around the rectangular hole where the stack passes thru the plates at the end? I suppose so. Certainly has the advantage of simplicity, even if the plastic in Coroplast is perhaps a bit thick. Link to comment Share on other sites More sharing options...
Onoff Posted June 28, 2018 Author Share Posted June 28, 2018 43 minutes ago, PeterW said: Yep probably should have said in the ceiling as the flow would be one to the other unless it was directed properly. I’d (theoretically) put the inlet on the outside of the bathroom door and extract above the shower and then the flow is near perfect for taking warm moist air away. Thanks. So the extract takes in moist warm air and displaces with air drawn from under the bathroom door? 19 minutes ago, jack said: I suppose so. Certainly has the advantage of simplicity, even if the plastic in Coroplast is perhaps a bit thick. Right then, onto fan size. Do I just get into Part F to size the extract as normal as in the lpm requirements? Would it need to be bigger to account for losses and restrictions through the exchanger? I guess I'd also want to take my diy Geberit pan extract pipe to this? Otherwise I'd be losing the heat from that warm, moist pan air! Link to comment Share on other sites More sharing options...
Onoff Posted June 28, 2018 Author Share Posted June 28, 2018 1 hour ago, PeterW said: You could always just get on with tiling .... ? Anything to delay finishing and having to talk to SWMBO! More loft time this way plus it's FUN! Link to comment Share on other sites More sharing options...
ProDave Posted June 28, 2018 Share Posted June 28, 2018 1 hour ago, Onoff said: Anything to delay finishing and having to talk to SWMBO! More loft time this way plus it's FUN! I would actively be avoiding loft time in this weather. I guess you like it hot? Link to comment Share on other sites More sharing options...
Onoff Posted June 28, 2018 Author Share Posted June 28, 2018 Searching at random I found thin wall plastic tubes in 4' lengths: http://www.visipak.co.uk/tube_packaging.shtml Link to comment Share on other sites More sharing options...
Onoff Posted July 1, 2018 Author Share Posted July 1, 2018 (edited) I've got various 110mm soil fittings destined for the tip where the rubber's perished. Thinking they might do for a tubular heat exchanger if I ditched the rubber and just CT1'd them onto the tubes? Is longer better with a tubular heat exchanger? I'm not sure what inlet outlet is which as in what goes inside / outside? Tube Inlet - In the bathroom, takes in warm moist air? Tube Outlet - Vents externally in soffit? Shell Outlet - ? Shell Inlet? Ta. Edited July 1, 2018 by Onoff Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 1, 2018 Share Posted July 1, 2018 I've seen old estate agents signs used as reasonably effective counter flow heat exchangers. Just space layers of them so that all the internal holes run in the same direction, with the gaps between them running in the opposite direction. You end up with a square heat exchanger where the airflow in and out is on each diagonal face. Link to comment Share on other sites More sharing options...
Onoff Posted July 2, 2018 Author Share Posted July 2, 2018 Is there an ideal size on this twinwall polycarb (Coroplast) stuff? I think I can get some 16mm for free. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 2, 2018 Share Posted July 2, 2018 3 minutes ago, Onoff said: Is there an ideal size on this twinwall polycarb (Coroplast) stuff? I think I can get some 16mm for free. Go with what you can get, but I'd say the thinner stuff, with gaps that are around 3mm to 4mm wide might well be best, as that corresponds closely with the sort of spacing in commercial units. Here are some plans for a coroplast MVHR from years ago: https://www.builditsolar.com/Experimental/DIYHRV/DIYHRV.htm Link to comment Share on other sites More sharing options...
jack Posted July 2, 2018 Share Posted July 2, 2018 16 hours ago, Onoff said: Is longer better with a tubular heat exchanger? To a point, but longer also means more resistance. 16 hours ago, Onoff said: I'm not sure what inlet outlet is which as in what goes inside / outside? It'll work either way around - look at the arrows in this diagram - one end of the tube is stale air in and fresh air out (interior) and the other is stale air out and fresh air in (exterior). You do need to arrange things so that it's easy to guide any condensate to a drain. 16 hours ago, Onoff said: Tube Inlet - In the bathroom, takes in warm moist air? Tube Outlet - Vents externally in soffit? Shell Outlet - ? Shell Inlet? If tube inlet and outlet are as you say, then the shell inlet is fresh air from outside, and the shell outlet is fresh air to the inside of the house. One possible approach (not to any sort of scale): Orange is air from inside the house being extracted, blue is fresh air from outside. The other coloured blocks are fans. There's no reason why you couldn't have the branches both facing down - I just did it this way because I copied the layout of the unit above. If you tilt the right hand side down, condensation naturally flows out without the need for a separate condensate drain, although of course you'd need to figure out how to direct it somewhere if you don't want it just dripping on the ground. You'd also need to think about insect screens. The main thing that gives me pause about this whole project is how you're going to access the heat exchanger to clean it. I think this is an important consideration. 1 Link to comment Share on other sites More sharing options...
Nickfromwales Posted July 2, 2018 Share Posted July 2, 2018 I would already just be buying one tbh. They're just not that expensive. Link to comment Share on other sites More sharing options...
jack Posted July 2, 2018 Share Posted July 2, 2018 20 minutes ago, Nickfromwales said: I would already just be buying one tbh. They're just not that expensive. Have you met Onoff? 1 Link to comment Share on other sites More sharing options...
Ferdinand Posted July 2, 2018 Share Posted July 2, 2018 1 hour ago, Onoff said: Is there an ideal size on this twinwall polycarb (Coroplast) stuff? I think I can get some 16mm for free. 16mm is probably the thickness of the 2 layer stuff, so you may be in luck and have two lots of tube in one sheet. The single layer stuff is normally 10mm for polycarbonate if that is what this is. F Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 2, 2018 Share Posted July 2, 2018 @SteamyTea made one, using thin plastic sheet, IIRC. I'm pretty sure he made some measurements of it's performance, too. I can't see why a coroplast counterflow heat exchanger couldn't be made to work every bit as well as a commercial unit, as long as you're prepared to put some time in to optimising it. For an MVHR that has to fit within a defined space an shape a DIY one may well be a better option, especially if you can make use of some of the space available to form a part of the casing. Variable speed fans are readily available cheaply, at least for a unit that doesn't need to service a large volume. I bought some PC cooling fans a while ago that had PWM speed control and even a tacho lead output to give feedback on the set speed. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 2, 2018 Share Posted July 2, 2018 4 minutes ago, Ferdinand said: 16mm is probably the thickness of the 2 layer stuff, so you may be in luck and have two lots of tube in one sheet. The single layer stuff is normally 10mm for polycarbonate if that is what this is. F I don't think it's polycarbonate, more like some sort of polyethylene or perhaps polystyrene. I know that it's stocked in model shops, as when I was buying stuff to make architectural models our local model shop had sheets of it for sale quite cheaply. IIRC, the sheets were around 5mm thick overall, as I went around measuring all the easy to cut sheet material they had to get a combination that gave the right scale thickness for the models I made (I settled on foamboard laminated with Liteply to give the exact scale thickness for our outside walls) Link to comment Share on other sites More sharing options...
Onoff Posted July 2, 2018 Author Share Posted July 2, 2018 3 hours ago, JSHarris said: Go with what you can get, but I'd say the thinner stuff, with gaps that are around 3mm to 4mm wide might well be best, as that corresponds closely with the sort of spacing in commercial units. Here are some plans for a coroplast MVHR from years ago: https://www.builditsolar.com/Experimental/DIYHRV/DIYHRV.htm Identical to the one in my opening post! Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 2, 2018 Share Posted July 2, 2018 2 minutes ago, Onoff said: Identical to the one in my opening post! Sorry, missed that - didn't go back to last year to the first page... Link to comment Share on other sites More sharing options...
SteamyTea Posted July 2, 2018 Share Posted July 2, 2018 3 hours ago, JSHarris said: @SteamyTea made one, using thin plastic sheet, IIRC Yes I did, think PVC sheet. I did a bit of rudimentary monitoring and had an efficiency of around 80%, but as this was a bathroom MVHR, so often on for a short time, and then off, the efficiency was all over the place. I keep meaning to get the unit down to show how it was made. Was pretty easy. Link to comment Share on other sites More sharing options...
Onoff Posted July 2, 2018 Author Share Posted July 2, 2018 1 hour ago, SteamyTea said: Yes I did, think PVC sheet. I did a bit of rudimentary monitoring and had an efficiency of around 80%, but as this was a bathroom MVHR, so often on for a short time, and then off, the efficiency was all over the place. I keep meaning to get the unit down to show how it was made. Was pretty easy. How did you trigger it: pull cord, humidistat, occupancy sensor etc? Link to comment Share on other sites More sharing options...
SteamyTea Posted July 2, 2018 Share Posted July 2, 2018 It is activated by the light and has the usual overrun of a bathroom fan (because it uses two bathroom fans). Link to comment Share on other sites More sharing options...
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