MVHR with extract only mode

[MikeGrahamT21]

OK, will keep an eye out for a counter flow unit.

[jack]

The first two drawings here show the difference. In a counterflow arrangement, the fluids move in opposite directions. Theoretical efficiency is 100%, although more than 90% in the real world is doing well.

 

Crossflow means the fluids move at 90 degrees to each other. This simplifies the connections in a device like that shown above, but theoretical efficiency drops to 75%.

 

A parallel flow, where both fluids are moving in the same direction, is at best 50% efficient.

 

The one linked above is crossflow, but with a double-back path (blue arrows) that will tend to bump the theoretical efficiency up from 75%:

 

 

 

To turn this into a full counterflow unit, you would change the divider strip layer to look like this, with the parallel dividers being parallel to the coroplast tubes:

 

 

22 minutes ago, PeterW said:

If you mean the Correx type stuff it will never work as it’s just too thick and does not have a guaranteed sealed channel structure. 

 

I'm not sure that a lack of sealing in the channels is likely to be a big issue, but I agree about the thickness. You could always use flat sheets of thinner material and alternate the pattern above with the pattern below:

 

 

 

The problem is that anything thin enough isn't going to be very rigid, so might be annoying to work with. Some sort of jig would help, and once the first few layers are done it should stiffen up and make things easier.

 

Sounds like a lot of work, but hey, it's not like you've a bathroom to finish @Onoff

 

9 minutes ago, richi said:

The "area" Vent-Axia I used in the garage reno uses what looks like a load of plastic straws bundled together (and, no, I didn't pay anything like £720 for it!)

 

Another diy option is to make something like this, using thinwall plastic tubing:

 

 

Make the (white) plates out of something like XPS (perhaps melt holes through).

 

To be honest, I love thinking about this sort of thing, especially when I have a big bit of horrible work that I'm trying to get done today(!), but I'm less sure about relying on diy solutions for bringing fresh air into my home long term.  

 

 

[jack]

26 minutes ago, MikeGrahamT21 said:

So they quoted 89% efficiency i think for this unit, how can it only be 60-65? I'll keep on the lookout for counterflow, makes sense air at 90 degree angles to each other would pass more heat. All good information this!

 

If they quoted 89%, it's counterflow. Anything with more than about 70% real world efficiency will necessarily be counterflow. 

 

Incidentally, counterflow is 180 degrees. It's crossflow that's 90 degrees.

[MikeGrahamT21]

OK, looking at those diagrams, i'd say all of the ones i've looked at so far are counterflow.

[Onoff]

32 minutes ago, PeterW said:

 

For what ..? The enclosure for a heat exchanger ..??

Yep

[PeterW]

Ideally it needs to be insulating so a lot are foam or lined with foam. 

[MJNewton]

10 hours ago, MikeGrahamT21 said:

I think my first goal if I can get a reasonably cheap but good MVHR unit second hand, will be to mount in the loft, and give it external air, and then basically just see what comes out of the inlet, if it smells of smoke I can investigate further then, before I go cutting a load of holes in the ceilings and walls.

 

Have you considered just using an inline extractor fan for this test? It ought to give the same result (I'd say you only need to emulate the supply side) but would do so without the commitment of a full MVHR unit.

[Onoff]

10 hours ago, jack said:

The first two drawings here show the difference. In a counterflow arrangement, the fluids move in opposite directions. Theoretical efficiency is 100%, although more than 90% in the real world is doing well.

 

Crossflow means the fluids move at 90 degrees to each other. This simplifies the connections in a device like that shown above, but theoretical efficiency drops to 75%.

 

A parallel flow, where both fluids are moving in the same direction, is at best 50% efficient.

 

The one linked above is crossflow, but with a double-back path (blue arrows) that will tend to bump the theoretical efficiency up from 75%:

 

 

 

To turn this into a full counterflow unit, you would change the divider strip layer to look like this, with the parallel dividers being parallel to the coroplast tubes:

 

 

 

I'm not sure that a lack of sealing in the channels is likely to be a big issue, but I agree about the thickness. You could always use flat sheets of thinner material and alternate the pattern above with the pattern below:

 

 

 

The problem is that anything thin enough isn't going to be very rigid, so might be annoying to work with. Some sort of jig would help, and once the first few layers are done it should stiffen up and make things easier.

 

Sounds like a lot of work, but hey, it's not like you've a bathroom to finish @Onoff

 

 

Another diy option is to make something like this, using thinwall plastic tubing:

 

 

Make the (white) plates out of something like XPS (perhaps melt holes through).

 

To be honest, I love thinking about this sort of thing, especially when I have a big bit of horrible work that I'm trying to get done today(!), but I'm less sure about relying on diy solutions for bringing fresh air into my home long term.  

 

 

 

The bathroom is in effect an all round insulated, sealed via vcl box. The only gap will be at the door. 

 

Quite keen to have a bash at a DIY single room solution. The one thing I do have is space for a "long" unit like the tubular one.

 

It would have to go in the uninsulated space above the bathroom ceiling but the tube itself could be externally insulated.

 

I'm pretty close to the soffit to site the intake and extract. Where to site the other two ducts?

Edited June 27, 2018 by Onoff

[jack]

If you're serious about it, I have some closed cell PE tubing left over from our MVHR installation, which might be of use as an outer housing for a tube-based unit.

 

I think I have a length of 180 and another of 200 or 220. Happy to send it your way if of interest.