Heat recovery ventilation. Interesting design.

[SpeedEvil]

http://www.ewp.rpi.edu/hartford/~pastor/MEP/Other/References/Counter%20Flow%20Heat%20Exchanger.pdf documents an interesting heat exchanger.
It's not small, but...

Comprised of 1.8m*1m*10 of 4mm twinwall polycarbonate sheeting.
It even defrosts without extra heat.

External dimensions would be around 2.4*1.2*.25m
150m^3/hour, at 90% efficiency.

Should be doable with - say - 2 sheets of kingspan 50mm, 10 sheets of twinwall, and some DIY valveage, for under 300.
Construction gets rather easier if you cheat so that it can't handle prolonged freezes, but needs a bypass for a few minutes an hour to defrost.

I haven't quite worked out the edge details.

[dhaslam]

The main problem with using these sheets  for heat recovery would be  making the connections and also dealing with  condensation.    There may not be much saving over using conventional   heat exchangers that are made for the purpose.   I don't think freezing is an issue  in  our climate, the  outgoing air should be warm enough to deal with  all but very extreme conditions.

[Richard Owen]

I think the edge detail would become easier if you changed the outgoing air to move across, rather than along, the line of the incoming air.

[SpeedEvil]

Make that 200, not 300, just found http://www.theplasticshop.co.uk/corrugated-polypropylene-sheet-2951-0.html

Condensation is dealt with easily in the above - it simply runs down to a drain.
(even in freezing conditions).
And I'd prefer one that'd work in freezing conditions - it was below zero, often lots below zero 5 weeks last year.
An efficient exchanger will freeze up at the outlet end when it gets below -2C or so.

I think I understand how the edges work now, and it's quite easy to construct.
I need to have a play, and then write it up.

And yes, crossflow is lots easier to make than counterflow, but it does have an efficiency hit.

[dhaslam]

I don't understand how the  heat exchanger  can freeze  in a  moderately cold  climate.     The moist warm  air from the house  meets the warmest incoming air first  in the exchanger  and  then moves along to the colder part of the exchanger.  As the outgoing air cools it loses some moisture  but the water should be collected in a moisture trap.   Because the air outlet  is  large  it wouldn't be easy to freeze it and in any case the water  should already be drained out.   

Incidentally it is just around freezing here (Irish Midlands) at present and I was surprised to find that, measured at the  outside vents,  the outlet air is  very similar to the inlet air at about -1C.       In the house the air at the vents is about  18C  and most rooms are over 20C.   

[martin W]

I wounder if this could be scalled down to use as a bathroom heat exchanger, say 1/2 the size. I don't see why not, but maybe 1/2 size will only save on material costs, labour time is probably going to be the same.

I am going to need to replace our bathroom extractor one day soon and was looking at something like this:

http://www.dealec.co.uk/acatalog/vent_axia_hr25_range.html  costs about £200 by the look of it.

but if this larger heat exchanger can be made for say £200, and some fans and simple ducting for another £100 or so I could maybe do the bathroom and bedroom for a few quid more.

No idea how I would speed up extraction from a bathroom when it is in use, maybe use the light to triger fast fan speed.

[Baz]

In DIY mode don't get too fixated on using the multicel plastic as its features are not being used and plastic is a poor heat conductor. I'd think more in terms of making somthing using tinfoil as the separator, preferably the heavier guage stuff htat used to be available as 'builders foil' but doesn't seem to be around now that plasterboard is ready foiled.

[Monty]

Had similar thoughts on diy heat exchangers, got the heavy duty foil to try and also found these packs of straws for £1-2 per pack, contrary to belief the relatively lower heat transfer  coefficient properties of the plastic don't affect the efficiency of a heat ex as badly as you might expect.

The trouble with diy and especially working with thin sheets of ali is the amount of time needed to adequately mock up something reliable.

[Baz]

Any thoughts on surface area of heatX related to volume of house/room/m3 or something?

[skyewright]

I'd think more in terms of making somthing using tinfoil as the separator, preferably the heavier guage stuff htat used to be available as 'builders foil' but doesn't seem to be around now that plasterboard is ready foiled.

YBS Foiltec?
http://www.ybsinsulation.com/products_floor_foiltecsingle.htm

[SpeedEvil]

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.


The heat conductance of the plastic isn't the big issue, it's mainly the air that is the insulator. (page 4 shows the thermal resistance of the plastic is a tenth that of the air)

As to scaling.
If you look at fig 3 on page 4, you can see the effect of scaling.
The whole thing flows 150m^3/hour.
If you half that flow, you get 75m^3 an hour, and you can look at the 'both sections' line in the graph, while only using one section.
(each section is then 55cm wide).

So, a 55cm*50cm tall exchanger would have 70% efficiency, a 55*1m 85%, ...

If you want to reduce the flow, then you get the same figures by just reducing the width - _not_ the height.
So - 38m^3/hour in 27*50cm at 70% or ...

If you are making one section - without the option to isolate one for defrosting (simply bypassing the incoming air and allowing the outflowing air to pass through the exchanger will do that), it possibly makes sense to make it 'square'.

Instead of 10 sheets of plastic, with an overall depth of around 100mm, and 55cm wide, you could do 20 sheets of half the width, for an overall 'square' section.

Fig 4 on page 5 shows what happens as you vary the flow away from the 150m^3/hour, or 75m^3/half exchanger per hour.
As you decrease the flow, the efficiency improves markedly,  approaching a hundred percent at 50m^3/hour.
On the other hand, if you want to increase the flow to 2.5*, it only drops to 85% or so.
As to how much air 'should' be changed - well...
It's up to you, but building regulations have a suggestion of half the house volume an hour, from memory.

I'm planning on _totally_ sealing up all the little gaps (how well this works in reality will of course be a question, but as I'm re-dry-lining the property, I have hopes) and having this as my only air inlet/outlet.
(at least in winter, in summer, vents may be opened elsewhere)

[stephendv]

For what it's worth, viking-house will sell their fine wire heat exchanger unit on its own: http://viking-house.ie/fine-wire-hrv.html  which is the approach I took.  Slapped together a box out of OSB and stuck 2 x 12V 120mm computer fans on it.  Not pretty, but it works (I hope).  Need to do some testing on exactly how much airflow I'm actually getting, because the fiwi offers some resistance.  Haven't moved into the house yet, so don't know what the efficiency is like yet either...

[SpeedEvil]

For what it's worth, viking-house will sell their fine wire heat exchanger unit on its own: http://viking-house.ie/fine-wire-hrv.html  which is the approach I took.  

Interesting - what was the damage for the pictured unit?

[dhaslam]

It would be easier to have a smaller number of bigger sheets, perhaps just two  8'X4'  if they can fit into the attic.  The ends would bend a bit to connect the pipes at the ends.  The connecting  pipes need to be about the same cross section area as the  sheets.  The cross section area is quite big, with a 1/4" gap  there is  12 sq inches, roughly a 4" pipe.   It is quite hard to get a good seal onto a pipe.   It would be possible to replace the polycarbonate with Correx that would be less expensive.    

[stephendv]

180 odd euro for the black heat exchanger only.