Heat recovering extractor fans

pete formulated on Friday :

Yes, ventilation should always be cross flow - two opposite ends.

Reply to
Harry Bloomfield
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Matty F coughed up some electrons that declared:

Ah - I see. That looks like it will turn into something that will be easy to make due to many identical simple parts.

Do I understand it correctly: the air goes down one of the triangular sections and fans into every alternate layer, coming out in the opposite diagonal corner? The other air path is the other two diagonals and between the layers of foil used by the first airway?

If so, the only comment I would dare to offer is that you're running the air past two fairly sharp 90 degree bends, so you might need beefier fans?

For the foil, what about brass sheet? It would be fairly stiff for a given thickness and resistant to corrosion.

I don't know about the gaskets - maybe square solid brass tube soldered, or solid square plastic section cemented together - both with a bit of rubber glued on one face and clamped with through-studs?

This sounds interesting - please let me (us) know how it develops.

Cheers

Tim

Reply to
Tim S

With the (small) thickness involved, would polythene sheet be conductive enough? It would be strong and corrosion sresistant.

Reply to
<me9

I am in two minds about these. Typically when responding to a shower, it seems that a humidistat controlled fan will work just as well and in some ways better...

Yes a ordinary fan will throw away some heat, however that is quite often desired after a shower to return the room to some semblance of normal temperature rather than it remaining super heated from the shower.

Reply to
John Rumm

The air doesn't fan into every alternate layer, it just goes to the next single alternate layer. The air uses the triangular sections just to skip past only two sheets of aluminium. So the hot air goes past each alternate layer one at a time, heating up the cold air

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cold air passes through triangular sections that are not shown in the diagram.

I'd suggest the air is going past more than 100 sharp bends. Sure, but make the device as large as you want. I think it should have two fans, and the inlet and outlet be widely separated in the room.

I imagine that brass would cost a lot more than aluminium. Almost anything would do as long as it is stiff, the thinner the better, and preferably a good conductor of heat.

The gaskets don't have to be perfectly air-tight. I'd just assemble the sheets and gaskets together and put them in a box that squashes them all together. Then they are easily taken apart for cleaning.

Reply to
Matty F

I have suggested that the sheets have to be very stiff, but if it's supported along the edge by the triangular transfer channels, something more flexible (i.e. conductive and cheap) would do.

Reply to
Matty F

Matty F coughed up some electrons that declared:

OK - so like one really long air path then - but swap the directions of one of the airflows on your diagram; you've got hot air and cold air coming in together - so you'd get warm air back into the room and warm air at the same temperature being blown outside. Sure that was just a drawing typo.

Yes - you'd need two fans whichever way you do it - though you may be able to get a single motor double blower unit (probably cost more that 2 fans though)

Reply to
Tim S

Well, that would probably be better, but the flow of air in the room won't be so simple as to result in the "newly heated air just being extracted again - as it's closer to the extractor than the manky stuff". The unit sucks on one side and blows out of the other, so there'll be a tendency for the air in the room to circulate (likely to work best in a cylindrical room (!), and a squarish one is going to do better than an oblong one). There'll be mixing, but in the bath/shower room I've seen with one of these units (HR25H) installed, it seemed to work pretty well. Note that "seemed" is just that; the owners were happy with it because there was nolonger a condensation problem, but no double blind controlled experiments were performed ;-)

Reply to
Jon Fairbairn

I think you need one of your airflows reversed to get cross flow operation - it would greatly enhance the efficiency.

Reply to
John Rumm

Surely that would depend on the the temperature difference between incoming cold and out-going hot air. If the air was -10 outside and +30 inside would it be more efficient than only a 5 degree temp. difference?

Reply to
www.GymRatZ.co.uk

Once upon a time in the early 80s we bought a new built house that had double glazing to the south side but single to the north, (and blown in cavity insulation).

Surprise, surprise, we had major condensation problems. Initially, though money was tight, I purchased a dehumidifier - the name escapes me but it was the major one that didn't become the "Hoover". Initially I placed this on the landing which resolved the condensation issues upstairs. Having resolved that, I thought I'd use it downstairs for a couple of days to at least prevent further condensation of the N facing windows. To my surprise, we kept the upstairs clear of condensation. Removing it from the hall to the kitchen, I found that we resolved all our condensation problems. Yes, we did have open windows in the kitchen whilst cooking. However, despite four humans emanating moisture upstairs and generating moisture in the bathroom, the principal source was (and still is) the kitchen.

YMMV

Reply to
Clot

I meant to add that the power used is significant and that the heat generated is retained within the house.

Reply to
Clot

Well, I didn't think that through, you are probably right. As I see it, this kind of device would be useful to slowly change the air in a room - replacing the CO2 with oxygen and getting rid of water vapour, at an extremely low running cost, perhaps only a few percent of the cost of other devices such as dehumidifiers and air conditioners. I'm not at all concerned about the size of the device. Cheapness and ease of build is the most important. And it must be able to be cleaned easily, unlike ones with thin pipes. The next task is to invent a good cheap way of making the gaskets. If they were rigid enough, cheap plastic sheet could be used.

Reply to
Matty F

Matty F coughed up some electrons that declared:

Hi Matty

Thus is one of those times I'm prepared to say I'm absolutely right.

What you want for max efficiency is to have the last of the cooled exhaust air meet the coldest incoming air, to suck the last joules out of it.

And conversely, you want to finish heating the new incoming air at the end of its flow with the first and hottest of the exhaust air. It's the same principle as that shower water reheater driang thingy in the Wiki.

This will be interesting. I was thinking, if you leave some temperature probe holes in all 4 ports, one could take 4 readings and directly measure the efficiency (mass airflow speed assumed constant).

Good luck!

Cheers

Tim

Reply to
Tim S

I bought one of these fans a few weeks ago, and will fit it soon. The heat exchanger is remarkable similar to the one in the tumble dryer.

The unit I have has a fan at each end - not sure if they are both blowing or both sucking, but one deals with the air out, and the other with the air in. I believe it is not supposed to be balanced, creating a slight +ve or -ve pressure.

The whole unit is in a 6" plastic pipe, which is sloped slightly downwards towards the outside. Condensation then drips out.

Reply to
Jason

Yes, it drips outside. Make sure the tube the unit is in slopes slightly downwards towards the outside.

It is all about direction - the inlet directs the air into the middle of the room, while the outlets suck it from alongside the wall. I expect the air will be pretty well mixed by the time incoming air gets sucked back out again.

The manual in mine also says not to locate it where the incoming air can blow the humid air away, e.g. don't direct it towards the cooker. It needs to be directed to some out point in the room so it then flows past the cooker on the way back out again.

-- JJ

Reply to
Jason

Be in two minds no longer: they generally do have a humidistat too, and boost their output accordingly. All all that heat - you've paid for it. It is a bit of a shame to throw it all outside.

I don't think the fact that the bathroom is hot has ever been an issue (certainly is not an issue to me). It is always the condensation that you are trying to deal with.

-- Jason

Reply to
Jason

That's why I wonder about the energy ratings of condenser dryers. It always seems to be lower, because it uses more energy running the fan. On the plus side, the heat stays in the house, so energy rating of individual items needs to be looked at in terms of the bigger picture.

-- Jason

Reply to
Jason

I don't doubt you for a moment. I was trying to solve the hard problems of using cheap materials and ease of construction and maintenance, leaving the simple decisions for later. The direction of flow could be decided on later. For an efficient exchanger, clearly it's best to heat the last of the incoming air using the hottest air from the room. I have avoided looking at existing heat exchangers so as not to constrain my thinking. It's now time to see how others do it. But why do they charge so damn much for them?

A heat exchanger is of interest because of its huge potential for other people around the world, but is not necessary for me. I live in an excellent climate. I put an electric heater on for a few weeks in winter and sometimes turn a fan on in summer and don't need an air conditioner. There's no real incentive to get or make anything that reduces my heating bill. I'm busy on a number of other large projects at the moment.

Reply to
Matty F

Matty F coughed up some electrons that declared:

Good plan :)

I do wonder. Perhaps 185 for a one off purchase of a self contained unit is below the pain potential of many people - but the good whole house units can cost amounts that are on their way to 1k. There they don't have any real constrains of form factor or size - they need a big case, 2 big fans and a heat exchanger and 4 ports.

It's of massive interest to me which is why I'm rather glad you mentioned it. I was already looking at commercial solutions. I like my fresh air - I can't stand this stupid modern obsession with building draught free buildings - I am certain (with no real empirical proof though) that being shut up in a box with no air makes everyone in my family more diseased in winter.

The design of my new bathroom (landlocked) and shower room (glass block tile windows) requires mechanical ventilation - so I could be very partial to a system where I can blow loads of fresh air into my house all the time without my gas bill going totally mental.

Lucky you! Where BTW, if you don't mind the question?

Cheers

Tim

Reply to
Tim S

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