One way heat exchanger?

[David WE Roberts]

Further to moving warm air (due to sunlight through patio doors) from the back of the house (S. facing) to the front of the house (N facing).

Architect seems to think this might work as long as there is an intumescent joint in the ducting to guard against fire transmitting along the duct.

Current (today) plan for routing it is from the ceiling down behind a false fireplace, under the floor, and up through floor vents in the receiving room just like proper blown air heating.

Given that it is passing through the false fireplace, which is designed to hide the ugly chimney bits associated with the solid fuel stove, it is also passing very close to the stove.

This offers the opportunity to pick up some of the heat from the stove when it is lit, and transfer that to the front room. However a simple heat exchanger would potentially lose heat back to the room when the fire is not in.

I am now trying to work out if:

(1) This won't matter because the whole rear room should be roughly at the same temperature (although the stove area may be cooler because of the chimney). (2) There should be two routes for the air, one bypassing the stove and the other using it for heat. (Warming to this.) (3) If there is such a thing like a one way heat exchanger - a bit like a Zenner diode which allows electrical current to pass in one direction only.

This is part of us trying to design into the new building work stuff that would be easy and relatively cheap, fun and useful, but harder to do after the building work was completed.

So does anyone have a solution to (3)?

Cheers

Dave R

[NT]

It does help some, but warm room air is at far lower temp than the output of a hot air boiler, so for the same noise level you get way less heat movement.

Intumescent compounds have limited expansion, and take enough prompting to expand. If you want to block the duct in a fire I'd consider a grid coated with an intumescent product.

When the fire isn't lit, the stove is at the same temp as the rest of the warm room, so heat wont be lost back to the warmer room.

If you're going to the trouble and expense of ducting warm air, I can't help thinking it would make more sense to take the air from a hot air solar panel, of the shadecloth type, mounted outside. You get far greater temperature and far more solar gain, plus you can thermostatically control the duct fan in autumn, and you can block the panel output with a panel of insulation for winter. Better results all round.

And if youre going to do that, do make more use of it by including a fine filtering system to cut down on house dust and reduce the need for cleaning.

NT

[NT]

If you then add the stove in circuit, you could put the stove on its own branch of duct from the main duct, with its own fan, again run by thermostats.

NT

[Andrew Gabriel]

The other problem with using air to heat is that any movement results in wind-chill effect on people, so if the air isn't heated much in the first place, there's the potential to feel like you're sitting in a draft which feels cold, even when it's warmer than the surrounding air. This means the system has to be well designed to generate very draft-free air movement. It's possible, but needs more design work than just using a fan and some ducting.

[John Rumm]

A Zenner lets current flow in both directions... that and breaking down at a known voltage when reverse biased is what makes it special ;-)

I think you may be trying to over think this one. Lots of effort for negligible gain...

[Tim Lamb]

Yes.

There are commercial systems sold to move warm air from lofts to more useful places. You might pick up some design ideas from their adverts.

Otherwise you could fit manual *dampers* to control airflow routing.

regards

[Tim Streater]

Well you can reverse it - that's what a fridge does, but you have to put in energy to do so.

It's a *statistical* law. Just much more likely to flow that way than the reverse.

By "much more likely" I mean that the laws of physics don't prevent an ice cube forming in a glass of cool water, with the water getting hot. But you'd have to wait billyuns of times the age of the universe to see it happen.

[David WE Roberts]

Manual dampers are an option. Also a seperate fan and a one way flap for the airflow over the stove.

Please remember that the extraction of heat from the stove is just icing on the cake. The main aim is to encourage air circulation between two rooms during the summer and also during any sunny days in autumn, winter and spring where the room temperatures are significantly different. For instance at the moment the temperature in our current dining room can rise to between 23C and 24C on a sunny day whilst the temperature in the front room is around 19C. Heat will not flow naturally between the two rooms. The hallway allows any heat coming out of the dining room to rise up the stairwell and heat upstairs.

Volume of the lounge is around 1200 cu ft. To completely change the volume of air in the lounge with air 5C warmer in

30 minutes would require a flow of 42 cu ft/min. Damn - everything is rated in cu m/hr 1200 cu ft is around 34 cu m

Quick Google for kitchen extractor fans suggest that they extract at around

230cu m/hr. So that would in theory change the air in the room in less that 10 minutes. This seems a bit agressive, but I suppose that you need a large airflow to get cooking fumes out of the kitchen.

Hmmm...bathroom fans are around 75-90 cu m/hr. Complete air change every 20 - 25 minutes? That sounds a bit more reasonable and less likely to cause a major draught.

Seperate air inflow required for the kitchen area to allow the extractor fan(s) to use localised airflow and not compete with the fire (when lit) and the heat transfer system. Ensure that the heat transfer system does not draw air directly from the air feed to the fire - we want warm air from the room not cool air from outside. Assume that the system will run with lounge door open to allow cool air to escape, or that a ventilator is fitted in the door. Ideally lounge and kitchen/diner door are both open to allow cool air to return to kitchen/diner from lounge.

So - summer (sun heat) intake vent(s) well away from the fire and air inlets and also away from the kitchen. Winter (fire lit) vent(s) near to the fire but not near to the fire air inlets or the kitchen - this is possible with the planned layout.

Bathroom extractor fans - one near the inlet(s) and one under the floor to keep things moving. Underfloor ducting running within the underfloor insulation but underfloor fan obviously not insulated.

It is sounding doable.

The general principle is to input a reasonably large volume of slightly warmer air instead of creating a localised hot spot within the room (radiator) which requires a far greater temperature difference to work effectively.

Cheers

Dave R

[David WE Roberts]

Ah - Zenner diode popped into my minds as I replaced a broken charge regulator on a motorbike with one many years ago. The principle as I understood it was that the diode would not pass current below a certain voltage so only once the battery was charged would it allow any further output from the generator to flow directly to earth. Didn't realise it potentially flowed both ways :-) So ignore my lack of electronics - just wondering if there was any material which allowed a biased flow of heat.

An alternative would be to have a moveable floor of the air duct (or the complete duct) so that it was moved away from the heat exchanger by the fire to create an air gap when the fire was not lit.

However having two ducts and a gate between them seems easier at the moment - either heat exchanger or room air.

Cheers

Dave R

[The Natural Philosopher]

Maxwell's demon?

[John Rumm]

Yup, commonly used in power regulation...

That's the slightly odd thing about them, you wire them opposing the current flow so they are reverse biased, and they then breakdown at a specified voltage...

Material, not so sure - although convection (aka gravity circulation) can move heat from low to high - so you could induce passive circulation using ducting and the chimney effect - an out ducts from low to high running from the warm room to the cooler, and then a low level one to return cooler air.

[Chris J Dixon]

Speaking of rewriting the laws of physics ;-)

Had a caller at the door yesterday trying to sell me one of those roof treatments. I would have sent him off straight away, if his leaflet hadn't made great claims about energy saving.

When I asked how it achieved this, he explained it was a sort of sealant (some name like "Thermilate") which then stopped the heat flowing through the tiles. (1)

Asked for more information he repeated his script. No evidence, but he was clearly impressed by the name, which I remarked was exactly what you would choose to enhance a dubious claim.

I enquired about his technical qualifications - Engineer?, studied physics?, to see if there was any point in further discussions but at this point he flounced off.

I pointed out that if he had heeded my "No cold callers" sign, he could have saved himself the bother.

(1) There seem to be a lot of claims like this around, and all seem totally bogus to me. I can't see how a thin coat of anything, even if it does have a handful of magic powder in it, will have any measurable impact on heat loss.

Typically, have a look at:

According to the Energy Saving Trust you may be losing 26% of the heat produced in your home through your roof.

in your home during the Winter months.

I did contact the ASA about a similar site earlier this year, but they didn't feel a claim of "Save up to 30% of your energy bills" was misleading, so I gave up..

Chris

[Steve Firth]

Chris J Dixon wrote: [snip]

So you refuse to use loaded thermal paste between the CPU on your computer and the heat sink? How many CPUs do you get through in a day?

[Chris J Dixon]

;-)

Back in the day when the shop floor were facing for the first time the mounting of "hockey puck" power semiconductors, and the heatsink was 45 kg of cast aluminium (1), a spate of device failures on test was traced to the pot of thermal paste being contaminated with swarf, which didn't really help :-(

(1) BR Class 314 EMU

Chris

[David WE Roberts]

Just sparked a wayward thought. There is now glass which can, at the flick of a switch, change from passing light to not passing light - or at least significantl;y reduce the amount of light passing through. Used as insanely expensive upmarket curtains.

Now wouldn't it be nice if you could do the same thing for thermal conduction as you could for light transmission. Flick a switch and re-arrange the molecules so they transformed from a heat conductor to a heat insulator.

Given that you can do such tricks to tinker with the transmission of light and electricity why can't you do the same with the transmission of heat?

Or perhaps just have a cell structure which you can fill with a conductive liquid to transmit heat, or force out with an insulating gas to provide insulation?

[Tim Streater]

Light transmission is the passage of photons through the material and whether they are absorbed or not. Heat is the vibration of the molecules of the material (well, temperature really, but the temperature

*difference* between one molecule and its neighbour leads to conduction).

[David WE Roberts]

So design me a material which has two states - one with a rigid molecular structure and the other with a wildly vibrating molecular structure. Simples!

[John Rumm]

produced in your home through your roof.

contained in your home during the Winter months.

Neither of those statements seem at all contentious. They make no claim as to how effective it is, just that it makes "some" difference.

[Chris J Dixon]

produced in your home through your roof.

contained in your home during the Winter months.

So, consider the second claim. Of the heat lost through the roof, what level of reduction would you be happy to accept as justifying the claim? 20%, 10%, 1%, 0.1%?

Chris

[John Rumm]

produced in your home through your roof.

contained in your home during the Winter months.

0.1% would be adequate to make the claim "safe".

Don't misunderstand however, I am not suggesting this is a good idea, or would yield a worthwhile saving - just that you can't take it to the ASA as an erroneous or unsupportable claim.