Ecofans - are they worth £100?

[snow hope]

Thanks for the feedback. I spoke to the UK division of Caframo, the Canadian manufacturers and asked if they had sold many in the UK and was told that in the last 4 years they had sold 2 million!! Yes, I found that hard to believe too, but that is what they said.

There are a lot of reviews on DODGY TAX AVOIDERS, most being positive. So I have decided to go ahead and purchase - probably on ebay unless my local supplier will match the price including delivery. Will be buying the lates 812 model which is meant to push 50% more air - up to 150 cubic feet per minute. More expensive but might as well go for the most efficient model.

Will feed back in a couple of months and tell you how good I think it is.

[vtsnowedin]

The thing I often see is people trying to push air back against the natural convection loop which can make even a large electric fan a total waste. The classic case was a room full of civil engineers with a wood stove trying to push the excess heat out into the outer office with a 24 inch floor fan sitting on the floor in the doorway pointed to the outer office. To convince them that they had it wrong I taped thin strips of paper to the top of the door casing. With the fan as they had it the paper hung nearly plumb but switching the fan around to blow the cold air at floor level into the heated room immediately started a flow out the top of the door that held the paper strips to better then a forty five degree angle.
I hate to say how many times I came back into that office and found the fan reset the wrong way.
They make small electric fans to mount in doorway tops to push air from room to room. They are about four inches in diameter mounted in a five inch square frame. I find them annoying as they buzz a bit and I am tall enough to hit them with my head if I am not paying attention.
I would solve the problem of even heat distribution the most economical way possible then discuss the merits of ecofans with my guests in a comfortable room.

[lurker]

Cool The thing I often see is people trying to push air back against the natural convection loop which can make even a large electric fan a total waste. The classic case was a room full of civil engineers with a wood stove trying to push the excess heat out into the outer office with a 24 inch floor fan sitting on the floor in the doorway pointed to the outer office. To convince them that they had it wrong I taped thin strips of paper to the top of the door casing. With the fan as they had it the paper hung nearly plumb but switching the fan around to blow the cold air at floor level into the heated room immediately started a flow out the top of the door that held the paper strips to better then a forty five degree angle.
I hate to say how many times I came back into that office and found the fan reset the wrong way.
They make small electric fans to mount in doorway tops to push air from room to room. They are about four inches in diameter mounted in a five inch square frame. I find them annoying as they buzz a bit and I am tall enough to hit them with my head if I am not paying attention.
I would solve the problem of even heat distribution the most economical way possible then discuss the merits of ecofans with my guests in a comfortable room. Wink

If they had an external air kit or room sealed stove or at least an air brick behind the stove they could have the fan the way they say i think.

[mobbsey]

Personally, I've always found that any product that's prefixed "eco" or "green" is a load of cac compared to what you can do with some junk and a little creativity.

I spoke to the UK division of Caframo, the Canadian manufacturers and asked if they had sold many in the UK and was told that in the last 4 years they had sold 2 million!! Yes, I found that hard to believe too, but that is what they said.

Possibly -- I've seen quite a few of these sitting in woodburner and "green" shops, but not that many in people's homes (just because they've sold them wholesale doesn't mean they've been purchased by the public!).


I've gone for the free passive alternative to a fan. Find old junk high-power hi-fi amplifiers, power supplies and the like, rip out the heatsinks, and arrange them on top of the stove to increase the surface area of matal in contact with the air:



I've tested the change in convection using joss-sticks and it definitely increases the speed of circulation. In the picture above (from our living room) the square centre one is an old Pentium 4 heatsink, the side ones are from an old 50W stereo amplifier, and the front one is from an ex-CB radio 12 volt power supply.

Cost -- nothing (apart from a few minutes disassembly); working brilliantly for the last three years with no maintenance (other than a little dusting).

[biffvernon]

According to my understanding of the laws of thermodynamics, your decorative stove-top sculptures will decrease the temperature of the stove top metal reducing the radiant energy emitted into the room by about the same amount as the increased convection adds.

If you put your pot of soup on the top instead you at least get a free lunch.

[mobbsey]

According to my understanding of the laws of thermodynamics, your decorative stove-top sculptures will decrease the temperature of the stove top metal reducing the radiant energy emitted into the room by about the same amount as the increased convection adds.

No, because the thermal incline doesn't start with the metal it starts with the 'high potential reservoir' of the combustion gases, and ends not with the heatsinks but with the 'low potential reservoir' of the gases in the room; and the heat transfer rate across that system will be proportional to the difference in temperature between the metal and the combustion gases (lower the difference, low heat flow, and vice versa).

The black matt finish of most wood burners has a low emissivity compared to the bare metal of the heat sinks; e.g., think flat plate collector in solar thermal set ups -- very good at sucking heat in without re-radiation. Consequently changing the temperature of the metal doesn't directly correlate the heat radiated. You have to get the stove matal very much hotter to increase the direct radiated energy, and convective output is going to respond slowly to changing metal temperature as a result.

In contrast putting "metallic" heatsinks on top increases the emissivity compared to the black surface, as well as increasing the surface area of metal in contact with the air to increase convection. If this causes the temperature of the stove metal to drop then what will happen is that heat transfer between the gases in the "secondary" combustion chamber (between the top surface and the internal baffle plate) and the metal will increase.

If there is a drop in the temperature then I think it's likely to be the exhaust gases leaving the stove into the chimney rather than the surface temperature of the stove itself. In other words, more heat dumped into the house, less dumped directly to atmosphere.

If you put your pot of soup on the top instead you at least get a free lunch.

Yes, we take them off to do that and stoke the fire -- but of course you can still toast with the doors open, and roast on a bed of embers using a small Le Crueset pan stood on half a brick inside the firebox, with the heatsinks in place.

[biffvernon]

I'm sure you are mostly right.

The black matt finish of most wood burners has a low emissivity compared to the bare metal of the heat sinks... changing the temperature of the metal doesn't directly correlate the heat radiated.

Is that why black body radiation comes from black bodies? And why my electric kettle is shiny silver?

The kettle we keep on the stove is Le Creuset, rather like this: http://www.lecreuset.co.uk/Traditional-Kettles.aspx but green and cost a fiver in a car boot sale rather than the £65 quoted.

[vtsnowedin]

According to my understanding of the laws of thermodynamics, your decorative stove-top sculptures will decrease the temperature of the stove top metal reducing the radiant energy emitted into the room by about the same amount as the increased convection adds.

If you put your pot of soup on the top instead you at least get a free lunch.

I can see a long thread developing here that might be quite useful to those who have not considered the topic before. If we could keep it in layman's terms more viewers might actually fallow the chain of thought and gain from the discussion.
If you have a solid fuel mixing with air and combusting inside a restricted vessel (like a wood stove) the excess heat from the combustion can leave the vessel by radiation (not nuclear but radiant) out through the walls of the vessel or get wasted by exiting out the exhaust stack. An efficient stove manages to get more of the available heat out through the walls of the stove and sends less up the chimney.
At one time any wood stove with more then ten feet of stove pipe between the stove and the chimney was considered a high efficiency stove as the thin walls of the stove pipe were excellent radiators of heat.
To increase the radiating surface area of a stove buy adding on anything you could think of, (that didn't melt ),you are most likely increasing its ability to dissipate heat into the living space and increasing it's efficiency. A pot of water on the stove dose do a good job of this and helps to keep the wood furniture from drying out and falling apart.

[mobbsey]

Is that why black body radiation comes from black bodies?

That's all to do with high energy physics -- objects heated to a few thousand Celsius, not the 150 to 300 Celsius of the average wood stove. In the long-wave infra-red band black objects absorb energy, and so will reach an equilibrium between emission and re-absorption -- e.g. in solar thermal this is called the "stall temperature" of the plate (e.g. stall temp. of flat plate is about 80C, and evacuated tube is 130C -- which is why it can heat the water more effectively because there's a higher differential between the ambient and stall temperature).

[mobbsey]

At one time any wood stove with more then ten feet of stove pipe between the stove and the chimney was considered a high efficiency stove as the thin walls of the stove pipe were excellent radiators of heat.

Years ago for a favour (arrghh!) I spent a long afternoon with pipe benders to make a coil of copper pipe to fit over a long stove pipe. We fixed the coil into place on the stove pipe with fire cement to get good thermal contact, and then used it as the heat source to create a thermosyphon system that ran a few radiators.

The problem with long stove pipes, especially when you wrap water jackets around them, is that they have a tendency to creosote/tar up on the inside, and this reduces the efficiency of heat transfer -- and they're a real problem if they build up a thick layer that subsequently catches fire (heat can warp/damage the pipe). Burning well seasoned deciduous wood helps, but tarry pine and unseasoned wood really gums up the works!

[biffvernon]

That's all to do with high energy physics -- objects heated to a few thousand Celsius, not the 150 to 300 Celsius of the average wood stove. In the long-wave infra-red band black objects absorb energy, and so will reach an equilibrium between emission and re-absorption...

You appear to speak very authoritatively but that in itself does not ensure compliance with the laws of physics. Wikipedia is another font of learning: http://en.wikipedia.org/wiki/Black_body

[snow hope]

This is very interesting and no doubt the concept of adding heat sinks will increase the radiation of heat to the air in the room. We sometimes boil a flat base kettle on the stove - coffee seems to be nicer made this way!

My actual requirement is different, hence why I am still intending to buy the Ecofan. I want to cause the air in the room to be slowly expelled by a constant postitive pressure to the rest of my house. This will be done (I think/hope) by the use of a one-time purchase cost, but with zero ongoing, running costs. The stove is lit every night in the winter and it is always warm (too warm for me sometimes) in the room when the stove is lit.

I think it will work and I also think it may cut down on the condensation that collects in some of the bedrooms - we shall see. I will feedback based on my results.

[biffvernon]

Paul's mention of the stove-pipe makes one think that the best place for your ecofan is where it causes an airflow past the stove-pipe, cooling it down a little.

(But don't trust his black radiation stuff until after you've studied and inwardly digested the Wiki article.)

[mobbsey]

I want to cause the air in the room to be slowly expelled by a constant postitive pressure to the rest of my house. This will be done (I think/hope) by the use of a one-time purchase cost, but with zero ongoing, running costs. The stove is lit every night in the winter and it is always warm (too warm for me sometimes) in the room when the stove is lit.

Fan will just move air around the room. To make it shift through the house you need a thermocline -- a difference in temperature between rooms with heat sources at the base of the house, extending upwards using the hallways as the heat exchange system (doors/curtains help regulate this process). By changing which doors are open, or using curtains as 'leaky' barriers to heat movement/where the heat can migrate, you can control the temperature through the house. If run in tandem with central heating, thermostatic valves are essential so that you don't upset the balance by heating upper rooms to a higher temperature than lower ones.

The whole process is very similar to passive solar venting/heating, of which you can find all sorts of ideas in books/on-line.

In our house our single wood burner is at the front of the house. We leave a small gap open into the attic. As the warm/damp/stale air slowly seeps into the attic the heat from downstairs (the kitchen, at the back, is the other big heat source -- cooker is in use a lot, e.g. on bread making days we don't need the fire going) is drawn up through the house. Cooling from the bedroom windows causes some air exchange with the warmer air in the hallway into the bedrooms. Then finally the seep to the attic takes it up and out through a small roof vent.

You can get technical, but the simplest way to see how the air moves is to light a joss stick, and then walk around looking for how air moves in and out doorways, whether doors and windows are leaking (smoke is sucked out/pushed away from window/door). Also a good way to check for fugitive drafts through partition walls, and even some light/power fittings that provide an opening into wall/floor voids.

The drawback with getting a reliable thermocline is that you have to have good draught proofing around other doors and windows to prevent cold air leaks disrupting the process. Note also that in summer, by varying which side of the house has an open window (preferably the shady side -- depending on wind direction), you can also bring cooler air into the house using this same method.

Of course, this all requires that you get involved in your house and how it works -- it's not like an AC system where you program in setting for each zone and let the electricity grid do the work for you.

[Catweazle]

A couple of points to consider.

1) Manufacturers of heatsinks for electronic devices often anodise them black to increase the amount of heat they can radiate.

2) Car radiators are always painted black for the same reason.

3) If a bare, hot microchip could lose heat as fast as one equipped with a heatsink then manufacturers would save the cost of an aluminium or copper heatsink.

In other words, heatsinks work, especially if they're black.