Wood Heaters – Radiant or Convection?

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Wood Heaters – Radiant or Convection?

Postby 470rigby » Tue Jul 13, 2010 4:21 pm

There has been some discussion re Wood Heaters on another thread and it has prompted me to ask a question that has been nagging away at me for years. So rather than hijack that thread; here goes!

Woodheaters come in two basic variants; Radiant and Convective, with some that combine elements of the two types of heat transfer. The vast majority of them on the market are convective and some of these are fitted with “booster” fans.

I suspect that most people buying Wood Heaters would hardly know the difference between the modus operandi of the two types; purchasing decisions usually being made on cosmetic considerations or at best by the clearance advantages of the convection types.

But, (IMO) there are other important considerations, the most important being building layout, ceiling type and building construction.

If I can use my case as an example; the house is mudbrick with large floor to ceiling mudbrick room dividers in the main living area that act as thermal masses. The insulated ceilings are Cathedral type and the maximum height is 5 metres. Windows are not double glazed.

But, the previous owner decided to install a purely Convection Heater! So what happens to the heat? Heads straight for the ceiling! And being Off-Grid, running Ceiling Fans to bring it back down is not possible. And I have to burn tons of wood to keep the house at a comfortable 20 degC. Minus 5 deg frosts are quite common here in NE Victoria.

I am replacing it with a Radiant Heater. Wall/drape/furniture clearances are not an issue, and we have no wish to heat the rest of the house; only the main open plan Kitchen/Dining/Dining area. My rationale is that the direct radiant heat will provide personal comfort, while heating the walls/dividers etc at lower levels where it will be re-radiated and also transferred by convection.

I believe that I am doing the right thing, but it does beg the question; how many designers are taking into account the type of heating to be installed when designing energy efficient houses?

For example, if one wanted to convect heat from a Wood Heater to other rooms, then lower Ceiling Heights would seem desirable as would a floorplan that assisted. But what about external wall construction? In this case, for example, would reverse “Brick Veneer” actually be detrimental?

Your thoughts please?
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Re: Wood Heaters – Radiant or Convection?

Postby Gordon-Loomberah » Tue Jul 13, 2010 4:43 pm

Many years ago when I lived in town I welded myself a wood burning firebox heater from 6mm plate steel, and whilst it heated the living room at one end of the house very well by radiation and convection, not much of that heat made it to the far end of the house on cold winter nights. I then added a boxed in section so air could be blown in at the bottom front, which was then ducted up the back and across the top, for a real blast of hot air. More heat made it to the far end of the house. Then I tried using a ceiling fan, and the whole house became quite cosy. The very hot air that was trapped just under the ceilings, but above the doorways, was able to be shifted.
So, given your high ceilings, I think a ceiling fan would do wonders for your heat distribution, even just within that main living area if you dont want any going to other parts of the house. From memory (late 1980s!) the fan I was using drew about 120W. I didnt have to run it all the time, just gave it a burst on high for a few mins every now and then did a good job of evening out the heating.

I reckon its worth considering :
1/ a fan to blow across the convection heater, small fans that only draw 20 or 30 watts can shift quite a bit of air. Extract more heat before it vanishes up the chimney!

2/ a ceiling fan to be used intermittently to distribute the heat more evenly.

Less expensive than buying a new heater, maybe even if you need to add another PV panel to power them, if you are running close to capaicity already.

These 2 things are probably worth considering for a radiant heater as well, as the near-ceiling area will still become hotter than lower down.
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Re: Wood Heaters – Radiant or Convection?

Postby 470rigby » Wed Jul 14, 2010 9:29 am

Gordon-Loomberah wrote: given your high ceilings, I think a ceiling fan would do wonders for your heat distribution,

Hi Gordon,

Thanks for your input.

I had thought of Ceiling Fans but was concerned about the high cost of installation. I am only 3 hours out of Melbourne, but the only way to get an Electrician here is at the point of a Gun! Also, it is hard to model how many/what speed/how often; not forgetting the extra load on the PV system. I don’t rule it out entirely though, and it may be the fall back strategy if a Radiant Heater is not the total solution. If I thought that they would be of any value in Summer it might just tip the balance in favour of them, but they would have to be mounted very high and so would be pretty useless.

Some of the Radiant Heaters on the market (Nectre for one) have provision for adding a fan to the rear that blows air through the heat shield and directs it forward, in the event that radiant heat emission alone is not suffice.

There is another reason for the Heater upgrade. The unit I have is an older model that predated the new emission standards, and does not burn as efficiently. Consequently, I have large build-ups of Creosote in the Flue that requires constant cleaning. A dicy proposition on a wet roof in a gale! In addition, the prospect of a full overnight burn with a more efficient Heater and an uninterrupted nights sleep without having to get up at 3 hourly intervals to stoke the Heater would be BLISS!

Rgds,

Wayne
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Re: Wood Heaters – Radiant or Convection?

Postby ghind » Thu Aug 05, 2010 5:42 pm

I don't think changing heaters is going to do much for you.

You need air circulation and the ceiling fan is the cheapest/easiest way to do it. Their power draw is not that bad.

However there is a much more effective way to do it.

If you suck cold air from a remote location at FLOOR level and dump it above the heater, your whole house will mix and become one temperature. The difference it makes is fantastic.

Many people think you want to take hot air from above the heater and dump it into the cold location. In fact, this is far from the most effective way.

Given that you are concerned about power consumption, you could do a few tricks.

1. Oversize your ducting to reduce drag
2. Put your fan on a cheapo timer. Run it only for 15 minutes in every hour. Or 5 minutes every 20 or similar.

Newer wood heaters simply have greater air input (you can't choke them down as much). This is how they improve emissions. It also means they are hard to make burn all night.

Is your flue tall enough and clear of the roof gable?
Is your flue clean
Are you burning high quality, dry wood. It needs to have been cut at least a year before you are burning it. Some types of firewood just clog up flues.

A ceiling fan has to move a lot of air to mix it. A duct can take air from one area a long way away and put it somewhere else. If you take cold air and put it somewhere hot, the hot air must come back to the cold location. You get a slow moving wall of warm air instead of a wind blowing down on your from the celing.

The right duct setup will use less power than a ceiling fan and do a much better job.

Fix whatever is wrong with your heater's flue but I wouldnt' change it. Better to install more batteries and solar and run a duct system. I think you will be disappointed by your new heater.

This kind of double barrel system works amazingly well (I have run it for 5+ years) http://www.youtube.com/watch?v=DDfefjZn5IE

It is MUCH more efficient - lots more heat for less wood.

I have a baffle in my bottom fire and also in the top too. The smoke/heat must go the long way to get to the chimney.

Greg
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Re: Wood Heaters – Radiant or Convection?

Postby 470rigby » Sat Aug 07, 2010 9:57 am

ghind wrote:You need air circulation and the ceiling fan is the cheapest/easiest way to do it. Their power draw is not that bad.


Thanks for you input. The reliance on electric fans is still a bit of a worry for me. Adding extra PV panels is not the answer unless I upsize the Battery Bank, and that effectively means repacing the bank that I have. Expensive! But a bit of tooling around on the net came up with some non-electric fans that sit on top of the Heater.

One is a Stirling Engine Powered Fan. The Stirling cycle power plant obtains its power from rapidly heating and cooling the same volume of air. When the air is heated, it expands, pushing a piston upward; when the same volume of air is rapidly cooled, it contracts, pulling the same piston downward, providing power. The same volume of air is heated and cooled rapidly converting the heat energy to mechanical energy which is then used to turn the fan blade. It is designed to sit on top of a woodheater. (See Vulcan Stove Fan http://gyroscope.com/d.asp?product=VULCANSTOVEFAN).

The other is the ECOFAN which generates electricity from stove's heat. (see http://www.amazon.com/Ecofan-Airplus-He ... B000VHDS92. Presumably this employs the Themoelectric Effect (Peltier /Seebeck Effect) in reverse to the way is is used in THermolectric Fridges.

I also had a look at the Double Barrel Heater (http://www.youtube.com/watch?v=DDfefjZn5IE), but I'm afraid they don't win any medals for cosmetics. After a long tirade from my wife, ending with the words "over my dead body" I feel that descretion, indeed will be the better part of valour!
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Re: Wood Heaters – Radiant or Convection?

Postby ghind » Sat Aug 07, 2010 12:00 pm

Hi Wayne

The barrel heaters are a bit ugly for inside a house but the concept really works. I love the concept but the looks are no good for home. I have one in a large hardware store and customers cannot believe how well it works. When we first put it in, I was amazed at the performance.

At home I have a Saxon 600 wood heater and it the best I've ever seen for home use. Even in a Tasmanian winter in a 30+sq house it does the job very easily.

However efficiency is always a good thing. I'd love to get another identical second hand heater and mount it on top. I would put a large baffle plate in the centre , cut a hole for the chimney to enter and set it up. It would always be a talking point but I think it would look great done right. I would estimate a performance improvement of 30%. For example if current efficiency is 60% and this lifts the overall efficiency to 80% then it is actually a 1/3 improvement in retained heat. Huh you say? If I currently keep 6 units of heat and I will get to keep 8 then I get 2 extra units. 2 extra units is a whole 33% more units than the 6 units I used to get to keep. 33% more heat output.

I think this is quite realistic, the baffle plate is small and not that far before the chimney. It certainly diverts the heat a little but the double barrel system I run at work has two huge baffle plates with long paths to get around them and they are so much better. The double barrel fire at work is heating a 1000m2m, two storey 180 year old building. It isn't balmy but we don't want for any more heat.

Can you show us a photo of your current heater and its installation?

The fans you refer to look really interesting. They may improve things. However your real problem is circulating the air that is 5 metres above the floor back down to floor level. Your house is not a normal 8ft ceiling and so those fans won't work as well for you.

As an experiment one night, get a standard oscillating fan, the sort you see in summer hardware catalogues and lay it on its side. Put it on the opposite side of the room to the wood heater and point it towards the ceiling right above the wood heater. Or the highest point in the room. Turn it on flat out and make sure you can feel air movement at the top of the ladder at the highest point in the room or above the wood heater.

As another experiment get a dual digital thermometer http://www.jaycar.com.au/productView.as ... rm=KEYWORD or two thermometers. Put one sensor at the highest point in your room and another at the lowest.

In my last house, with 4m cathedral insulated ceilings the temperature would often be 40 degrees at ceiling level and 5 degrees at floor levels. This was due to bad installation of a heat pump before I bought it which had both the air return at duct outlets at the highest points in every room. I was able to greatly improve it by putting 2 air outlets at floor level, I didn't get the time to move the large air return down to properly fix it before I sold it.

In my old work building, we had similar problem, a heater which had plenty of grunt. Tall ceilings. 5 degrees at floor level and 30+ at ceiling level. A thermometer at head height said 15 degrees but it was still freezing.

70% of the solution to that problem was a 60 metre long duct which took cold air from the a far point in the building at FLOOR LEVEL and dropped it at ceiling level directly above the heater. That single 300mm duct has two inline fans in it. I fixed the other 30% a different way but in such a large building to do it properly a 600mm duct with much larger fans would have done it. Due to cross sectional area, a 600mm duct would probably flow 5 times as much as a 300mm duct. So I cheated and used what I had. But the idea is simple.

Take cold air from a really cold far away spot within the same building and dump it at the hottest point. The heat then falls down in a gentle wall. You feel no breeze but very quickly the whole building from floor to ceiling evens out in temperature. People stop getting cold ankles and sore joints. Much more pleasant all around. Also more efficient too. Your ceiling insulation will be a lot more effective when it is exposed to 25 degrees inslide and -5 out than it was when it used to be 40+ inside against the ceiling and -5 out.

I can't imagine why you can't expand your battery bank by putting some more in parallel. Is that type of battery no longer available? Can you show us a photo of your setup?

If you were going to run this kind of setup 24 hours per day, via an inverter, you would need about 2 to 3kwh. That is a lot of power. If you went back to 20 minutes per hour from 4pm to 8am then you may only need 0.5kwh.

In the dead of winter, you'd still need about 3 by 80w or 3 by 100w panels to provide this kind of power output plus batteries and inverter. If you really can't expand your current system and if you really don't have any spare capacity then that is a bit of a cost.

I'm currently building an ultra green shopping centre. In the supermarket side, there has been a lot of innovation in fans. Compared to everything else, fans didn't used to seem to be a big deal. Who cares about a 40w fan?? When you have to generate the power like you do, you see it adds up. Inside fridges, running a 40w fan also means you are creating 40w of heat which you have to remove so it starts to matter a lot more.

Anyway, EBM Papst make some awesomely efficient "EC" fans. You could get a 12 volt or 24 volt one and save the inverter losses. They are also just generally a LOT more efficient for the volume of air moved. If you do go with 12 volts or even 24, use oversized wires to minimise voltage drop and maximise efficiency.

http://www.ebmpapst.com.au/media/conten ... _9_WEB.pdf

I would have though 400m3 per hour (111 litres per second) airflow would be enough for you - this can be achieved with 18 watts of power with a suitable fan. You would certainly want to run oversized duct and longer sweeping corners to keep the efficiency up but it would make a huge difference.

No harm in trying the fans you mentioned before. I'm sure they will help. However I don't think they'll do enough as they are not mixing your hottest air with your coldest air. That is how you create the best comfort in a heated home.

Also, if you have to refill your wood heater every 3 hours then it must be getting a lot of air in to be able to burn that quickly. Or is it a small, crappy heater?

Greg
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Re: Wood Heaters – Radiant or Convection?

Postby 470rigby » Mon Aug 09, 2010 9:16 pm

Thanks for very considered reply Greg.

if you have to refill your wood heater every 3 hours then it must be getting a lot of air in to be able to burn that quickly. Or is it a small, crappy heater?


Yes, it is a crappy Heater. An old Norseman "Tile Fire" that came with the house. It predates the new Australian Standard and is almost totally convective, having an outer "skin". I can put my hand on the back and sides without getting burnt. The top has a grille which convects heat, and the only radiant heat is what is emitted from the front door and the flue.

In my old work building, we had similar problem, a heater which had plenty of grunt. Tall ceilings. 5 degrees at floor level and 30+ at ceiling level. A thermometer at head height said 15 degrees but it was still freezin


I have done the tests. The temperaure differential from "head height" to cieling is only 5-8 deg C, which surprised me. I put that down to the mudbrick walls and Thermal Masses in the Living Room, which are probably radiating heat that has been convected to them back into the interior. (I should also say that measuring temperatures where radiant heat is "variable" is problematic because of the influence if reflectivity of temperaure sensors, so these temperature could just be experimental artefacts).

What I am trying to do is radiate more heat from a Wood Heater to the wall/masses etc at a lower level so that they will re-radiate heat to the occupants. Hence the interest in radiant type heaters.

I would have though 400m3 per hour (111 litres per second) airflow would be enough for you - this can be achieved with 18 watts of power with a suitable fan. You would certainly want to run oversized duct and longer sweeping corners to keep the efficiency up but it would make a huge difference.


My house is on stumps, so using a sub-floor ducting system is dooable. The issue devolves to how to pass the colder air over/through the Wood Heater. Some of the radiant heaters on the market (Nectre Mk 1 is one) have a facility to fit a fan which blows air throught the rear heat baffle and across the top of the heater. The issue then becomes; how much heat can be transferred in this way without reducing the firebox temperature to the point where efficiency is compromised. This is the same problem that Wetback Water heaters in the firebox cause.



As an experiment one night, get a standard oscillating fan, the sort you see in summer hardware catalogues and lay it on its side. Put it on the opposite side of the room to the wood heater and point it towards the ceiling right above the wood heater. Or the highest point in the room. Turn it on flat out and make sure you can feel air movement at the top of the ladder at the highest point in the room or above the wood heater.


I will give that a go. My problem is that even with an 10 deg C differential floor to ceiling (typically the ceiling temp is 28 deg.C and the floor is 18 deg.C), I would have to get a virtual 100% mix of the air in the room to make much of a difference.

The heater that I have is "stuffed" and I need to get a new one. The conundrum I have is whether to get another (more efficient) convection heater, or one that radiates more heat. If the radiant scenario does not work then I guess there are some fallback strategies such as what you have proposed that I can employ to improve its performance.

Cheers,

Wayne
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Re: Wood Heaters – Radiant or Convection?

Postby ghind » Mon Aug 09, 2010 11:37 pm

470rigby wrote:The issue devolves to how to pass the colder air over/through the Wood Heater. The issue then becomes; how much heat can be transferred in this way without reducing the firebox temperature to the point where efficiency is compromised. This is the same problem that Wetback Water heaters in the firebox cause.


Wayne, I don't blow my cold air onto the fire. You do need to keep the box temperatures up. In addition you need to properly circulate the air in the room. If you simply blow floor level air onto the firebox, you are not mixing the air from floor to ceiling so it will continue to layer. It is much better to blow the cold air to the hottest part of the room, typically above the fire at ceiling height.

My suggestion (which I've thoroughly tested myself) is not to blow the air onto the fire box. Let it convect upwards. Then use the cold air duct system to dump air above the heater.

Since you have to replace your heater anyway (filling it every three hours and it still has no horsepower seems to be your summary of it), maybe you can pick up a good convection type for a modest second hand price and save some money towards the ducting.

I'm a little sceptical of radiant type heaters being the solution to your problem (but I do NOT have the experience to tell you it is just what I think). I think the heat will still rise and you'll have an improvement but not a solution with radiant

The bigger Saxon heaters are excellent. If you can find a good second hand suitable sized unit I wouldn't worry if it was radiant or convection.

Ideally, somewhere in the house you will have a way of getting your underfloor ducting into the ceiling. This may be via a cupboard. The longer your run, the bigger your ducting should be to reduce friction drag. The flexible ducting is cheap but the solid ducting creates a lot less drag for the same size duct.

For me to demonstrate this experiment. I can just go grab an inline duct fan and some spare ducting. The ducting to test this would cost maybe $300 for a 6m length or two (you don't even need insulated ducting for the test and the fan). Is there anybody locally you could borrow this off for a small fee to trial it. Suck cold air from the floor and dump it at the roof above the heater.

When builders refit buildings they commonly throw away huge amounts of really good flexible duct. Can you talk to an office/commercial reno company or find a skip and raid it for parts?

I enjoy projects like this and don't mind buying basic parts to test them as they usually work out way cheaper and better and more fun to experiment than to pay somebody who is a "pro".

In your case, given the "cost" of power you need a high quality fan. But for testing a basic reliable cheapo like http://cgi.ebay.com.au/EZI-AIR-4-POLE-I ... 335b16bad4 would do.

uninsulated is fine for testing and for internal use (inside the insulation envelope).

http://cgi.ebay.com.au/300MM-X-5M-SILVE ... 2ea6c89c8e

For under the floor or in the roof cavity (outside the insulation)
http://cgi.ebay.com.au/300MM-X-6M-SILVE ... 3353eea169

Just a starting point. That is not where I get any of my gear but you get the idea.

12 metres of duct, an inline fan with a good flow rate and a couple of thermometres would get you a lot of information about how your house should be set up.

Greg
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Re: Wood Heaters – Radiant or Convection?

Postby 470rigby » Wed Aug 11, 2010 12:21 pm

ghind wrote:My suggestion (which I've thoroughly tested myself) is not to blow the air onto the fire box. Let it convect upwards. Then use the cold air duct system to dump air above the heater.......

.....to demonstrate this experiment. I can just go grab an inline duct fan and some spare ducting. The ducting to test this would cost maybe $300 for a 6m length or two (you don't even need insulated ducting for the test and the fan). Is there anybody locally you could borrow this off for a small fee to trial it. Suck cold air from the floor and dump it at the roof above the heater.


Once again, thanks for your input Greg.

Unfortunately the Cathedral Ceiling and open plan layout do not lend themselves to doing this. I looked at going underneath the floor to the outside wall then come back through the wall over the Heater, but the mudbrick is load bearing and it would be quite a job.

hmmmm....
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Re: Wood Heaters – Radiant or Convection?

Postby ghind » Wed Aug 11, 2010 2:55 pm

Often times you can come up a wall behind or in a cupboard. Do you have any double stud walls? Any way to hide a box which is 100mm deep but say 800mm wide or wider?

Or how about coming up behind the heater into a tube which is hidden by the heater. This tube would exhaust the cold air upwards towards the ceiling. It is important that you can feel air movement of this air at the ceiling height so that it falls back down.

I think it would be worth a trial run to see how much difference it makes while it is still winter. Then you have all summer to work out how to build something that looks right.

Any photos of the room you can show us?
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