Author Topic: Wood burners  (Read 2948 times)

Offline kay

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Re: Wood burners
« Reply #25 on: January 21, 2021, 04:34:00 pm »

I'd offer a word of advice to anyone thinking of getting a stove; go for one with a flat top and get a 4 pint farmhouse kettle to stand on it. It gets hot enough to brew up with at a pinch. You can even cook a basic meal on a flat topped stove, if needed. A flat topped stove can also have a stove fan placed on it, which recovers extra heat that would have gone up the stove pipe.


I think you're underselling the cooking capabilities. Any slow cooking meat can be cooked on a wood stove - if ours is lit, it often has a heavy cast iron casserole on top cooking the evening meal, and a kettle providing hot drinks during the day. Griddle scones are also good. Hotter cooking things work but may mean the stove puts more heat into the room than you want.

I was a bit puzzled by "recovers extra heat that would have gone up the stove pipe" but presumably you mean that it moves the air around, directing warmer air into the room and allowing colder air to surround the stove, thereby  extracting  more heat from the stove?

Online cavemanmike

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Re: Wood burners
« Reply #26 on: January 21, 2021, 04:40:37 pm »
Cosy. And even dries the laundry

Offline martinb

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Re: Wood burners
« Reply #27 on: January 21, 2021, 04:40:45 pm »
Interesting thread as here in France, EVERYONE has a wood burner or a wood burning stove as € for € electricity/gas/fuel oil is as expensive as the UK (well maybe not electricity as France generates close to 80% of its needs by nuclear power)

Over here, wood is normally seasoned over 2 to 3 years before burning, many locals have small holdings and grow trees for fuel! We had a small wood burner in the UK and the bloke who sold the house to us was one of the few people who seasoned his wood, and made a point of telling us not to burn green wood.

Over the 3 years I've been here, I've taken down quite a few trees on my plot for one reason or another, and I've only just burnt logs from a tree I felled in early 2018. I have a 9Kw wood burner, I'm just off to light it now, but it keeps my whole house (80sq m+) nice and toasty.

I know councils and/or government are looking at a tax in the UK on wood burners, but they need to look at gas patio heaters first - much more energy ineffecient.

Offline Pitlamp

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Re: Wood burners
« Reply #28 on: January 21, 2021, 04:47:12 pm »

I'd offer a word of advice to anyone thinking of getting a stove; go for one with a flat top and get a 4 pint farmhouse kettle to stand on it. It gets hot enough to brew up with at a pinch. You can even cook a basic meal on a flat topped stove, if needed. A flat topped stove can also have a stove fan placed on it, which recovers extra heat that would have gone up the stove pipe.


I think you're underselling the cooking capabilities. Any slow cooking meat can be cooked on a wood stove - if ours is lit, it often has a heavy cast iron casserole on top cooking the evening meal, and a kettle providing hot drinks during the day. Griddle scones are also good. Hotter cooking things work but may mean the stove puts more heat into the room than you want.

I was a bit puzzled by "recovers extra heat that would have gone up the stove pipe" but presumably you mean that it moves the air around, directing warmer air into the room and allowing colder air to surround the stove, thereby  extracting  more heat from the stove?

Agreed (1st paragraph) and yes, almost (2nd paragraph). A stove top fan is perhaps the simplest way of extracting some heat from the pipe which would otherwise go out to atmosphere; there are of course proper stovepipe heat exchangers which are commercially available and many DIY designs (You Tube is your friend.)

Online PeteHall

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Re: Wood burners
« Reply #29 on: January 21, 2021, 04:58:20 pm »
Hotter cooking things work but may mean the stove puts more heat into the room than you want.
Indeed! I accidentally had mine glowing red a few weeks ago for the first time  :o
It was in fact far too hot to cook on, simply because you could hardly go into the room, let alone get close enough to the stove for the constant stirring needed for high temperature cooking!

I was a bit puzzled by "recovers extra heat that would have gone up the stove pipe" but presumably you mean that it moves the air around, directing warmer air into the room and allowing colder air to surround the stove, thereby  extracting  more heat from the stove?
I think that a stove is supposed to be more efficient if you can get the heat away from the outside of it, so as well as directing heat into the room, the fan should also help with the efficiency. I installed ours half way out of the fireplace recess for this reason (it also meant that I could fit a back-exit flue, leaving the top free for cooking). While I'm sure the fan must help, I've not done anything scientific to prove it!

Offline Ian Ball

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Re: Wood burners
« Reply #30 on: January 21, 2021, 05:16:56 pm »
I will admit to seeing the wood burner as the height of awesome but not really until the kids stop being clumsy oafs.

I tried to offload a hundred kilos or so of wood from our tree a couple years ago, quite hard to get rid of it really, luckily there was no one around when I dumped it in front of a well known caving club  ;)


Offline ogofmole

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Re: Wood burners
« Reply #31 on: January 21, 2021, 05:46:28 pm »
We love our Woodburner, great when we have power cuts, just stick the kettle on the top for a brew and even done some cooking on it. Plus we have our own 10 acre woodland  ;D

Online cavemanmike

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Re: Wood burners
« Reply #32 on: January 21, 2021, 06:58:29 pm »
It's also important to note that you should have a flue liner and an insulated chimney or your stove will spend most of YOUR fuel heating the huge void in your chimney before it heats the room /house /cental heating. Don't want to sound obvious

Offline SamT

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Re: Wood burners
« Reply #33 on: January 21, 2021, 08:02:58 pm »
It's also important to note that you should have a flue liner and an insulated chimney or your stove will spend most of YOUR fuel heating the huge void in your chimney before it heats the room /house /cental heating. Don't want to sound obvious

Not sure thats true at all.  The fire box gets hot, metal radiates heat out into the room. Any heat then left in flue gas as it goes up the flue, insulated or not, just goes up the flue, It doesn't somehow magically go back down and into the room.

However, you *should* insulate your flue, but for a different reason.

Its all about the flue temperature.

In order for a wood stove to work efficiently, it needs to be running at about 600 degree C.

Providing enough O2 is supplied to the stove, at this temperature, all the volatile fumes and char should be burnt off, leaving little in the way of particulates and flue gas comprising Nitrogen and its oxides, CO, CO2, and H20.

If incomplete combustion occurs due to lack of O2 or low temperatures, your flue/chimney ends up full of particulates/soot, Tar,  Sulphur and Nitrogen oxides which will (if the temp is too low in the flue) mix with the H20 vapour to create Suphuric and Nitric Acid. Hmmm nice. Which will rot your nice expensive flue liner  (this is also why dry seasoned logs should be burnt, to reduce the H20 element as much as possible).

So insulating your flue, will keep the flue gasses hotter and prevent cooling leading to nasties condensing on the lining. 

Ideally, you should use a small 4" flue liner (for a small 5 or 8 kW stove) to keep the velocity of the flue gasses higher, ensuring they get up and out the top of your flue before cooling.  However, building regs stupidly says its got to be 6 inches, for some unknown reason.

So I'm not sure I'd want to be trying to squeeze out the last of the heat from my flue gases with heat exchangers an the like, bit miss guided I think.
« Last Edit: January 21, 2021, 08:13:10 pm by SamT »

Offline SamT

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Re: Wood burners
« Reply #34 on: January 21, 2021, 08:17:07 pm »

Also - this is the reason that back boilers in a wood burner might not be such a good idea - essentially your constantly water cooling the fire box, which makes it difficult to get it up to the temperatures needed to burn wood cleanly.

Offline SamT

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Re: Wood burners
« Reply #35 on: January 21, 2021, 08:21:19 pm »
It's also important to note that you should have a flue liner and an insulated chimney or your stove will spend most of YOUR fuel heating the huge void in your chimney before it heats the room /house /cental heating. Don't want to sound obvious


Err - sorry - re-read.  Yes you should use a flue liner in a chimney. And you should insulate it if possible.  People in the past have poured vermiculite down the chimney to fill the void around the flue, helping to insulate it, but I have heard that this has caused big problems with damp.

https://www.stovesonline.co.uk/leca-insulating-backfill

We have a 6" liner going up through a inglenook in Wales (about 8 feet across at the bottom), but we've lined the flue with purpose built flue lagging.

Loads of good info and knowhow on www.stovesonline.co.uk

Online PeteHall

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Re: Wood burners
« Reply #36 on: January 21, 2021, 09:08:40 pm »
I'm sure you are right about flue insulation Sam, however I'm not sure it's such a big problem if the stove is kept alight for a reasonable length of time.

If the stove is only alight for a short time, I expect the issue of condensation is far greater than when the stove is alight for several hours and the flue has time to reach temperature and hold temperature long enough to clear any condensation.

As I mentioned in an earlier post, the warmth from the central chimney keeps most of my house really warm, and because of the thermal mass, it stays warm all night, where the stove cools within a couple of hours. I assume that insulating the flue liner would reduce the heat transfer into the masonry, with more of the heat going straight up the chimney and out the top. It might help the flue to get up to temperature quicker, but beyond that, I see it as a general negative, though of course I may be naïve to the problems I'm stocking up for myself.

I'm pretty sure the recommended flue size for our 5kW stove is 4", but I installed a 6" in line with building regs. Ironically, when the building control officer came to sign it off, he didn't check anything, I had to keep asking him if he wanted to check this or that. The only thing he asked off his own back was about the CO alarm, which was on a shelf; he said it needed fixing to the wall, I said I'd fix it while he waited, but he said it was ok he'd sign it off anyway!

Offline SamT

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Re: Wood burners
« Reply #37 on: January 21, 2021, 09:39:20 pm »
As an aside - Building control in this country is just shit.  North of the border they're a bit more on it, with their own building regs etc and the inspectors seem to want all the i's dotting etc.

I spend an in-ordinate amount of time explaining the building regs to them which is surely their job. 

When the guy popped round to sign off on our new office (converted/refurbed) from on old shop he just popped his head in the door, wobbled the glass balustrade a bit with his hand, sucked his teeth a little and said cheerio (the glass balustrade is meant to have a metal capping rail, which presumably gives it much more strength, but the office owner didn't like the look so left it off).

Problem is, the government privatised them all.  So the building control are effectively working for, and paid by the developer. Hence everything gets passed off, presumably no questions, else they won't get repeat work.  If there isn't a conflict of interest there, then I don't know what is!

Online cavemanmike

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Re: Wood burners
« Reply #38 on: January 21, 2021, 09:46:09 pm »
I was under the impression that the heat in your fire would be pulled up the COLD chimney first before heating up your stove and therefore you're room. Please correct me if I'm wrong but I've had a conversation with a hetas engineer.
And also my chimney is 500 years old and 5 foot x2 which takes a lot of heating if not installed correctly

Online PeteHall

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Re: Wood burners
« Reply #39 on: January 21, 2021, 09:56:30 pm »
I was under the impression that the heat in your fire would be pulled up the COLD chimney first before heating up your stove and therefore you're room. Please correct me if I'm wrong but I've had a conversation with a hetas engineer.
Sounds reasonable, though it's a while since I studied thermodynamics and I missed most of the lectures anyway as I was busy caving  :-[

Quote
And also my chimney is 500 years old and 5 foot x2 which takes a lot of heating if not installed correctly
I grew up with one like that. The bottom of the chimney was sealed off with a bit of fire cement board (with an access hatch) and the stove pipe went about 6 foot above that into the cavernous chimney. I doubt it was very efficient, but we had access to plenty of wood  ;D

My current chimney is about 50 years old and 9" square, with a 6" liner up the middle...

Offline SamT

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Re: Wood burners
« Reply #40 on: January 21, 2021, 10:13:18 pm »
I was under the impression that the heat in your fire would be pulled up the COLD chimney first before heating up your stove and therefore you're room. Please correct me if I'm wrong but I've had a conversation with a hetas engineer.
And also my chimney is 500 years old and 5 foot x2 which takes a lot of heating if not installed correctly

Not sure what you're getting at.  A cold chimney/flue never draws well, once its warm the hot gases in the flue will all rise, creating a much better draw.  An insulated flue liner heats up quicker, thus produces a good draw quicker.  I remember as a child trying to get fires 'drawing'.  Occasionally ending up with the living room full off smoke from back drafts/down drafts.  Once the chimney is warmed through, no bother. 

To try and visulise it, think if the cold air in your flue as water, and the warm air in the fire box as air bubbles.  Initially, with only a few small bubbles, these will rise up the chimney, fighting against the restriction all the way up, and some of the cold water will sink back down the flue to replace the 'space' the bubble vacated below.  Once up to temp. the whole flue is "all bubble" rising up, with no more water in.

Online ChrisJC

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Re: Wood burners
« Reply #41 on: January 22, 2021, 06:42:44 am »
And that's where you let yourself down.  That's that attitude that over the decades has gotten us where we are,  the "Fuck it" type of attitude that pumps my nads.

Perhaps my point isn't clear. What I am trying to say is that just having a well meaning piece of legislation to 'green' something isn't good enough. It must take a holistic view of what it is trying to achieve. In the case of Drax, the idea of running a power station on biomass is laudable. However, if it comes from green timber chopped down in the US, kiln dried with gas powered kilns, then shipped 3000 miles to the UK on lorries and ships, then burned, perhaps the overall CO2 emissions from the process are actually worse than the coal that it is supposed to replace. So the net effect is worse for the planet, and worse for the coal mining industry.

Sadly we have a habit of making well meaning legislation that fails to look at the big picture, particularly environmental legislation.

Chris.


 
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Offline SamT

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Re: Wood burners
« Reply #42 on: January 22, 2021, 08:32:50 am »

Quote
It must take a holistic view of what it is trying to achieve. In the case of Drax, the idea of running a power station on biomass is laudable. However, if it comes from green timber chopped down in the US, kiln dried with gas powered kilns, then shipped 3000 miles to the UK on lorries and ships, then burned, perhaps the overall CO2 emissions from the process are actually worse than the coal that it is supposed to replace. So the net effect is worse for the planet

We can certainly agree on that.  Hence my comment re the Port Talbot plant.  (the stuff burnt at drax was local).  But its still no excuse to be burning coal and whilst you may have some whistful sympathy for the coal miners of the 80s, that industry is (in this country at least), and in my view rightly so, consigned to the history books.  We have the technology, the economic means and just lack the government wherewithal to implement low carbon alternatives.

Online Fulk

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Re: Wood burners
« Reply #43 on: January 22, 2021, 05:00:01 pm »
SamT:
Quote
Ideally, you should use a small 4" flue liner (for a small 5 or 8 kW stove) to keep the velocity of the flue gasses higher, ensuring they get up and out the top of your flue before cooling.  However, building regs stupidly says its got to be 6 inches, for some unknown reason.

Well, we have a wood-burning stove rate at a nominal 4 kW; the manual that came with it specified that the flue should be 5 to 6 inches in diameter.

Online Fulk

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Re: Wood burners
« Reply #44 on: January 22, 2021, 06:49:17 pm »
incidentally, ours is an 'inset' stove (not what we really wanted, actually), but it's very efficient at heating our siting room.

Offline SamT

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Re: Wood burners
« Reply #45 on: January 23, 2021, 11:38:27 am »
SamT:
Quote
Ideally, you should use a small 4" flue liner (for a small 5 or 8 kW stove) to keep the velocity of the flue gasses higher, ensuring they get up and out the top of your flue before cooling.  However, building regs stupidly says its got to be 6 inches, for some unknown reason.

Well, we have a wood-burning stove rate at a nominal 4 kW; the manual that came with it specified that the flue should be 5 to 6 inches in diameter.

Shrug  -  principal still applies, narrower the flue the greater the velocity of the flue gasses, less the chance of tar a nasties building up.   Obviously it has to be suitably large enough to cope with the volume of gases being produced.

Offline SamT

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Re: Wood burners
« Reply #46 on: January 23, 2021, 11:52:14 am »

Just as an aside - can everyone stop using the word 'efficient'  incorrectly.    :furious:

I have to deal with it every day at work and it pisses me off.

Your stove is very 'effective'  or 'good' at heating your room.


To use the word efficient, you always have to be considering two quantities, a ratio, a percentage.   In this case, its raw energy in (logs), heat energy out.   I suspect your log burner is not at all "efficient" as a large proportion of heat energy goes up out the flue, not into the room, especially if its an inset.

But that's not to say it doesn't effectively heat up your room to a nice cosy temperature, since the proportion going into the room is sufficient to heat it.

[/rant]

Also - human thermal comfort is a funny and fickle thing.  We actually feel quite comfortable when presented with 'radiated' heat.  The warm sun on your face, even though your on top of a ski resort at 3000m  in the alps,  stood in front of a bonfire on Nov 11th, even though the air temp all around you is only 3 deg.

Big black iron stoves 'radiate' the heat straight at you (and the surounding walls furniture), which in turn heat faster.  We 'feel' that heat on our skin/face etc and it makes us 'feel' cosy and warm.  Whereas a white central heating radiator (Ha!) radiates virtually nothing, especially when its hidden behind the couch, and warms the air in the room (slowly) which convects around the place(can lead to drafts) and doesn't necessarily make the occupant 'feel' warm.  Stone walls will still feel cold to the touch, and thus an occupant can 'feel' cold as heat radiate from them, out to the walls.






Online cavemanmike

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Re: Wood burners
« Reply #47 on: January 23, 2021, 12:19:08 pm »
You do like a rant don't you Sam  ;D ;D ;D

Online ChrisJC

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Re: Wood burners
« Reply #48 on: January 23, 2021, 12:34:32 pm »

Just as an aside - can everyone stop using the word 'efficient'  incorrectly.    :furious:


Are we allowed to say that the woodburner is much more efficient than an open fire?

And out of curiosity, are they more or less efficient than a gas or oil boiler?

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Offline pwhole

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Re: Wood burners
« Reply #49 on: January 23, 2021, 12:53:39 pm »
My physics teacher at school used to regularly rant that if everyone painted their radiators matt black instead of gloss white, they would be so much more efficient. Anyway, I have central heating, and it takes 15 minutes to warm up my entire (2 bedroom) flat for about three hours before it's noticeably chilled off. And I have the window open much of the time when I'm smoking. I just put a jumper on if I'm cold. The last winter quarter gas bill (Jan-April) was £72.19, and that also includes all my cooking and all my hot water. Between April and October I never have the heating on as it's too warm.

 

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