wood burners

[Simon]

I know this is not strictly electrical (until someone raises the control system ) but. As I cut down and shred a large tree I wonder about the practicalities of having a woodburner that feeds in pellets or shavings to burn. What really turns me off wood burners is that usually they are just furnace boxes that burn wood and like a regular fireplace still send most of the heat out of the flu. What I would propose is a burner that can feed in at the correct rate for the energy demand and be fanned by an electric fan that will push the exhaust through a heat exchanger that exchanges with air or water. Does such a device already exist ? are they stupid money or something worth considering.

I grew up in italy where people used olive pip burning furnaces to heat their homes much like i am hoping to do although I have no idea how they did the heat exchanging and they were big units.

[dr.diesel]

Those things are all over the place in rural US, commercial units run about $5k, but doesn't include the plumbing and installation costs.  Most of the units here are an outdoor unit with a glycol system to carry the heat inside.

[Simon]

Ah right, so they are quite efficient ?

[dr.diesel]

Not at all, for the most part.  I've not read anything recently, but a typical non catalytic burner was somewhere in the 50% range, but varies depending on load.  But also spews tons of particulate matter into the air, smoke like crazy.

These things are regulated by the air inlet, when the inside gets warm and the burner gets choked off, they smoke/smolder even more, at a super low combustion efficiency point.

They do make some fancier units, that once up to temp can burn the particulate/smoke in a catalytic chamber in the upper exhaust, but I've never seen one of those in action.  Probably 3x the cost as well.

[mib]

A typical wood burning stove should be roughly 75% efficient. 75% of the potential heat from the wood should end up heating your home.
That sort of efficiency does require it to be installed and used correctly, something that many people fail to do.

You can also get very efficient pellet boilers here in the UK that will burn any biomass that is cut into small enough pieces and appropriately dried. They are 2-3 times the price of an equivalent gas boiler - mostly due to the much smaller market. They are a little less efficient than a condensing gas boiler due extra losses in the fuel loading and ash removal systems. But they can be 85-90% efficient.

The biggest problem with pellet burners is the physical size of the unit plus space for the fuel, which rules out their use in most homes here.

[Simon]

I see, well outside I have plenty of space and I shred any wood matter that I need to dispose of as I generally allow it to compost but I don't have the sort of supply that might make it worth while. I don't really see why they can't be made very efficient by simply passing the exhaust through a heat exchanger. This idea comes from an article i read about a guy that used an oil burner to generate electricity with peltier cells and then fed the exhaust through a radiator and effectively achieving near 100% heat recovery.

I am currently cutting down a rather large apple tree hence my interest, again not worth getting a burner for but i wonder if there is a market for my shredded chippings.

[fourtytwo42]

What really turns me off wood burners is that usually they are just furnace boxes that burn wood and like a regular fireplace still send most of the heat out of the flu.

Like someone else said depends how there used and installed, I am lucky that my flue runs up the middle of the house and also passes one side of the airing cupboard so heating the brickwork is very useful.  I inherited the burner 13 years ago and immediately knew something was wrong as it had no internal baffles meaning as you say most of the heat went up the flue.  I made some heavy gauge baffles to force it into a serpentine path and now so much better. Proper control of the top vs bottom air also works wonders My only wish is that it had a back boiler but cannot have everything

[Simon]

To be honest I can't see anything not passing the exhaust through a proper heat exchanger as efficient. It is just a fact that most heat is carried away by the fumes so you have to recover it from there.

[fourtytwo42]

Your right but I didn't want to restructure my building and I would be very concerned about tar buildup, as the gasses cool then tars in the wood condense so your heat ex changer efficiency may well drop off rather rapidly, however I am sure someone has done it. I guess it also depends what you want to burn, in my case it's multi-fuel meaning anything I can get for free

[Simon]

yes that is true I suppose hence chimney's need cleaning. Maybe there is a way of burning the tar off by injecting some sort of liquid and highly flammable fuel.... kind of reminds me of the soot burning off that diesel cars do (or are supposed to in the case of VW)

[fourtytwo42]

Burning the tar off (aka chimney fire) is what happens here when city dwellers light the woodburners in an old house for the first time, they seem to think the more it roars the better then cannot understand why it roars and roars without any wood!! Many fine thatches have been lost that way too

[Simon]

Well clearly not something I should go dabbling in, if anyone is interested in some of the finest apple chip fuel let me know, I'll be storing it up nice and dry and hoping to shift it to someone that can use it as it will be too much for me to compost and the council aint having it.

[Ian.M]

It looks like good clean dry apple wood chips go for about £10/Kg for barbecuing and home smoking. 

[Simon]

Wow, I've cut down a gold mine [emoji6]. Do you think it has to be leaf free or does that add to the taste? I once sold a small quantity of apple for 40 quid I think I know why now....

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[mib]

You'll be disappointed in the value of your prunings...

I have a large garden, and I do maintenance on a number of neighbour's equally large gardens. I can easily produce 20+ cubic metres of shreddings over the next few months. So I made some serious queries about whether they had any value. The simple answer is my idea of a large quantity is industries idea of too little to bother with.

If you can provide 10 cubic metres or more of green waste all at once then you'll probably be able to find a local company that will take it away for free (actually they'll usually pay you a token £1 for it or something like that so it's not taxed as waste). That's probably the best you'll find. They only start paying when you can provide 100s of cubic meters on a regular basis.

If you're cutting down an apple tree then the wood does have value. Larger pieces can be sold to wood turners as long as they aren't rotten in the centre, which is rare with an old tree. Failing that any bits larger than 2" diameter cut into 8" lengths are worth £20-£30 per cubic meter as firewood. You can at least double that by storing it in a dry place for a couple of years and selling it as seasoned firewood.

To sell it as chips for smoking you'd need start with the bigger bits, remove all the bark, leaves etc, then kiln dry it and chip it - you're not going to make any money doing that on a small scale.

[Simon]

Oh so I won't be buying solar panels on what I make out of the tree [emoji6].yes there will be some nice size pieces for wood work but no I'm not going to that effort for the chippings.

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[Ian.M]

If you are keeping the wood for turning, you need to dip the ends in melted paraffin wax to prevent rapid moisture loss causing cracking.    For the rest of it firewood sounds like the best option. 

[Simon]

ok, is paraffin wax readily available, there will be a few nice pieces and the trunk is really thick, the house was built in 1947, I am guessing the land was an orchard and this is one, or the only remaining tree, would be nice for it to be turned into something nice (pun intended?) instead of just going for firewood. The small branches bub 30mm will be shredded, still good as fuel and too much to compost so will see if anyone wants it for burning.

[jpanhalt]

I didn't see this thread until now.   I happen to have a wood burner for "auxiliary" heat in my home.  It is an outdoor version. 

The burning unit itself is a large double walled boiler.   Fire is in the inner chamber and that heats water  (400 to 500 gallons) in the supply between the two walls.  The water in the outside boiler is at atmospheric pressure, and the burner operates at somewhat reduced oxygen.

 A circulating pump brings the water to the house.   There is a neat little heat exchanger in the house that heats the water used for domestic heat.  If the wood burner is off, domestic heat is provided by a propane boiler.  It is probably a bad idea to use a single water supply.   I have not seen that done.

Efficiency is of little concern as the fuel is waste wood and would probably be burned in a pit outside just to get rid of it.  No non-commercial individual I know actually buys the wood used.   He have a real surplus of dead ash at present.

The circulator motors are quite small, usually 1/25 hp to 1/4 depending on the BTU.   My propane house furnace is 100,005 BTU and uses a 1/25 hp circulator.   The circulator to the wood burner is 1/4 hp, but the pipe is larger (1-1/4") and the distance and water volume greater.

A medium size home will go though several cords of wood in a  Winter, so wood from a single tree would hardly make a dent.  Maintenance is minimal, except the larger circulator pumps are expensive when they need to be replaced.

As mentioned, they are quite popular in rural areas that otherwise depend on propane for heat.  One downside it that the propane dealers are allowed to charge more, if you have a wood burner.  That additional charge is in the form of disallowing certain discounts.   Nevertheless, a home that might take $2000 to $3000 to heat all season, will get by with less than $500 in propane.   A neighbor says his propane bill is only $100.

The other downside is the need to add wood on some miserable days.

[Simon]

In italy they burn olive stones that are available by the sack after the olive pressing season ends. they are usually installed in a cellar of rural houses (mains gas has arrived to most places). they have a big funnel shaped tank with an endless screw in the bottom so you can fuel up for an amount of time and leave it running like any other boiler.

I know what you mean about oh it's only scrap wood who cares about efficiency but meh, people in the UK think that installing wood burners is being green but ultimately they amount to boxes with pipes out the back and up the chimney, they are slightly easier than a fire and in my opinion just as inefficient, if we all installed them in the UK how would we fuel them all sustainably?

[G7PSK]

https://www.stovesonline.co.uk/wood_burning_stoves/Pellet-stoves.html

Easily available in the UK complete with subsidies.

[Towger]

It is better in Northern Ireland where there are big subsidiaries.  People with empty warehouses heat then just to claim the subsidiary.   Google Ash for Cash for details.

[Simon]

https://www.stovesonline.co.uk/wood_burning_stoves/Pellet-stoves.html

Easily available in the UK complete with subsidies.

Oh god not another solar FIT scam ? I'd struggle o fuel one really where I m so probably not worth it.

[Delta]

A wood burning stove is nearly an order of magnitude more efficient at heating a room than an open fire.  Do not be deceived by the simplicity and old fashioned-ness.

I assume you are using renewable energy to power your garden shredder of course...

[Simon]

Oh I assume they are very good, but I don't have a consistent supply of fuel and frankly the idea that using these is classed as renewable is ridiculous, yes renewable is a few thousand people lived in the UK, not a few million, when we gobble up everything we have what do we burn then ?

Well when my powervault arrives I could be running my shredder off 50% solar, maybe 100% if i do it when the sun is out so I have 1.6KW from a powervault and 1KW from my solar but that would take some timing and weather.