Wood Burner Problem

[delta9]

Hello guys,

Wondering if anyone can help me out with this wood burner.

Basically I've built a wood burner out of a gas bottle and it does
not work as I intended... Bit of a disaster! lol

I did not follow any plans/dimensions as such... Just youtube/google for some ideas.

The main problems:

1. The primary intake does not work at all, no air is being sucked in via
this design, I know its not working because I have to keep the door open slightly
to keep the fire going... I made it this way in hope of keeping the glass clean, but
clearly this has been a massive fail.

2. Secondary air intake not working either, no secondary combustion/flames taking place and lots of smoke from flue.

Not sure what I have done wrong, perhaps I need to put the primary air at the bottom
of stove instead?

The secondary air manifold was made from 30mm square tube (3mm wall)... There are 3.4mm holes on all 4 sides of the tube.

I know flue length plays a factor in the draught/vacuum and I'm using 125mm mild steel tube (3mm wall), as pictured - this runs 1250mm... After this, the flue then carries on a further 2000mm with galvanised 125mm tube... So total flue length is 3250mm.

Maybe because this flue is not insulated could be a factor I dont know! Perhaps it
isnt long enough either.

The wood used for the test burn was dry too, it was offcuts of pine battens.

Any comments or suggestions would be most appreciated,
Thanks

 

[addjunkie]

The long uninsulated flue will cool rapidly as the gasses rise, making them more dense, possibly comming back at you, stopping the draw and filling your space with carbon monoxide among other things.

Stick the fire outside with out the flue extension, does it burn well with the door closed.

This will confirm or not the flue issue. If it burn badly try putting the logs on a small grate, see how it burn, you shouldnt with logs, but its all about adequate air at the point of combustion. Secondary air is a dark art.

 

[Mikes]

Primary air needs to come in under the grate, secondary air just above the fire level. Primary air will give you the combustion, secondary air is the 'fine tuning' to ensure complete combustion and limit smoke. Trick is to fine tune the secondary air such that you achieve the foregoing without wasting heat up the chimney. It looks like your primary air is fed from the top down in the box sections. If that is correct then the airflow will try to go the opposite way as it warms due to convection. Commercial manufacturers try to keep the glass clean by bringing the air in just below such that it tends to pull dirt away from the glass.

 

[delta9]

@addjunkie

Thanks for the info, yes I have been testing it outside, but I am intending to put
this burner indoors (garage) once it works (carbon monoxide alarms will be used)... Due to the height of garage most of the flue (2.5m) will be indoors, thats why I thought I could use the cheaper (uninsulated) option.

Initally I did test burn with just the 1250mm flue - But No it didnt burn with door closed, then I added the 2000mm flue onto it, this resulted in the same outcome - Fire will not burn with door closed... Opening/Closing of primary air valve does nothing.


@Mikes
Thanks for that,
The primary air was supposed to come from the top right (see pic), down and over glass.

The box sections (manifold) is the secondary air intake.

Maybe the hot air outlet of the secondary air manifold is too high above the fire?
It is 300mm from grate...

Cheers

 

[rtbcomp]

Normally with wood the air comes from the top, with coal &c it comes from underneath.

Did you allow enough time for the flue to heat up?

 

[Gareth J]

The inlets (i.e. where air from the room enters the stove) for both your primary and secondary should be very low.

Where are they on this?

Also, I'd guess (and it is a guess) that your outlets (i.e into combustion chamber) are very small.

The pressure differential isn't huge, especially before things have heated up so small holes may be too small to get any decent volume through. I would think you would be better with bigger holes and controlling flow at the valve.

Good woodburners "wash" the glass with the preheated secondary air. Coupled with double glazing of the window and the inside pane gets hot enough, and has enough excess oxygen, to burn off soot/tar

 

[julianf]

How long are your test runs?

The job won't work until the flue is hot. The hotter the flue, the less heat is taken out of the flue gas, and the greater the difference in density between it and the surrounding air.

If the top of the flue stack is still cold at the end of your test, that is going to be your primary problem.

Lighting a load of screwed up paper to start a fire does two things and only one of them is lighting your kindling. The other thing is it rapidly throws a load of hot air up the stack and creates the draw. Try using a whole load of screwed up paper (ie keep chucking it on) until the stack is hot and drawing well, and then try closing the door with plenty of kindling burning.

Solid fuel fires are a bunch of feedback loops. The hotter things get the hotter they try and get (and vice versa)

 

[delta9]

@rtbcomp

Cheers, that's what I thought air from the top was best, but its not sucking any air in.

Only ran it for about 1 hour, how long should it take?

Thanks

@johnser
Thanks, both inlets are about knee high, the round tube on the side/front
is the primary inlet, the square box section at the back is the secondary.

You could be right about the outlets holes being too small... I'll drill
them bigger now, worth a go

Cheers

@julianf

Thanks for info, hopefully its teething problems

I'll test it tonight...

Thanks

 

[rtbcomp]

I would have thought an hour was plenty long enough to warm the flue. Mine gets going after about 15 mins or so.

 

[julianf]

Even a week of smouldering isnt going to heat the flue.

And if the flue isnt hot, then it may be that it never gets beyond smoldering, unless some quick (energy) release fuel is used.

What i guess i mean to say is its just too complex to put simple figures to?

Get it hot - hotter than you think it needs to be, and then try. From the photos it does not look like its had much of a burn at all.

 

[Gareth J]

Try extending the inlets to floor level, a temporary test bodge should be easy enough to sort if you want to try it

 

[ibrooks]

I think the first thing I'd do is open up the hole to the flue, I can't see those small holes flowing enough air to allow any draw unless the flue is practically glowing.

Thinking about it and as a test whilst it's outside..... try throwing a pile of burning paper down the flue to kick the draw off and see if that helps.

 

[julianf]

I think the first thing I'd do is open up the hole to the flue, I can't see those small holes flowing enough air to allow any draw unless the flue is practically glowing.

Thinking about it and as a test whilst it's outside..... try throwing a pile of burning paper down the flue to kick the draw off and see if that helps.

is this the gas exit point -

If so, there is 99.9% of your issue!!!

 

[steviec_lj]

Was about to mention the above, that if your outlet is the other side on that circle of holes, then you need to remove that section, with all or most of it..

The heat you’re generating is heating the firebox, and that’s about it

 

[gaz1]

For start off your chocking your fire I believe with those silly holes I your baffle plate make it a bigger hole

PRimary air needs a push as incoming air needs to rise same with your secondary air so piping them back to the lowest point of your system or via an outside source

Also suspect not enough holes for primary air either some times a slit is better

Look for caotrpheus on you tube look at his vids hes done the sameish build like you

 

[Screwdriver]

I would expect to see a primary air inlet right at the bottom front. What you label as "primary air" is way too small, way too high and more like a secondary inlet. It looks like a really good build and I think you have a good chance of making it into a decent stove but it needs air. If it was me, I'd have a huge slot underneath with a sliding gate. You can always choke off the inlet if it's too big but if it's too small, you'll always be choking the fire.

 

[waddycall]

If those holes are the flue exit then it is choked and can’t draw air in. Probably best not light it again if that’s the case as itll probably be spilling flue gasses too.
The trick to designing a good stove is to get the gasses to circulate inside, mixing the air and gassified fuel. A baffle plate which forces the flue gasses towards the front an inch or two away from your “primary” air inlets will help with that. You want it to tumble forwards and get mixed in with the air coming in, this is how airwash works in commercial Stoves. Downwards not up.
Many continental wood burners don’t have primary air. They just have secondary and tertiary which is what you have.

 

[Screwdriver]

@waddycall That's interesting. I was tempted to suggest a baffle plate (I fitted one to my first attempt) but seemed like too much info in one go. Never knew how those foreign stoves worked, you wouldn't happen to have any design shematics by any chance?

 

[sardine]

To repeat what others have said, your third picture shows little holes where the flue joins the top plate ?

It should be one unrestricted hole the size of the flue pipe.

That has to be done before any other problems can be addressed.

Such a stove will draw happily with two metres of uninsulated flue pipe, narrow boats have them for a start.

 

[delta9]

Thank you all for the input :-)

Well you were right, this stove was starved of air/oxygen... Or flue problem...

I made the primary air outlet hole bigger and this made a massive difference (I also took away the rope on the primary inlet for a bigger aperture) - The fire is now keeping alive with the door closed.

Big improvement there, however it is still on the smoldering side, so needs a few more adjustments to make thanks to all your recommendations.

Yes, the holes on the baffle plate is where the flue gases exit (as pictured red circle).... So I'm going to cut that baffle plate and do away with the hole design... Not sure where I got that idea from .....

With regards to the primary and secondary air, I intend on using this stove
indoors, so I want these both air inlets to be outside - to bring in fresh air... in that case do I still need to extend these vertically towards the ground (outside) as pictured?

Thanks again, hopefully nearly there with it!

See how it runs tonight with the flue restriction holes removed,