Anyone with experience of Stovax Stockton wood burner?

[David]

I'm trying to work out why the secondary air jets which are supposed to
help the clean burn aren't currently working.

I've got as far as this.

The air jets are in a plate in the back of the stove.

They are in an area which is just below the baffle plate across the top of
the inside of the stove which draws the exhaust gasses to the front of the
stove before they flow over the baffle plate and into the flue.

This seems to be a reasonable design for a clean burn, and usually seems
to clean the glass (more or less).

However there are secondary air jets at the back and I am struggling to
work out what might be blocking the air flow to these.

Diagram is shown within
<https://www.stovax.com/appliance/stoves/wood-burning-stoves/buying-wood-
burning-stove/how-a-stove-works/>
under the heading "Stovax Cleanburn System".

I've removed the baffle plate but I'm...errr...baffled how to confirm
there is an air flow from the base of the stove (where I can feel the
inlet) up the back and out through the air jets at the top.

There was a bit of crud there (but that may have come from the baffle as
it was removed) and I have vacuumed it out but I can't seem to feel any
obvious opening for the air to come up through. No sign of any crud
dropping out through the bottom of the rear vent either.

The parts breakdown shoes that the rear metal plate (behind which magic
happens) is a replaceable part but that seems like very major surgery, and
I so far haven't found any instructions on how to remove and replace it.

So I am asking if anyone else has got their hands dirty with the Stovax
Stockton range and has any light to shed.

Cheers



Dave R

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[David]

Updated to say:

Further detailed investigation (OK scrabbling about inside) showed that
the secondary air supply didn't go anywhere near the baffle but came up
inside the hollow fire back.

I did have some fun with my Aldi endoscope thing which I had forgotten
about and which is good for investigating inside stuff.

So no real idea what is going on.

I do have a much clearer idea about how the whole thing is supposed to
work, though.

I've put it all back together and just lit the fire so we shall see if it
has made any difference.

I could possible use a smoke source (recently extinguished candle?) to see
if any air is being drawn in through the bottom vent, I suppose.

A smoke pellet would probably choke the room up with smoke.

Still looking for anyone who has experience with the Stockton range.

[Tim+]

Unless you have enough heat to drive convection I doubt a smoking candle
will tell you anything useful.

Tim

--
Please don't feed the trolls

[Jeff Layman]

It's almost certainly not Stockton unique. I have a Scan Andersen 4-5,
which, from the webpage you linked to would seem to have an identical
mode of action. Its manual also mentions Cleanburn, and has an airwash
to keep the glass clean (it's a multifuel stove). I can't answer your
question, but would be interested if anyone can explain why logs are
said to burn best on a bed of ash - with no air flow from beneath - but
solid fuel needs to have a grate to burn well.

I can't understand how wood is supposed to burn well when air is
introduced only from above. It makes no sense to me, especially as that
Stovax webpage said the heat output of wood and solid fuel is basically
identical. From my experience, it is very difficult to get added logs
burning well unless the door is left ajar for a short while. And it
seems very important to keep the chimney warm so there is a good
convection flow to draw air in.

Usually the stove works well, but sometimes it seems to have a mind of
its own and is difficult to keep alight. I might have to add kindling to
get it going again.

--

Jeff

[Steve Walker]

On 06/02/2021 16:28, David wrote:

Many years ago, the company I worked for used to use glass tubes with
chemicals in. IIRC we had to shake them a bit and then snap the end off
and they gave a minute or so of smoke, but not huge amounts. The trouble
was, as most of our work was in food factories or printworks producing
food wrappers, we had to get special permission to take glass onto the
shop floor each time.

There were also smoke matches - which it seems Toolstation still sell.

[Andrew]

Gazco are down in Devon so you can phone them and talk to an engineer,
which I have done in the past when I had a query with the seals they
use for their balanced flue gas stoves.

[Lawrence Milbourn]

On my Stockton 4 the air for the back jets doesn't come from below but from a hole in the rear plate. It is covered bu a small oval plate secored bg a single screw. The instructions were to pivot this plate to get the required air flow. Has this hole got blocked or has the oval plate turned to block it?

[AJH]

On 06/02/2021 19:46, Jeff Layman wrote:
> would be interested if anyone can explain why logs are said to burn best
> on a bed of ash - with no air flow from beneath - but solid fuel needs
> to have a grate to burn well.

Firstly I have little experience of the Stockton 5, one of which is
installed at my daughter's house. I bought it as it was affordable and I
wasn't sure it would be used much. In the event my granddaughter doesn't
use it all. I disliked it because the handle got too hot to re stoke
without a glove on. I have a Morso s11 at home and I am delighted with
it, it is so much better than the Jotul 602 it replaced. Were I less
happy I would look to buy a Charnwood Aire.

A couple of points about these modern insulated firebox DEFRA approved
stoves with preheated air wash; the air paths are convoluted so until
the flue is good and hot (above ~110C) they will not draw well, also if
you allow them to die down such that there is a not a blazing flame they
can smoke badly before a flame re establishes.


Yes when you open the door to reload there is a spike of particulates
into the room.

Your point about not requiring under grate air is because 70% of the
energy available from burning wood comes from the volatiles that offgas
from the wood as it pyrolyses, this is what burns in the diffuse yellow
flame as it reacts with the preheated downwash air that also keeps the
door glass clean. The jets of air from the middle back are also
preheated in the metal plenum they emerge from and these are normally a
fixed tertiary air supply to make sure the fire cannot be choked down
and smoulder. The smoke from smouldering wood being products of
incomplete combustion i.e. wasted chemical energy as well as pollution.
The airwash sweeps down the glass in a circular path and thus also
provides enough air to oxidise the char directly to CO2 in a glowing red
bed with little blue flames of carbon monoxide being visible

A smokeless coal fire is different in that it is largely solid carbon
with fewer volatiles given off so air is introduced under the grate and
nearly all the heat comes from oxidation of the carbon. To make this
happen fast enough the oxygen in this primary air first dissociates on
the glowing carbon and combines to form CO2 and a lot of heat, the hot
coals radiate and convect heat to the upper layers and drives off what
volatiles there are but as the bed of coals deepens there is a further
reaction of reducing a portion of the CO2 back to carbon monoxide, which
cools the bed as it is endothermic. The equilibrium of these two,
oxidation and reduction reactions is determined by the bed temperature
and depth.

The mixture of gases from these reactions rises above the bed where it
meets the downwash secondary air where the CO is oxidised with a bluish
flame above the red hot coals.

[Jeff Layman]

Thanks for spending the time to write such a detailed explanation. I had
a look at the manual for your Morso S11, and found it is somewhat more
informative than that for the Scan Andersen 4-5 (which, by the way, is
now sold as the Jotul 145). It is interesting that, although much is
identical, there are obvious differences between the two manuals. In
particular, when first lighting a wood fire, the Morso says that only
secondary air is used; the Scan says that "The ash grate should be
half-open when the fire is lit". So that involves partial primary air.

Although I think much of the explanation you gave mentioning gas
released from the wood makes sense, and I'll return to that in a minute,
it didn't answer the question as to why the wood is supposed to burn
better on a bed of ash. The Morso manual states "always keep 1cm of
ashes in the bottom of the combustion chamber", but does not explain
why. I thought this paper was interesting, but it does not mention ash
<https://erc.cals.wisc.edu/woodlandinfo/files/2017/09/mt198405hr.pdf>. I
would have thought it would have if it was that important. Quite a few
pages state that the ash insulates the wood from the body of the stove
and helps it keep warmer and so burns better. I can't see it making much
difference; once the stove is hot it is the chimney convection draught
which affects the burning (an aside - both the Morso and Scan manuals
mention a pressure of 12 or 13 pascals in the chimney for efficient
burning. I'm not arguing, and it's impressive to give a figure, but I
most certainly have no idea what that means in practice! I do, however,
have a thermometer on the chimney at the stove outlet and would agree
that if the temperature falls below 100°C or so burning tends to be
inhibited).

The gas burning from the secondary or tertiary air makes sense, and I
have most certainly seen the air flow from the tertiary vents at the
back "light up" as they contact the gas. But that gas wouldn't care
where the air came from - top or bottom - to allow it to burn. I feel
that it is probably easier to exclude bottom air as a confounding factor
which might make the fire burn too hot, and so leave it to the secondary
air wash (which can be controlled) and the tertiary air which can't. The
other point is that the ash grate soon gets blocked with ash, so air
could not enter the burning chamber through it. The only way round that
would be to cut away some of the ceramic rope seal between the ash
chamber and the burning chamber, and use the primary air control to
change the supply of air.

I also get smoke when door is opened. I think you are better off with
your Morso, as it has a 45 deg slope from the back to the centre of the
top. The "smoke flow" there has half the baffle at the top to help it
slow down before it reaches the front vent to the chimney. The Scan has
a baffle at about 40 deg from the back just above the tertiary air vents
to the front vent to the chimney, so it has a slope all the way, and the
force of air coming from the tertiary vents help it go right past the
vent to the chimney and out the top of the front!

--

Jeff

[David]

On Sun, 07 Feb 2021 05:53:00 -0800, Lawrence Milbourn wrote:

<snip>

> On my Stockton 4 the air for the back jets doesn't come from below but
> from a hole in the rear plate. It is covered bu a small oval plate
> secored bg a single screw. The instructions were to pivot this plate to
> get the required air flow. Has this hole got blocked or has the oval
> plate turned to block it?

On the slimline Stockton 8 there is a sliding flap underneath the stove
and a vent at the rear which is exposed or blocked off.

Our Stockton 4 multi-fuel which is in the shed still waiting to be
installed after 7 years has the arrangement at the back.

[AJH]

On 08/02/2021 13:20, Jeff Layman wrote:
> In particular, when first lighting a wood fire, the Morso says that only
> secondary air is used;

I find that the lever that controls air jams with ash and has to be
tapped to move it. As I never use it with coal I suspect the primary air
passage is blocked because if I push the lever left the fire goes out.

Anyway I find lighting this fire almost impossible without the air
control fully to the right and the door seal cracked open.

[Tim+]

That suggests to me that you’ve got a fairly serious blockage to your air
supply somewhere. When we had our wood burner installed, we started having
trouble with it being difficult to light and I eventually spotted that it
had moved backwards a ¼ of an inch which was then occluding the rear air
intake.

[AJH]

On 08/02/2021 17:13, Tim+ wrote:

> AJH <ne...@loampitsfarm.co.uk> wrote:

>> Anyway I find lighting this fire almost impossible without the air
>> control fully to the right and the door seal cracked open.
>>
>
> That suggests to me that you’ve got a fairly serious blockage to your air
> supply somewhere. When we had our wood burner installed, we started having
> trouble with it being difficult to light and I eventually spotted that it
> had moved backwards a ¼ of an inch which was then occluding the rear air
> intake.
>

Yes as I said I suspect the primary (under grate) air has become blocked
but the two others, the air wash and the fixed jets at the back, are
fine. The air control has a central null position and when to the lift
supplies a small amount of primary air and the secondary air for burning
coal, set toward the right it just controls the downwash.

If you think about it when everything is just lit the downwash opposes
the flow of combustion gases to the flue (the two slots are adjacent. By
cracking the door seal you are creating a simple updraught to kindle
the fire until the chimney has a bit of buoyant warm gas in it to create
a draw. Last time I measured it I think it drew 1/4" of water. Morso
suggest lighting with the door ajar.


It's rated at 4kW(t) and has been running at nearly full output and is
keeping this small semi above 20C downstairs while it is minus 1.5C out,
we have solid walls with no insulation (I keep meaning to do something
about this but get worried about damp condensing in the insulation to
wall interface). It's about 18C in the rest of the house and the chimney
breast will keep the whole hose above 15C from when the fire goes out
till 8am.

[AJH]

On 08/02/2021 13:20, Jeff Layman wrote:

> why the wood is supposed to burn better on a bed of ash.

My thought on this is a bit fuzzy but in general supplying one unit of
oxygen and 4 units of nitrogen will release about the same amount of
heat whether the fuel is wood or coal. Too much primary air will gasify
more char and require more secondary air to burn the CO. so you could
end up with too much heat and insufficient oxidation of the products .

Only varying the downwash air means that all the volatiles get burnt
first and then any air left gets to the char.


This can get quite pronounced with dry wood burned at high power, like
now, a char bed builds up a few inches thick and then burns away before
more wood is added. In fact I can produce about 1kg of charcoal a day if
I wanted by smothering it once the flame goes out.

If you consider oven dry wood is about 50% carbon and the remainder is
hydrogen and oxygen in near the same proportion as water and has about
15-18MJ/kg you will see this is close to burning the same 0.5kg of coke
with 30MJ/kg.

[Jeff Layman]

On 08/02/2021 17:03, AJH wrote:
> On 08/02/2021 13:20, Jeff Layman wrote:
>> In particular, when first lighting a wood fire, the Morso says that only
>> secondary air is used;
>
> I find that the lever that controls air jams with ash and has to be
> tapped to move it. As I never use it with coal I suspect the primary air
> passage is blocked because if I push the lever left the fire goes out.

Fortunately I haven't had ash block any lever.

>
> Anyway I find lighting this fire almost impossible without the air
> control fully to the right and the door seal cracked open.

Mine will light with the door closed, but when I top it up with logs I
have to leave it open to get the new wood alight properly.

The worse problem I had with the Scan is a safety design issue really.
One day I topped it up and closed the door after the new wood was
roaring away. When the door is closed the fire usually goes down
somewhat, but this time it didn't. It was just like a few weeks earlier,
when I'd had to replace the ceramic rope as it had become compressed,
and air was leaking in past it. But as I'd replaced that, it couldn't be
the cause. I then saw the fire inside, through a gap between the stove
body and the door. I thought I hadn't shut the door properly, so
refitted the removable handle and turned it to reclose the door. It made
no difference. I shut down the airwash completely, but like yours, there
is no control over the tertiary air. I then had to hold the door closed
until the fire went down. After the wood had burnt low, I opened the
door and found the bolt holding the latch was loose. I tried tightening
it, but it just turned. I removed it completely to find the end threads
were stripped! Luckily I had a spare 5mm bolt of the right length and
use that to hold the latch. It worries me that, if that screw fails,
there is no way the latch will hold the door closed, and no way anything
else can hold the door closed as the catch for the latch is internal and
inaccessible.

--

Jeff

[AJH]

On 08/02/2021 19:14, Jeff Layman wrote:
> The worse problem I had with the Scan is a safety design issue really.
> One day I topped it up and closed the door after the new wood was
> roaring away. When the door is closed the fire usually goes down
> somewhat, but this time it didn't. It was just like a few weeks earlier,
> when I'd had to replace the ceramic rope as it had become compressed,
> and air was leaking in past it. But as I'd replaced that, it couldn't be
> the cause. I then saw the fire inside, through a gap between the stove
> body and the door. I thought I hadn't shut the door properly, so
> refitted the removable handle and turned it to reclose the door. It made
> no difference. I shut down the airwash completely, but like yours, there
> is no control over the tertiary air. I then had to hold the door closed
> until the fire went down. After the wood had burnt low, I opened the
> door and found the bolt holding the latch was loose. I tried tightening
> it, but it just turned. I removed it completely to find the end threads
> were stripped! Luckily I had a spare 5mm bolt of the right length and
> use that to hold the latch. It worries me that, if that screw fails,
> there is no way the latch will hold the door closed, and no way anything
> else can hold the door closed as the catch for the latch is internal and
> inaccessible.

I had something similar at first and it was ash that had built up at the
back of the ash pan such that the ash pan did not fit as far in as it
should have. This caused the door to contact the ash pan and be slightly
distorted when the lever shut the door, allowing a small leakage of air
passed the bottom seal.