Stove chimney advice

[MtB]

The logic in the article is fundamentally flawed and wrong in my opinion. 

His premise is that an insulated flue pulls better so the fuel burns 'better'. This overlooks the fact that even a short and uninsulated flue in a boat pulls too hard and has to be throttled back to prevent the stove burning too fast and hot. If the bottom vents on a boat stove are left wide open it will burn really fiercely and hot, and on single door stoves if the ash pan door is left open the stove can easily glow red hot.

Lots of us older boaters know about this red heat risk from personal experience, and safety is the reason most stoves are made with a single door these days. 

There are other reasons an insulated flue might be a good idea e.g. to prevent acid corrosion at the top of the flue as Bizz pointed out earlier, and improved safety from reduced flue surface temperature, but increased fuel efficiency due to greater flue draw cannot be one of them. 

 

Edited April 15, 2018 by Mike the Boilerman
Interfering spell checker

[Robbo]

6 hours ago, Mikexx said:

The article contradicts itself. It says "The hotter the inside of the chimney (and the taller it is) the more vigorously the smoke and gases inside rise,
and the more vigorously fresh air is pulled in against the fuel, making it burn", yet the unisulated chimney provides initially more heat to start with

Where does the article contradict itself?  It’s quite clear to me and makes sense.

[Robbo]

5 hours ago, Mike the Boilerman said:

The logic in the article is fundamentally flawed and wrong in my opinion. 

His premise is that an insulated flue pulls better so the fuel burns 'better'. This overlooks the fact that even a short and uninsulated flue in a boat pulls too hard and has to be throttled back to prevent the stove burning too fast and hot. If the bottom vents on a boat stove are left wide open it will burn really fiercely and hot, and on single door stoves if the ash pan door is left open the stove can easily glow red hot.

If you read it again, you’ll find that the insulated flue has better draw when the fire is banked down and is more controllable.  A more modern defra approved stove is around 80% efficient compared to old stoves of around 60-70% so less heat goes up the flue.  More efficient stoves require a good draw as they won’t keep the flue as hot.

[Neil Smith]

People talk about the insulated v uninsulated flue all the time but at the end of the day they both work well and it comes down to pay your money and make your choice.

Neil

[Robbo]

39 minutes ago, Neil Smith said:

People talk about the insulated v uninsulated flue all the time but at the end of the day they both work well and it comes down to pay your money and make your choice.

Neil

If I had a old stove I prob wouldn’t bother, if I had a modern efficient stove I prob would.   

[Dr Bob]

9 hours ago, Mike the Boilerman said:

The logic in the article is fundamentally flawed and wrong in my opinion. 

His premise is that an insulated flue pulls better so the fuel burns 'better'. This overlooks the fact that even a short and uninsulated flue in a boat pulls too hard and has to be throttled back to prevent the stove burning too fast and hot. If the bottom vents on a boat stove are left wide open it will burn really fiercely and hot, and on single door stoves if the ash pan door is left open the stove can easily glow red hot.

I think the article is quite accurate in what it says but its what it doesnt say is the issue.

Mike is also right on stoves which have a good draft but not all do. We've got a boatman that has a really good draft when lit from cold (and has been well cleaned out) but after a couple of weeks of 24/7 is quite clogged up with ash and is boardline 'good' draw. However in this state it stays in overnight with no problem but is not as controllable as I would like. If it had a double skin flue then the pull would be increased and it would be more controllable...... but our works so why worry?

....BUT, the flue on our stove has an area of almost the same as the stove surface itself (4.5sqft for the flue viz 6sqft for the stove). That means we have almost twice the area to emitt heat...which is important when it is cold on the boat. You can see in the graph in the article that the heat put into the boat is more with the double skin but over double the time so the ability to heat the boat quickly is far less. To get the heat in, you would have to run the stove much hotter which long term cant be a good idea - can it? No wonder they can burn anthracite, it must be running a good deal hotter.

So bottom line, better fuel savings with the insulated flue and more control but less ability to get heat into the boat when needed and the need to run the stove hotter. If there is enough draw on your current stove then I think it is bad to have a double skin, but if your stove struggles to stay in overnight - as many people seem to complain....and most recently Roland.... then a double skin flue sounds a good plan.

[rusty69]

3 minutes ago, Dr Bob said:

but if your stove struggles to stay in overnight - as many people seem to complain....and most recently Roland.... then a double skin flue sounds a good plan.

....Or a decent stove like a Morso Squirrel (other stoves are available)

[Dr Bob]

6 minutes ago, rusty69 said:

....Or a decent stove like a Morso Squirrel (other stoves are available)

Maybe that is why this issue arises. If the Squirrel always has a good pull then that article is more for the peeps who have inferior ones. We got a boatman fitted as there was a big crack in the squirrel that came with the boat. As it welded steel rather than cast iron (is that right?) it is not likely to crack but although it is no where near as controlable as a squirrel, it has worked fine for us apart from a few days when I didnt clean the ash out well enough. A bit more draw would be nice but not worth faffing about with.

If one of your squirrels has a good pull, how do you stop your two getting together and producing a range of squirrels?

[Robbo]

The standard squirrel is around 76% efficient, so on the low end compared to a decent modern stove.  Although I don’t know if this figure is for wood of solid fuel (solid fuel tends to be lower)

Edited April 15, 2018 by Robbo

[rusty69]

4 minutes ago, Robbo said:

The standard squirrel is around 76% efficient, so on the low end compared to a decent modern stove.  Although I don’t know if this figure is for wood of solid fuel (solid fuel tends to be lower)

Is wood not a solid fuel then?

[Detling]

On 03/04/2018 at 13:30, Mac of Cygnet said:

Just to throw fuel on the fire, research has been done which seems to show, counterintuitively, that an insulated flue makes a stove more efficient at heating the space around it.

http://www.tigerboats.co.uk/images/forum/insulated-flues.pdf

(I have a single wall flue.)

but to get the same heat output in to the cabin  the fire burns fasted so will need stoking more often and be a b***er to keep in overnight.

[Robbo]

14 minutes ago, rusty69 said:

Is wood not a solid fuel then?

No, wood tends to burn more efficiently.

[rusty69]

3 minutes ago, Robbo said:

No, wood tends to burn more efficiently.

Oh.I didn't know that wood wasn't considered to be a solid fuel. I do love burning oak,specially this time of year.

Edited April 15, 2018 by rusty69

[Robbo]

16 minutes ago, rusty69 said:

Oh.I didn't know that wood wasn't considered to be a solid fuel. I do love burning oak,specially this time of year.

It’s still a solid fuel, but most decent stoves will quote both efficiently figures, and in wood burning stoves you can’t burn “coal” type fuels.  You will find most stoves classed as multi-fuel (ie wood & coal types)

Edited April 15, 2018 by Robbo

[Robbo]

2 hours ago, Dr Bob said:

So bottom line, better fuel savings with the insulated flue and more control but less ability to get heat into the boat when needed and the need to run the stove hotter. If there is enough draw on your current stove then I think it is bad to have a double skin, but if your stove struggles to stay in overnight - as many people seem to complain....and most recently Roland.... then a double skin flue sounds a good plan.

The draw isn’t really down to the stove but the height and temperature.  As a lower efficiency stove like the Morso will keep the flue warmer when banked down then a single skin flue sounds good.   If the stove is more efficient (I’ve seen quotes of 87%) then a insulated flue may be required to keep the draw when banked down.   So if your stove struggles to keep in overnight it may be because you have a crap stove or need a insulated flue because your stove doesn’t send enough heat up it.

[Robbo]

In the article they say they use a Portway stove, thought I would look up the efficiency of the stove...   it’s quoted as 104%, how does that work?  https://www.portwaystoves.com/products/portway-marine

 

edit; found how it’s calculated; https://www.kiwa.co.uk/uploadedFiles/Our_Services/Testing/Step by Step Guide to ErP and Solid Fuel Part 1(1).pdf

its 78.8% efficient, the 104% is a energy efficient index number.

Edited April 15, 2018 by Robbo

[Dr Bob]

1 hour ago, rusty69 said:

Oh.I didn't know that wood wasn't considered to be a solid fuel. I do love burning oak,specially this time of year.

Do try and burn some of those Oak promiscuous mothy thingies at the the same time.

 

24 minutes ago, Robbo said:

In the article they say they use a Portway stove, thought I would look up the efficiency of the stove...   it’s quoted as 104%, how does that work?  

Are they using the same algorithm as Smartgauges?

[Robbo]

19 minutes ago, Dr Bob said:

Are they using the same algorithm as Smartgauges?

No they had a ecofan on top ;D

[Neil Smith]

18 minutes ago, Dr Bob said:

Do try and burn some of those Oak promiscuous mothy thingies at the the same time.

 

Are they using the same algorithm as Smartgauges?

If they are then you probably need 2 for checking the first one is working correctly.

Neil

 

[Dr Bob]

1 hour ago, Robbo said:

The draw isn’t really down to the stove but the height and temperature. 

Does the stove affect how much draw you get? I am not that well up on boat stoves but had a Vermont castings wood burner when we lived in Scotland (for 20 years) that was excellent - 2 door, great draught. This boatman of our is limited because it only has one door and now and again seems to suffer a lack of draw which I have put down to it being clogged with ash after researching longer chimneys or top hats etc.

[Robbo]

4 minutes ago, Dr Bob said:

Does the stove affect how much draw you get? I am not that well up on boat stoves but had a Vermont castings wood burner when we lived in Scotland (for 20 years) that was excellent - 2 door, great draught. This boatman of our is limited because it only has one door and now and again seems to suffer a lack of draw which I have put down to it being clogged with ash after researching longer chimneys or top hats etc.

Yes, the gases must stay hot to rise so the more heat goes up the chimney the hotter they will be.   

http://www.soliftec.com/chimneyeffect.htm

[Dr Bob]

6 minutes ago, Robbo said:

Yes, the gases must stay hot to rise so the more heat goes up the chimney the hotter they will be.   

http://www.soliftec.com/chimneyeffect.htm

Ok, I understand all the tech stuff about pressures, temps and flue dimensions but what about the stove itself. How does that affect the draw? I know the heat generated by the stove determines the fule temp but what about the design of the stove itself. How does that affect the draw?

[Robbo]

1 minute ago, Dr Bob said:

Ok, I understand all the tech stuff about pressures, temps and flue dimensions but what about the stove itself. How does that affect the draw? I know the heat generated by the stove determines the fule temp but what about the design of the stove itself. How does that affect the draw?

I presume that would be ‘route’ and ‘air supply’.

[rusty69]

48 minutes ago, Dr Bob said:

1 hour ago, rusty69 said:

 

Do try and burn some of those Oak promiscuous mothy thingies at the the same time.

I've heard of a hornet moth, but not a horny moth!