Using a Woodburning Stove Outdoors - Flue Length?

[Elliot Gold]

I've just aquired this disused woodburning stove that I'm intending to use as a patio heater. In the UK we're still restricted to meeting people outdoors due to covid and it's too cold to be comfortable in the garden. I thought that using a stove, rather than a fire pit, would be far more efficient in terms of heat & wood usage (and pollution!).

My question is about flue lengths. The instructions recommend a 4.5m flue, which is impractical with how I'd hoped to use it. The woodburner sits on a (fireproof) wheeled base and it's my intention to move it under cover when not being used, to avoid it rusting in the rain (ah the joys of British weather!). The maximum height that seems realistic for the flue is 2m. I'm wondering what the impact of that will be? I've tried searching the forum and googling for a better understanding, but so far the main issue seems to be the likelihood of smoke emerging when the fire door is opened to load wood, which will not be a problem outdoors. A reduced draw has also been mentioned, which I guess will make the fire more difficult to start, but I wondered whether that could be countered by simply opening the air-vents further? Most boat stoves I've seen have short chimneys, so I'm hoping this isn't a comletely silly idea!

Finally, I wondered if single-wall / double-wall / flexible flue liner would make any difference to this unconventional use of a stove? The latter being hugely cheaper!

Forgive me if questions are foolish. I've only ever operated pre-installed stoves, so this is all very new to me! I'm grateful for all comments, tips and advice!



I guess something similar to this is what I was envisaging:

 

[begreen]

It depends on the stove design, but yes, for one with just a simple baffle that would work. Modern low-emissions stove need more flue for stronger draft.

 

[Corey]

Interesting use case. I've been working on a patio fireplace for similar reasons, but I suspect a stove would actually throw more heat out to people near by.

As far as the pipe length, I suspect the formal instructions don't mean much at this point. The main reason for the specified length would be good draft to avoid excess smoke spilling out when you open the stove inside your home and compete with any 'stack effect' created by your house. But is some smoke spillage outdoors a big thing? Likely not. And you definitely won't have a stack effect.

I'd probably try to get the flue at least a few feet above the people and a few feet above roofs of any nearby structures - with the idea being you don't want smoke to linger around near the ground or be caught in a wind draft coming off a roof and swirl back to the ground. Double wall pipe would increase the draft for any given length of pipe and technically increase the stove efficiency a bit, but as this seems like an occasional burner, outside, those may not be key considerations. The only other factor would be near by combustibles or other structures. You'd obviously want to maintain all clearances specified.

 

[Elliot Gold]

It depends on the stove design, but yes, for one with just a simple baffle that would work. Modern low-emissions stove need more flue for stronger draft.

That's good news - yep, this one has a simple baffle

 

[Elliot Gold]

Interesting use case. I've been working on a patio fireplace for similar reasons, but I suspect a stove would actually throw more heat out to people near by.

As far as the pipe length, I suspect the formal instructions don't mean much at this point. The main reason for the specified length would be good draft to avoid excess smoke spilling out when you open the stove inside your home and compete with any 'stack effect' created by your house. But is some smoke spillage outdoors a big thing? Likely not. And you definitely won't have a stack effect.

I'd probably try to get the flue at least a few feet above the people and a few feet above roofs of any nearby structures - with the idea being you don't want smoke to linger around near the ground or be caught in a wind draft coming off a roof and swirl back to the ground. Double wall pipe would increase the draft for any given length of pipe and technically increase the stove efficiency a bit, but as this seems like an occasional burner, outside, those may not be key considerations. The only other factor would be near by combustibles or other structures. You'd obviously want to maintain all clearances specified.

Great, thanks!
It looks like 2m should just about do it then. I'll experiment and report back!

Out of interest - do you know if there is any efficiency difference between using the rear exit, or the top exit for the flue? Is one techically 'better' than the other?

 

[begreen]

I think that top venting will work better. It's a shorter more direct path for the flue gases.