Fitted a 5kW log burner in my 20ft cabin last winter — was it overkill?

Finally got round to writing this up after a few people asked me about it in the solar sizing thread. I put a Charnwood C-Five into my 20ft x 10ft timber frame cabin back in November. Bloke at the merchant talked me into the 5kW model over the 4kW, said I'd thank him later. Cabin's got 100mm Rockwool in the walls and 150mm in the roof, double glazed throughout, so it's reasonably well insulated but not passivhaus territory.

Honest answer? It's borderline too much heat for that space. On a cold night I'm only feeding it two small logs every couple of hours and keeping the air wash nearly closed. Gets up to about 22°C in there within 40 minutes, which is brilliant, but I'm definitely not running it anywhere near output. Wondering if a 4kW would've been the smarter call and whether I'd get a cleaner burn at a more natural throttle setting.

The flue's a 904 grade twin wall, 5 metre rise, draws like an absolute dream so I can't fault that side of things. Running it on kiln-dried ash, moisture below 15% on the meter.

Has anyone else slightly oversized their burner and found ways to work with it, or did you end up swapping it out? Curious whether the 5kW is actually fine long-term running at low output or if I'm asking for trouble with tar and creosote buildup.

DebbieEvans | 847 posts | Pembrokeshire

@CraftyWelder Great write-up, looking forward to reading the rest when you've finished it!

I've got a similar situation in our 18ft cabin and honestly went through the same deliberation. We ended up with a 4kW Hobbit stove and even that feels like a lot on milder nights — we're cracking windows open by 9pm sometimes!

The thing people don't always factor in is thermal mass. A well-insulated timber frame holds heat surprisingly well once it's up to temperature, so you end up over-firing just to get there quickly, then sweltering.

Did you go for the boiler version or just the room heater? And what's your insulation spec like — PIR or rockwool? That'll make a big difference to whether 5kW was genuinely overkill or actually sensible for your build. 🔥

RKE_Builds | 1,203 posts | Array

@CraftyWelder Watching this one — I've been wrestling with the same question for my narrowboat's back cabin. 5kW in 200 sq ft is... ambitious. The rule of thumb I keep seeing is roughly 1kW per 14 cubic metres for a well-insulated space, so a 20x10 cabin with decent ceiling height probably wants 3-4kW max before you're cooking yourself out.

That said, a slightly oversized stove you run cool is arguably worse than one you run hard — wet wood, slumbering fires, tar buildup in the flue. Did you end up running it full chat just to avoid that, or did the Charnwood's airwash controls give you enough fine-tuning?

Also curious whether you coupled it with any thermal mass — flagstone floor, that sort of thing — to absorb the excess and release it overnight.

FormerMechanic15 | 312 posts | Array

Had almost the exact same dilemma with my shepherd's hut. Ended up going 4kW and honestly it's borderline too much on mild nights — have to crack a window or it's unbearable within 20 mins.

5kW in a 200 sq ft space is definitely pushing it, but depends massively on your insulation spec. Well-insulated timber frame with decent PIR boards? You'll be roasting. Rough-and-ready stud wall with mineral wool? Might be spot on actually.

One thing worth considering — running a smaller burner closer to its rated output is better for the flue long term. Constantly damping down a 5kW because it's too hot means you're smouldering it, which is asking for creosote trouble.

Keen to see how @CraftyWelder found it through the colder months specifically.

Gill1982 | Array

Done almost exactly this in my shepherd's hut — went with a 4kW rather than 5kW and honestly even that feels like a lot for the space on a mild night. Main thing nobody told me beforehand: the thermal mass of a small timber building is basically zero, so it rockets up to temperature fast but drops off just as quick when the fire dies down.

The flue position matters more than the stove rating IMO — I had mine too central initially and lost a lot of usable floor space.

@CraftyWelder curious what your insulation spec is? That'll probably determine whether 5kW was overkill more than the stove itself. My hut is 100mm Celotex throughout and even the 4kW has me stripping layers off within 20 minutes.

DefenderSolar | 847 posts | Array

Running a similar footprint in my shepherd's hut conversion and the oversizing question kept me up for weeks before I committed. What nobody tells you is that a big stove throttled right down actually burns dirtier — you get that lazy, smoky, tar-building burn rather than a clean hot fire.

My 4kW Hobbit runs flat out for 45 minutes on a good seasoned oak log and the place is genuinely tropical. I dread to think what a 5kW would do — I'd be sleeping with the door open in January.

@CraftyWelder curious whether you're finding you're running it at very low output most of the time? That's usually the tell. Also — what's the insulation spec on the cabin walls? That changes the whole equation massively in my experience.

Foggy95 | 156 posts

Interesting thread this. One thing nobody's mentioned yet — have you noticed any issues with the flue drawing properly when you're only running it at tick-over to avoid roasting yourself out? Smaller stoves tend to handle low burns better than oversized ones, and with a 5kW in that footprint I'd imagine you're barely cracking the air controls open half the time. That can lead to tar and creosote building up quicker than you'd expect, especially through a longer flue run. Worth keeping a close eye on and maybe sweeping it more frequently than you otherwise would. How's your ventilation set up? In a well-insulated cabin that size, fresh air supply to the stove is often the overlooked bit.

MoorSeeker | Array

Slightly different angle here — has anyone considered how the burner interacts with EV charging on-site? I've got a small off-grid charging setup and I'm wondering whether running a larger stove affects the overall energy balance during winter months when solar yield drops. Specifically thinking about whether the heat output reduces the need for electric heating elsewhere, freeing up battery capacity for overnight charging sessions. Do any of you actually track this kind of thing, or is it more of a rough seasonal adjustment you make? Curious whether anyone's run the numbers properly.