Help with log burner

[Mike_scotland]

Hey,

 

Can anyone recommend a log burner? 

 

We seen online that your meant to do a calculation for your room size... e g height x width x length) so we have a large living area, kitchen, diner its about 7.5m by 7.5m at 2.4m high then divide it by 14 so i think we need a 9 or 10kw log burner.

 

No idea how much wood that would need and were not sure if its worth the initial outlay then the running costs but we want to have an idea from anyone who has had one.

 

Thanks in advance

[ProDave]

That calculation is meaningless without knowing how well insulated the house is.

 

Our WHOLE house needs a heat input of just over 2kW when it's -10 outside so even our 5kW stove would overheat the whole house if you left it burning too long at full power.

 

What are you looking for in terms of style?  Are you looking for ducted air intake to make it a room sealed stove?  Are you installing a twin wall flue or will it use an existing chimney?

 

I WILL recommend the stove we have, a Mendip Stoves Churchill 5.  The first stove I have ever had that genuinely keeps it's glass clean, and has ducted primary and secondary air intake.  It is available in larger sizes if you find it really needs to be larger.

[JohnMo]

If your well insulated get one with a small a kW as possible, otherwise you melt.  Our room is 6 X 6 and vaulted ceiling 6m high, half a load on min setting will have us opening the windows to cool down after about 10mins

[JohnMo]

Ours is a Heta Scan-line 8, with soap stone to slow down the heat output.

[SteamyTea]

34 minutes ago, Mike_scotland said:

We seen online that your meant to do a calculation for your room size... e g height x width x length) so we have a large living area, kitchen, diner its about 7.5m by 7.5m at 2.4m high then divide it by 14 so i think we need a 9 or 10kw log burner.

That will be about 134 m².

Or about 170 kg of air.

To heat that air up 1 K will take 170, kJ of energy, which is 47 kWh.

So a 10 kW burner would take 4.7 hours to raise the room up 1 K.

Timber has an energy density of about 4.5 kWh.kg^-1, but half of that goes up the chimney, so you would be looking to put 20 kg of timber in it.

 

(to me that seems an lot of timber to heat just the air up, so may have made a mistake)

 

Yes, I was 1000 out, so 17 seconds to raise the room up 1 K and about 200 grams of timber.

Edited November 28, 2021 by SteamyTea

[cwr]

I think you need to check your kJ to kWh conversion there...

[saveasteading]

10 minutes ago, SteamyTea said:

half of that goes up the chimney,

 

A modern burner is declared at about 80-85% efficient.

Then if you keep a large area of a metal flue exposed in the room, there is a lot of heat from that.

Presumably in real life it is less than that.

 

Meanwhile, "more than 60% of energy used for electricity generation is lost".   ie 40% efficiency from carbon fuel to kW output at end-user.

[saveasteading]

19 minutes ago, SteamyTea said:

That will be about 134 m².

I think our room of that size (with very poor insulation and a lot of cold thermal mass, heats by 4C in an hour, at full blast with dry oak. That would be 5 big logs of 1kg each?

After that the fire is turned down to minimum air or it gets too hot, and it is a log per hour.

[SteamyTea]

13 minutes ago, saveasteading said:

A modern burner is declared at about 80-85% efficient

There is a huge difference between running a stove in a lab and then running one in a house.

Maybe 50% up the chimney us a little harsh, but I doubt many will ever reach maximum efficiency for more than a few minutes at a time.

 

9 minutes ago, saveasteading said:

I think our room of that size (with very poor insulation and a lot of cold thermal mass, heats by 4C in an hour, at full blast with dry oak. That would be 5 big logs of 1kg each

So about 20 kWh of energy.

[SteamyTea]

18 minutes ago, saveasteading said:

Meanwhile, "more than 60% of energy used for electricity generation is lost".   ie 40% efficiency from carbon fuel to kW output at end-user.

Oh yes, but it is not really comparable is it.

Taking the same methodology, 99.75% of the solar irradiance is lost when growing trees.

 

Edited November 28, 2021 by SteamyTea

[jamieled]

1 hour ago, SteamyTea said:

1 hour ago, saveasteading said:

A modern burner is declared at about 80-85% efficient

There is a huge difference between running a stove in a lab and then running one in a house.

Maybe 50% up the chimney us a little harsh, but I doubt many will ever reach maximum efficiency for more than a few minutes at a time.

 

I'm happy to be corrected, but I believe quoted stove efficiencies refer to the combustion efficiency, not heat transfer efficiency. Clean (relatively speaking) burning stoves need a hot flue so 50% might not be far off the mark.

[Thedreamer]

Yesterday was colder than normal winter temperatures here and generally overcast so little in the way of solar gains.

 

It was 18c in the house when the stove went on and I heated the entire house (140m2, maybe take a bit off for our utility & porch) to 23.5c over the course of the evening. That was me burning one medium sized trug full of split alder/spruce logs and some smaller branches. Our stove is a Charnwood 4.8kw with air feed under the suspended floor. 

 

It's colder then yesterday outside and it was 19c when I woke at 10am and now its just above 18c. Stove will be lit in a hour and half and the cycle will begin again. 

 

Was outside collecting some wind blown wood this morning, in the end I had a total of three wheel barrows, that will eventually heat the house for a couple of weeks. Cost me nothing, but some time and effort. As I collected the sticks a robin who seems to follow me around, collected some worms from the soil. He was happy, I was happy, we were all happy.

 

 

 

 

[saveasteading]

38 minutes ago, Thedreamer said:

He was happy, I was happy, we were all happy.

And if you are happy then I am happy that you are happy.

 

Re the efficiencies, yes we know the difference between lab test and real life, in many fields.

I would intuitively say that our burner is probably 70% at best and 60% when not working efficiently, including the many kW noticeably coming from the metal flue.

And before this fire we used 4kW of electric heat, nearly 24/7 and it took days instead of hours to find the 5C rise we were looking for.

 

Does anyone know if it is working well when all a mass of orange glow, and no smoke to be seen?

[SteamyTea]

4 minutes ago, saveasteading said:

Does anyone know if it is working well when all a mass of orange glow, and no smoke to be seen?

I think in a home wood burner, visible smoke is more to do with the water content being driven off.

From the colour you can estimate the temperature, so if you know the area that it is radiating from, you can work out the power it is delivering.

Temperature (°C)    Colour
480    Barely red in the dark
600    Dark red
800    Cherry red
950    Orange, barely visible in sunlight
1100    Orange-yellow, visible in bright sunlight
1300    Light yellow, nearly blinding, welding goggles required.
1500    Nearly white, blinding
 

[saveasteading]

10 minutes ago, SteamyTea said:

From the colour you can estimate the temperature,

I hoped you might know. 

[Iceverge]

2 hours ago, saveasteading said:

Then if you keep a large area of a metal flue exposed in the room, there is a lot of heat from that.

Presumably in real life it is less than that.

 

I reckon an exposed flue does nothing to increase your efficiency. Rather the contrary. It demands more energy released from the stove to keep it at operating temperature and prevent tarring. 

 

Take 2 situations and 2 stoves. One with a single exposed flue in the room and one with a twinwall all the way from the stove to the ceiling

Set both to run at 5kw. (Simplistically) assume you get 4.75kW from the twinwall stove and 250W from the twinwall flue and 4kW and 1 kW respectively from the single wall.  The stove with the single wall flue will be running at lower temperature with consequently lower combustion temperatures and poorer burning efficiency. 

 

All you will achieve with an exposed (single wall flue) is taring and more logs burnt for equal heat output.

[Iceverge]

On another point I don't think that external air stoves are more efficient than room air ones. The stove will need to burn more fuel to compensate for the lower incoming air temperature to output the same heat to the room  

 

The main advantage is that you don't have a hole in the wall creating drafts. Unless you diligently close the air supply and damper on an external air stove when not in use the chimney will draft and draw cool air though the stove cooling the room so you'll still loose energy. I think you'd be better off with a normal operable vent near the stove. (Against Bregs obv)  

 

One way around this is a concentric insulated flue combining exhaust and intake and a completely sealed stove. Hard to get outside Germany and Austria. It is a mega expensive route though. 

 

https://www.poujoulat.co.uk/solutions/residential-applications/chimney-systems-for-residential-applications/twin-wall-insulated-chimney/efficience

 

 

 

 

[ProDave]

7 minutes ago, Iceverge said:

The main advantage is that you don't have a hole in the wall creating drafts. Unless you diligently close the air supply and damper on an external air stove when not in use the chimney will draft and draw cool air though the stove cooling the room so you'll still loose energy. I think you'd be better off with a normal operable vent near the stove. (Against Bregs obv)  

 

Yes that is the main advantage of a room sealed stove, they don't leak much air from the house and don't kill your air tightness.

 

And yes, a closable air inlet vent is not allowed under building regs.

[saveasteading]

20 minutes ago, Iceverge said:

I reckon an exposed flue does nothing to increase your efficiency

Interesting, and currently counter-intuitive...but I am here to learn.

 

I can't see how 2m of exposed flue before it is insulated for the rest of the journey can not provide spare heat...otherwise it is flying wasted to the sky. 

The early stage fire chucks flame and smoke up the chimney which would be wasted but heats the flue then room.

The later stage fire sits and glows very happily, and a lot of the heat inevitably goes up and away. Best capture some.

As long as there is enough heat and gas flowing up (to waste) the fire will burn well. 

 

Now for the real-time example: 

I am looking at ours now. Started with tree prunings from last year, an hour ago, and about to put the first proper log on. The flue is 1.5m long exposed before it is insulated (rockwool in plasterboard) and then goes into a chimney (stuffed with rockwool)

The heat coming off the flue feels like 2kW to me (and I did remember to mask off the heat rising from the stove).

 

The enclosed length of flue does not feel warm at all, which is just as well as it goes through a wardrobe.

 

The room temperature has risen from 17 to 21 in an hour.

[saveasteading]

2 minutes ago, ProDave said:

a closable air inlet vent is not allowed under building regs.

I can understand this rule as oxygen is good for us, and we can't ensure the skill set of the inhabitants.

 

However , for your own house, with a lot of understanding of these things, the open one might perchance change to a shuttered one? 

[epsilonGreedy]

7 minutes ago, saveasteading said:

However , for your own house, with a lot of understanding of these things, the open one might perchance change to a shuttered one? 

 

 

The problem is that your "understanding of these things" tends to decline when your brain is starved of oxygen.

[ProDave]

11 minutes ago, saveasteading said:

 

However , for your own house, with a lot of understanding of these things, the open one might perchance change to a shuttered one? 

I could not possibly advise changing something after completion so it no longer meets building regs.

 

But I have seen such closable air vents next to a stove.

[SteamyTea]

54 minutes ago, saveasteading said:

I hoped you might know. 

The easier way to to get someone that already has a stove to find out what the mean surface area temperature is and then assume an airflow around it (they can keep a small paper plane hovering, so not that great), and work it out from there.

It will probably be a lot less than stated.

[Iceverge]

39 minutes ago, ProDave said:

Yes that is the main advantage of a room sealed stove, they don't leak much air from the house and don't kill your air tightness

 

Another option would be to take your stove air feed via a duct so that it entered your heated envelope at the same height as your chimney. This way the air would be equally buoyant both at the cold and hot side of the stove (when it was unlit) and assuming you had a perfectly sealed stove and flue system (unlikely) there would be no airflow.  Equally it would be important to collocate intake and chimney so as to avoid airflow due to differential airflows at the terminus/chimneypot.  It's all highly theoretical of course and in most houses won't make a blind bit of difference. 

 

It was these marginalities that drove me to distraction when building our house. I was afraid of loosing that "feels like the heating has been on" feeling when you open the front door in the shoulder seasons and then having the satisfaction of realising we didn't have any heating on.  

 

I think the need for any external air for a stove is really just theoretical overcaution by the building regs. If you had a non particularly efficient 8kw stove in our passive house 175m2 ( 475m3)  burning flat out it might use 5.2kg/hr  (of 12% moisture ) which would correspond to about 20m3 of air per hour. We have very good airtightness at 0.31ACH50 but still PHPP predicts we would have 20m3/hr natural infiltration balancing this situation nicely.

 

However with 8kw of output on the coldest day of the year our house would be 85deg according to PHPP. The modelling must run out at some stage and this isn't realistic but you get my point. I'm sure at some point before I was donning a full heat suit to approach the stove to add yet another log while our house was like a steel foundry my family would have had me locked away.  

 

 

 

 

 

 

 

[Iceverge]

1 hour ago, saveasteading said:

I can't see how 2m of exposed flue before it is insulated for the rest of the journey can not provide spare heat

There are two markers to top efficiency.  Minimum exhaust gas temperatures and maximum combustion temperatures. 

 

A  90% efficient condensing gas boiler has about a 2000deg flame temp and about 40deg exhaust gas temp. 

 

By allowing some of your heat to come from the flue you haven't minimised the exhaust gas temperature from the combustion chamber or maximised it within.  Your fire will burn with more particulates and/or combustion will continue in the flue which it wasn't designed for. 

 

In short you should get a good quality small stove ideally with a damper and run it as hot as possible with a well insulated flue beginning as near as possible to the stove itself. 

 

You will still get the afore mentioned 5kw but just with less fuel, less soot and less dirt and wear on the chimney. 

 

This rather dramatic piece of equipment captures the principles well. 

 

https://www.youtube.com/watch?v=r8IGMwcyRNA&ab_channel=HydroToHeatConvertor