How efficient is an open fire?

[Ashbrooke]

14 minutes ago, Haironyourchest said:

Yeah its cold alright. Just took some readings, all at the same distance, about two meters.

 

Stovetop: 196 degrees celcius

Top of pipe: 40 degrees

Own hand: 34 degrees.

Sorry mines currently 300 Fahrenheit so about 150 Celsius 

[Haironyourchest]

I take the pipe outside once a year to sweep, so I can inspect it minutely - never had a problem with creosote buildup beyond the usual. But I season all my wood for at least a year, normally more as it would be lying around for a while before I get round to splitting and stacking.

[openspaceman]

2 hours ago, Al Cormack said:

Same as an engine. Colder air combusts better.

It's not an engine; an engine is limited by the mass of air it can cram into the cylinder, so more dense air=> more fuel to burn and higher pressure to expand it from. the pressure which the air expands through is directly related to power.

 

A stove is not limited by the amount of air it can take in, it tends to be limited by the surface area of fuel and surface area of heat exchanger.

 

Get your stove really going well and then look at the size of the air opening it needs to have good combustion. Typically a 5kW(t) wood burner will run at full power on just over a kilo of dry wood an hour. that needs 6kg of air plus the same again excess air on small stoves. So that equates to about 16m3 of hour passing through the stove. An occupied  room typically needs 5 air changes and hour. In this little room that equates to 145m3 of air an hour.

[surveyor]

Dad and I built a fireplace in his log cabin and used the 2" required gap around the firebox, in conjunction with a forced air fan to try to draw heat into the room, but only had limited success with that.   So now have replaced the fireplace with a wood pellet burning stove which draws outside air and am quite pleased with it.  It will fire up and shut down at any pre-determined time and only needs the hopper filled once a day.  Here is the fireplace before the change, notice the gaps on either side of the firebox, and the air vents above the mantle.

 

[matelot]

Density of air doesn't vary hugely with temperature. At 0 degrees centigrade air density is 1.29 kg per m3. At 20 degrees it's 1.2 kg per m3.

 

 

 

 

[Stubby]

53 minutes ago, openspaceman said:

It's not an engine; an engine is limited by the mass of air it can cram into the cylinder, so more dense air=> more fuel to burn and higher pressure to expand it from. the pressure which the air expands through is directly related to power.

 

A stove is not limited by the amount of air it can take in, it tends to be limited by the surface area of fuel and surface area of heat exchanger.

 

Get your stove really going well and then look at the size of the air opening it needs to have good combustion. Typically a 5kW(t) wood burner will run at full power on just over a kilo of dry wood an hour. that needs 6kg of air plus the same again excess air on small stoves. So that equates to about 16m3 of hour passing through the stove. An occupied  room typically needs 5 air changes and hour. In this little room that equates to 145m3 of air an hour.

Denser air still contains more oxygen than less dense air albeit not by much .

[openspaceman]

15 minutes ago, Stubby said:

Denser air still contains more oxygen than less dense air albeit not by much .

Indubitably, about 0.3% for every degree of coolth. My point being a stove doesn't gain anything by it.

 

I think I have mentioned before a stove and chimney is a very poor heat engine as the only motive power it contributes is circulating the gases up the chimney.

[Stubby]

9 hours ago, openspaceman said:

Indubitably, about 0.3% for every degree of coolth. My point being a stove doesn't gain anything by it.

 

I think I have mentioned before a stove and chimney is a very poor heat engine as the only motive power it contributes is circulating the gases up the chimney.

I respectfully disagree . The 3 components of combustion are Heat , Fuel , and Oxygen .  Take away the o2 and you wont get combustion . Add some and you will . Add some more and it will be more intense .  

Edited January 31, 2018 by Stubby

[Rough Hewn]

In very old cottages you can sometimes find a narrow channel dug in the earth floor, covered in rock, which leads to the fireplace. (Open fire).
Pull cold air through your house or through a pipe?
Also with a burner you can close it down to reduce airflow to very little.
This slows down the burn, and the hot gases escaping.
More heat will radiate from the burner while consuming less fuel and not pulling cold air in.
In France I've seen open fires with rows of 2" square steel tubes underneath, pointing forward, attached to a small blower at the back. Great for warming your feet and floor.
The best fire ive ever seen is a wood/multi fuel fired aga with back boiler and underfloor heating. Toasty.

[openspaceman]

1 hour ago, Stubby said:

I respectfully disagree .

Let's just disagree amicably then Stubby