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Steven P

What makes stoves so efficient?

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This is something I have been wondering and Google is being no help at all.

 

So what in its design makes a wood burning stove so efficient compared to an open fire?

 

The door I think helps a lot by limiting the amount of warm air the chimney can draw up it, but if that was the main factor for efficiency, then why not bolt a glass sheet on hinges to the fire place?

 

Smaller flue size than traditional brick chimney? all that does is keep the gasses warmer so they don't condensate as much creosote

 

Is it the baffle plate? or the stove bricks, maybe the air gap around the stove? I am not sure.

 

So what design feature makes a wood burning stove so efficient?

 

 

(The reason I was wondering is that the upstairs fireplace is small, fits a 12" grate, and would be nice if it was a stove.. but would need some work doing first, but if I could work something out to make it more efficient that could be good)

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Not drawing the cold air in through draughts like an open fire is the biggest thing imo. Other than that all the things you said

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I think the biggest thing is air inlet control, meaning you can can completely control the rate of burn, whilst also controlling the rate of warm room air that is going up the chimney.

Some open fires can be pretty good, with a damper of the flue and some control over the air that is allowed in to the bottom of the grate, but obviously there is still alot of air getting to the fire.

Stoves also work by convection, and because there is no significant draught up the chimney from the room, warm air circulates. Most fireplaces radiate the heat into the room, which is often hotter 2ft from the fire but will not get to the back of the room, like convected heat will. Again the best fireplaces well heat up and give some convection, and are angled to optimise this.

Open fires are amazing and beautiful, just a bit inefficient, but if the room isn't absolutely freezing or your not shy on wood, they are very bonny to have in a room.

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9 minutes ago, billpierce said:

Open fires are amazing and beautiful, just a bit inefficient, but if the room isn't absolutely freezing or your not shy on wood, they are very bonny to have in a room.

I agree with most of that but swap "bit" for "very", also can never be as clean burning.

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Correct me if I am wrong but open fires don't run hot enough so loads of the gases just escape up the chimney unburnt so completely wasted. Modern stoves are full of insulation to create a hot and clean burn making the most of the available energy. Yes, and having control over the air helps massively. Saw a study on open fires wherein a centrally heated home they could have an overall negative effect on the heating of the home due to the convection drawing all the heat out up the flue. 

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31 minutes ago, Steven P said:

This is something I have been wondering and Google is being no help at all.

 

So what in its design makes a wood burning stove so efficient compared to an open fire?

As you say air control , mostly preventing excess air being pulled from the room, heated and then sent up the chimney.

31 minutes ago, Steven P said:

 

Smaller flue size than traditional brick chimney? all that does is keep the gasses warmer so they don't condensate as much creosote

This doesn't impact combustion efficiency but it does increase flue gas velocity so less likely for tars to settle out.

 

The firebricks and baffles you mention are part of the way the stove design  implements a couple of very simple concepts:

 

The energy in the log is conserved after the combustion, so what goes into the room plus what goes up the chimney plus what is left in the ash are equal to the original energy that was in the log.

 

The rules for good combustion are the three Ts, Time, Turbulence and Temperature

 

Temperature: a high temperature allows all the products of combustion to burn out, any Products of Incomplete Combustion like smoke are caused by the flame being quenched before combustion is complete, apart from dangers to health they represent a loss of chemical energy up the chimney.

 

A high combustion temperature is  provided by the insulating effect of the firebricks. So the heat delivered to the room is through surfaces the combustion products heat after the combustion chamber and before going up the chimney. A flue temperature higher than necessary to prevent condensation in the chimney  is wasted energy. This is why an insulated chimney is used. I don't have an insulated chimney as I like my flue gases to heat the brickwork as they are vented. I should insulate the top 4m length but it would not be cost effective for me. Some of the most efficient wood burners are masonry stoves which burn fast and hot and the heat then soaks out from the brickwork over the following hours.

 

Time: A flame needs time to burn out, the baffle  forces the burning flame to take a longer route and thus stay in the combustion chamber longer.

 

Turbulence: jets of preheated air impinging on the flame mix the fuel gases and vapours together increasing the  probability of meeting  enough oxygen to completely burn the fuel.

 

Then there is air to fuel metering, this is far more complicated than a fuel injector in a car but suffice to say the nearer to the correct ratio of fuel to air you can achieve (stoichiometry) the higher the temperature can be as you are no longer heating up air which then has a free ride up the chimney.

 

With a petrol engine you can achieve pretty much a stoichiometric burn, with wood  (and diesel actually) you always need more than stoichiometric air for a clean burn, this is known as excess air, it isn't used but it is needed to increase the probability of a fuel molecule being oxidised.

 

Of course the other single big  thing that depresses firebox temperature is moisture in the log.

 

 

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Correct me if I am wrong but open fires don't run hot enough so loads of the gases just escape up the chimney unburnt so completely wasted. Modern stoves are full of insulation to create a hot and clean burn making the most of the available energy. Yes, and having control over the air helps massively. Saw a study on open fires wherein a centrally heated home they could have an overall negative effect on the heating of the home due to the convection drawing all the heat out up the flue. 


Is this news to anyone?
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My stove is efficient cos I burn any old sticks in it fr free an it warms me. Whilst gas boiler demands payment. K

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1 hour ago, Khriss said:

My stove is efficient cos I burn any old sticks in it fr free an it warms me. Whilst gas boiler demands payment. K

That's efficiency of converting your labour into heat rather than the efficiency of converting chemical energy of the log into heat which we were discussing.

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