Really OTT log-burner tech question

[FridgeFreezer]

WARNING: This is tragically anal and over-complicated but I won't hear the end of it until someone finds an answer...

Since I knocked up a log burner the lodger has been obsessing about optimising its performance. He has decided the 3x 12mm holes in the bottom are strangling its performance and need to be made larger. However, rather than just drill a few more holes and see what happens, he's started doing calculations based on airflow, exhaust diameter, gas expansion due to temperature change, etc. etc. Yes, he needs to get out more, I've told him, but he won't stop going on about it until he finds an answer that is satisfactorily scientific / accurate.

So, does anyone have any information / ideas on what a decent air intake size is for a log burner with a 3" bore chimney?

Oh and it doesn't have to be right, it just needs to sound scientific enough to shut him up and stop him from tinkering with something that works fine!

[Night Train]

Send him to the Centre For Alternative Technology where he will have a great time with similarly minded people. I am there myself on the Environmental Architecture MSc course.

[Les Henson]

he's started doing calculations based on airflow, exhaust diameter, gas expansion due to temperature change, etc. etc.

Kick his 'ead in

Les.

[muddy]

well the chimney can whip away approx (pie x 1.5" squared) 7.7sqaure" of gas so i guess about 6" square of 'oles would be about rite taking in to account his expansion malarky,

I'd forgotten how active a landrover keeps you

Or just stick his head in it and ask him if its burning hot enough or whether the heat energy being transfered to his eybrows is low enough to warrant an increase in effeciency.

Will.

[Guest mortus]

i think 8 12mm holes should make it burn better, but if you make anymore, it will become very efficient when lit... but it will struggle to light in the first place... so really petrol is the answer....i mean... you need a variable air inlet...

like a lighting postion.. which should be closed... very inefficient burning

normal postion... then MEGA HEAT LEVEL THAT GIVES MEGA POWAH!!!

think of it like a bunsen burner... you start it with the air inflow shut....

and if you want a really good fire.. you need a fan mounted in your chimney to suck out the fumes.

[jimmy_neutron]

Well, there are many factors, far beyond scientific reason.

1) which way the winds blowing.

2) how hard the winds blowing.

3) how high up in the air ya chimney is, and how large and close the nearest objects are to the chimney.

For example, huge trees around the chimney reduce updraught on ya stack and can even create downdraught.

4) what your burning on the stove.

5) how much air flow there is through the stove. For example, Anthracite large nuts will burn better than coal dust cos the air can't get through the coal dust.

Look at a coalbrookdale wenlock. http://www.firesonline.co.uk/acatalog/Aga_...fuel_Stove.html

Under the glass is an air adjuster which is just a large hole thats adjustable by winding a plate in / out on a 10mm thread.

Let him heat ya stove up 2 much and it'll cost ya a new grate, but at least ya'll be warm

As for formulas .. Formulas will give ya not enough heat when ya fire choked up and put a hole through ya grate when theres a howling wind.

Formulas wot ya feed ta yon calfs

[Astro_Al]

Fridge, give yourself the option of lots of air (more holes), and then regulate it down to what you want.

Oh, and kick him in the nuts...

Al.

[David_LLAMA4x4]

Send him to the Centre For Alternative Technology where he will have a great time with similarly minded people. I am there myself on the Environmental Architecture MSc course.

Tie it in with a trip to The Land Rover weekend whilst he is up this neck of the woods....

[Vanny]

will the difference in diameter between the burning cylinder and exhuast not have the effect of setting up a venturi thus accelerating air drawn up the chimney? Will fitting a correctly tuned venturi help efficiency?

actually, i've just realised this could become a very dull topic, so i'll stop!

[paintman]

A short length of medium diameter stove fuel applied sharply to the nape of the neck with a swinging motion should put his mind at rest for a reasonable amount of time.

[nige90]

Tell him the holes you have are designed to give sufficient heat output for the room size

and adjusting for peak performance would make the room to hot and waste logs. Not to

mention you'd spend more time stoking the fire than working

[Ryan]

Either threaten to drag him behind Eddie next time he comes offroading with you, or threaten him with a hair cut

[Old Spot]

Why not weld a piece of pipe (maybe 1 1/2) into the burner and then fit a gate valve to it - variable draw, shut it right down for overnight burning or open it up for draw.

You could get realy anal and extend the pipe outside the house and you eliminate the drafts caused by the burner.

[Tonk]

Why not weld a piece of pipe (maybe 1 1/2) into the burner and then fit a gate valve to it - variable draw, shut it right down for overnight burning or open it up for draw.

You could get realy anal and extend the pipe outside the house and you eliminate the drafts caused by the burner.

or connect the compressor to it and see if u can get it to glow........

or butane inject it, errrrrrr maybe not

[Fatboy]

We've had log burners for years.

The draft they require has to be variable, it takes a bunch of air to get them started, then once there is sufficient heat, you close them down. Ideally, for the fuel to last longer, you want the logs to burn like cigars with minimal flame and that needs next to no air, or they just burst into flames.

If you keep the air supply high, the logs will disappear in no time, the heat might be impressive though

[Bull Bar Cowboy]

Why not weld a piece of pipe (maybe 1 1/2) into the burner and then fit a gate valve to it - variable draw, shut it right down for overnight burning or open it up for draw.

You could get realy anal and extend the pipe outside the house and you eliminate the drafts caused by the burner.

Getting really anal would be to control the valve with a PIC ................. the programming would be based on the internal room temp, the outside air temp and the outside air density.....................I guess it might be possible to also collect data for the PIC & valve control by using a MAF sensor...................

Ian

[simonr]

The draft they require has to be variable, it takes a bunch of air to get them started, then once there is sufficient heat, you close them down. Ideally, for the fuel to last longer, you want the logs to burn like cigars with minimal flame and that needs next to no air, or they just burst into flames.

At last! The (from my POV) right answer!

I've found with mine - exactly this!

The stove gets hotter if you throttle it right down - presumably because the hot gas is spending more time in the stove rather than shooting up the chimney. If you get the throttle too low, it starts to die - so it's a fine balance.

You can buy flue thermometers which indicate (apart from the temp - obviously!) whether you should increase or decrease the throttle for max efficiency.

You could spend a lifetime trying to calculate it and still not be right because of the number of other factors such as the type and dryness of the fuel etc etc.....

I think a stove is best kept empirical (Relying on or derived from observation or experiment)

Si

[will_warne]

At last! The (from my POV) right answer!

I've found with mine - exactly this!

The stove gets hotter if you throttle it right down - presumably because the hot gas is spending more time in the stove rather than shooting up the chimney. If you get the throttle too low, it starts to die - so it's a fine balance.

You can buy flue thermometers which indicate (apart from the temp - obviously!) whether you should increase or decrease the throttle for max efficiency.

You could spend a lifetime trying to calculate it and still not be right because of the number of other factors such as the type and dryness of the fuel etc etc.....

I think a stove is best kept empirical (Relying on or derived from observation or experiment)

Si

Exactly what I've found with the one at home. You want loads of air to get it going and then, once up to tempetature throttle it right back to stop all the heat going straight up the chimney. Its amazing how many people believe that the more flames the more heat when it simply isn't the case.

Oh, and if you really want to get silly with the calculations get your lodger to factor in where the air that the stove is burning is coming from and how that's effecting the room temperature. There's a reasonably good case for saying that a normal open fire will actually lower the average temperature of a house as you're pulling in cold air from outside.

[Astro_Al]

Its amazing how many people believe that the more flames the more heat when it simply isn't the case.

I dunno - I think total heat output will go up, compared to it ticking away with a flameless amber glow.

When I was stripping out my old canal boat I needed to get rid of the wood cladding from the entire inside of the cabin (a lot). Where better than the stove?! I got it goinhg so damn fast that it'd consume the wood faster than I could cut it with a power saw and load it in. The furnace used to literally roar and you could see it glowing red (the cast iron, not the fuel) even in broad daylight.

Goddam that was hot work!

Al.

P.S. - I agree with you guys about normal use though, get some heat in then throttle back. John - add some holes to get it going then a means of strangulation (airflow, not housemate... or both).

[simonr]

I'm using it to heat water - and I have a thermometer measuring the water temperature. I've found I get the max water temp at about 50% throttle. At 100%, it roars, the stove door glows red hot and it burns the fuel in seconds - but the water temperature is lower than throttling it back.

At about 25%, I reckon I'm getting the best efficiency. The water temp will be 10% lower but the fuel will last 50% longer.

I have a 'turbo' fan blower on my heater. I don't use it now because it lowers the water temperature even further!

Today, the outside temp is showing as 7 degrees. The water temp is 59 degrees and the air temp (upstairs) a comfortable 22 degrees - and that's on a 25% throttle. That's burning dry hardwood.

The turbo makes a big difference burning coal though. At least until all the fuel is glowing red. Then it pays to throttle it down to 50% and the water temperature soars by about 30 degrees.

Si

[Night Train]

When burning wood the flames are caused by the flamable gasses coming off the hot wood (off gassing). It is more efficient to burn wood quickly, with a high draft to make sure all the gasses are burnt as well as the wood itself. If you allow a wood fire to be damped down by slowing the draft then the gasses given off (which is most of the fuel in a wood fire) goes up the chimney unburnt.

The most effective wood burners are batch burners. They burn logs or wood chips (biomass) very quickly and efficiently with a lot of flame and transfer that heat to a heat store (usually a water tank) to be used as required later. In a domestic installation it may only need to do a half hour burn in the morning and that will provide the heat and hot water for the whole house for the day if the heat store is sufficiently insulated.

[simonr]

OK, I can see the logic to that.

Edited to add:

Since posting this, I switched on the turbo fan on my boiler - which generates lots of flames in the fire box! In an hour, I have burned a whole days worth of fuel and at the same time the water temp in my heating has dropped from 70 deg to 39.8 deg C.

I've now almost closed the throttle and the temperature is rising again slowly.

I think that if you had a much more efficient heat exchanger, your statement would be correct. However, an average wood burning stove is pretty poor at extracting heat from the flue gas, so burning it hot and fast just wastes more!

Si

[Boothy]

Being up Norf, where us men are tougher (so they say, and ficker) we soon found out that when we really needed heat the wood burner is a ball ache, to much time spend pratting around with it,

so we adapted, indeed we evolved, and used something we call GAS,

its a bit like a Jap import, turn it on it starts, does the job, and can be left alone!, bit like us up North

[Astro_Al]

Simonr - thats just what I was going to say - there's no question that the heat output goes up. I've never see cast iron glowing red due to a smoldering-ember type log fire in it. Its bound to be the way you are harnessing the output.

Thankfully though, your most efficient running is also your most effective!

Al.

[Night Train]

Yeah, that's the thing. Batch burners are cool to the touch on the outside due to the insulation. Very little of the heat escapes from a short hot burn.

Stack a load of storage heater bricks around the log burner to about 3' thick and then wrap it all up with about 18" of insulation then put the whole lot inside a sealed insulated box and fit a balanced flue.

HTH