New laws on woodburners

[SteveA]

20 minutes ago, Paul in the woods said:

They're not really selling the concept, are they? A masonry stove is something I'd be interested in but I'd like some basic numbers first.

 

I understand the efficiency of wood burners is often net, i.e. they ignore the heat used to remove any moisture. (So, 85% efficiency becomes 70%) but that's the same as a masonry stove.

 

The difference is that a normal wood burner has to keep the flu hot and if you're not careful heat from the house also goes up the chimney at night. However, what is the difference if you have a well insulated chimney and let the fire die down at night and close it off (or have an outside air feed)?

 

 

I see that as a question/ answer session.... not an advert for the latest DEFRA stove.
If you want basic numbers check out Peter Van den Bergs research here: http://batchrocket.eu/en/workings

I dont think there is a single woodburner that can compare with those kinds of efficiencies and cleanliness of burn throughout the whole cycle.... perhaps some masonry stoves can achieve similar though?

The biggest battle with this kind of tech is the resistance to adopt, to change. That's human nature though.
Worth bearing in mind that this stuff has been happening elsewhere for years and the UK is dragging its heels (as usual).

Cheers, Steve
 

[SteveA]

1 hour ago, difflock said:

Bollocks ! ! !

Any decent respectable brand-name wood stove, that anyone with owt approaching brains ud buy,

has loads of cast iron to modulate the heat output,

and hitting 75-85% efficiency,

means a rocket stove must be 8*80%=640% efficient, which canny compute, ye ken, like ! ! !

regards

disgusted-of-Dervock

A bit of forum etiquette would be nice.

[openspaceman]

16 hours ago, Woodworks said:

I read something about how testing was done from a guy involved in the development of the Burley. The percentage figure is derived from measuring of the flew gases and the efficiency of conversion but nothing to do with how much heat ends up in the room. 

 

Edit. Did a search for where I read this but looks like I was talking tosh haha https://www.navitron.org.uk/forum/index.php/topic,10364.0.html

What makes you think it was tosh?

32 minutes ago, SteveA said:

It's accurate according to whatever method they use to calculate to get to that figure.... but in the real world there is a huge loss of the heat in a woodburning stove that goes up and out the chimney & very little mass to absorb heat from the actual stove itself.

 

If you start with a closed warm room  with a stove in it and a flue plus a means to replace air that is used in the stove then law of conservation of energy applies.

 

Say you burn 1.25kg of wood at 20% moisture content and have to use 100% excess air to do so cleanly. If we define cleanly as no visible smoke and 30ppm CO then we can assume chemical energy losses to be around zero. Also ignoring ash content of the original mass.

 

Then  the energy content of the wood higher heating value is about 21MJ/kg. To burn this wood the rounded equation is something like this:

 

(C5H7O3+1.45H2O)+11O2+44N2 =>5CO2+4.95H20+5.5O2+44N2

where the bit between brackets is the surrogate formula for wood at 20%mc wwb

The N2 is inevitable as it is about 79% of air so you have to have it to get the oxygen you need.

 

To burn the 1kg mass of wood with 20% moisture  to extract 21MJ of heat you need to exhaust 8.8kg of flue gas and that includes 0.775kg of steam which you don't want to condense in your flue.To guarantee it won't condense it can be slightly lest than 100C but that's another story, we'll say it leaves the stove flue at 100C.

 

 

We have heated up all this mass from room temperature to exhaust it at 100C that means we have raised the exhaust gases from about 20C to 100C, lets assume the same specific heat as  of air about 0.001MJ/kg. Therefore our losses up the flue are the difference in heat in the flue from the original air and wood input plus the losses in converting the hydrogen in the input to steam, this equals ~2.5MJ out of our  original 21MJ HHV and so the maximum stove heat efficiency into the room is 88%

 

Where a mass heater scores is it releases the heat from the wood as above in a short hot burn  but then no further mass flow occurs up the flue so there are next to no losses from a stove "ticking over" all the rest of the day.

[Woodworks]

3 minutes ago, openspaceman said:

What makes you think it was tosh?

 

I had it in my head that it was from measuring a chemical analysis of the gases to determine how complete the combustion was but in the thread on Navitron Ivan said  " The efficiency is calculated from energy content of fuel vs ratio of heat lost up the chimney vs heat lost from the stove. I'm not sure of the exact calculation, but it's a standard controlled test procedure." 

 

 

 

[SteveA]

6 minutes ago, openspaceman said:

Where a mass heater scores is it releases the heat from the wood as above in a short hot burn  but then no further mass flow occurs up the flue so there are next to no losses from a stove "ticking over" all the rest of the day.

Yep, there's that plus the exit (eg, chimney pot) temperature is much lower so far more of the heat is absorbed into the mass (when compared with a typical wood burning stove).

 

[openspaceman]

Just now, Woodworks said:

I had it in my head that it was from measuring a chemical analysis of the gases to determine how complete the combustion was but in the thread on Navitron Ivan said  " The efficiency is calculated from energy content of fuel vs ratio of heat lost up the chimney vs heat lost from the stove. I'm not sure of the exact calculation, but it's a standard controlled test procedure." 

 

 

 

You only need a complete ultimate flue gas analysis if some of the chemical energy is being dumped unburned up the flue, this is typically the case when smouldering damp wood. In a clean burn we can assume complete oxidation so all the chemical energy is release in the stove. Then the only heat loss from the system is what goos up the flue. This has two parts, the physical heat which is the specific heat of the mixture of gases times the difference between flue temperature and room temperature plus the latent heat of vaporisation of all the steam that goes as part of the flue gases.

[openspaceman]

2 minutes ago, SteveA said:

Yep, there's that plus the exit (eg, chimney pot) temperature is much lower so far more of the heat is absorbed into the mass (when compared with a typical wood burning stove).

 

Yes I heard that and although they were talking in Fahrenheit  I'd have to sit down and work the partial pressures of all the flue gas constituents to actually see if one could get that low (140F was mentioned which is only 60C) without water running back into your stove.

 

In practice the 350F =~170 C which they mention as being a test standard for stoves is far more realistic  for a flue exit temperature into a 2 storey chimney in order to avoid condensation in the chimney. If I change from the 100C figure I used to 170C the stove efficiency drops to 85%. It gets worse if you allow that the air actually comes from outside, as do the logs which may be at 0C.

[SteveA]

15 minutes ago, openspaceman said:

Yes I heard that and although they were talking in Fahrenheit  I'd have to sit down and work the partial pressures of all the flue gas constituents to actually see if one could get that low (140F was mentioned which is only 60C) without water running back into your stove.

 

In practice the 350F =~170 C which they mention as being a test standard for stoves is far more realistic  for a flue exit temperature into a 2 storey chimney in order to avoid condensation in the chimney. If I change from the 100C figure I used to 170C the stove efficiency drops to 85%. It gets worse if you allow that the air actually comes from outside, as do the logs which may be at 0C.

Interesting, I've not heard of any condensation issues in RMH's to date. Some people used to have issues with smoke backfill until the flue was up to temperature but those problems seem to have been ironed out (apart from where people haven't designed the system properly!).

 

I'm most interested in Peter Van Den Bergs batch box as it can be stuffed full of wood and without any tending, whereas the vertical fed or gravity ones need more care and attention.

Cheers, Steve

[difflock]

I did not appreciate your sensitivities Steve, nor did I mean the "bollocks" to be as offensive as it was percieved to be.

but

for anyone to state that a rocket mass stove, or whatever, can use 1/8 of the fuel to heat the same space is more than a tad incredulous, hence my "involuntary" exclamation.

P.S.

Openspaceman, impressive and convincing calcs you provided.

regards

marcus

[SteveA]

1 hour ago, difflock said:

I did not appreciate your sensitivities Steve, nor did I mean the "bollocks" to be as offensive as it was percieved to be.

but

for anyone to state that a rocket mass stove, or whatever, can use 1/8 of the fuel to heat the same space is more than a tad incredulous, hence my "involuntary" exclamation.

P.S.

Openspaceman, impressive and convincing calcs you provided.

regards

marcus

Hey no worries here - it would take alot more than that to offend me.

cheers, steve

 

p.s.... I'm not talking bollocks about the 1/8th amount of wood required. They are very different tech to wood burners.