Secondary (clean) burn temps

[Simmo6108]

Hi All - Long time lurker, first time poster, so please be gentle......

I am in the UK with a 5kw multi-fuel (coal and wood) DEFRA approved stove. I have secondary burn (referred to as clean burn in the manual). Much to my wife's annoyance, I have started trying to measure and monitor the temps actually inside the box (as opposed to the flue temps which I measure using a magnetic thermometer).

My query is, what temps do most people need in the box to bet the secondary burn to work most effectively? I am really just looking to see if I am optimising the efficiency as best as I can or if I am sending too much heat up the chimney. This is my first winter with the stove since it got installed so trying to get all the mistakes made this year!

Apologies if this is covered elsewhere, I did a search but found nought.

 

[FionaD]

Hi simmo. And welcome...

I'm assuming here that you're burning wood...?

I work by stove top temp, so forgive me for explaining things using stove top temps rather than inside the box temps. I guess you must have an infra red thermometer or something? You can use that to measure the stove top temps too. Aim for half-way between the middle and one of the top back corners.

If you load up with good dry wood (very important and quite a challenge in the uk unless you get a second mortgage!). Then open up the main air intake fully and let the fire really take, the secondaries should start to appear without any help on your part when the stove gets to a certain temp. It depends on the stove, and also on how cold it is outside, but my secondaries kick in around 350 / 180 . You could start very gradually dailing down the primary air intake around that point.. Or let the stove get up just a few more degrees, but not too much till you get to know how fast she heats up. I take the primary air intake down about 1/4 at a time, with few minutes in between. As you turn down the primary air, the secondary flames should show even more. It's kind of the dance between 'less primary air = more secondary flames' at this point and that's the trick really. Once those secondaries start rolling the stove top temps should continue to rise, cos the secondaries heat the stove more, rather than sending too much of your heat up the flue - and that, as you say, is what you want. Keeping the primary open for too long will send too much of your heat up the flue, but turning it down too soon will smoulder the wood before it's had a chance to take well enough, then you get a "un-clean burn"!

Again, depending on your stove (what kind do you have?) and how strong your chimney or flue draws, you may be able to shut the primary air right down once everything's ticking along nicely. I can, if it's windy out, but most folk seem to leave the primary a tad open.

In terms of optimum temps generally, your stove manual should tell you the best operating range, anything in that range should give you a clean burn if you've got secondary flames. Your stoves optimum operating range is probably between around 200 and 300 or 400 - 600 (I think in Celsius in all things but stove top temps, becuase of this forum! )

I hope that helps.

I'm one of the few Brits here and was also a total newb when I arrived here three winters ago. I'm still learning every day and love it. I hope you continue to feel the same! You could do a whole lot worse than visit here now and again, the guys here are great and really know their stuff. I was just lamenting here recently that British burners need to learn more about their stoves, we' re generally pretty undereducated in that respect, I guess we're newer at the wood stove lark.

Do come back if you have any other issues - and enjoy your stove.....

 

[jophysx]

I'm not sure what you mean about the temperature "inside the firebox". It's very non-uniform. The coals are glowing bright red and orange which corresponds to about 1300-1800F. The flames themselves are often yellow, corresponding to 2000-2400F, but they have very little actual heat capacity. The firebrick and any unburnt wood is much cooler.
As a practical matter, the exterior of the firebox is the place to measure temperature, since the metal conducts the heat well and provides more of an isothermal surface reflecting something proportional to the average temperature inside the box.

 

[branchburner]

My query is, what temps do most people need in the box to bet the secondary burn to work most effectively? I am really just looking to see if I am optimising the efficiency as best as I can or if I am sending too much heat up the chimney.

Apologies if this is covered elsewhere, I did a search but found nought.

I don't think you will find it here because it's a bit uncommon to measure internal stove temps, other than by using a probe to measure temps in a catalytic combustor stove.

Depending on the secondary burn technology (cat, burn tubes, or downdraft) you will find discussions of the theoretical temps needed in the firebox to achieve optimal secondaries, but no practical way to monitor or precisely control those temps. Obviously there is a minimum threshold your stove needs to reach simply to get the secondaries to fire off, but in the course of the burn cycle I would think the best way to see how far that threshold is being exceeded is by closely monitoring flue temps.

You want the flue to be above the minimum temps needed to keep creosote from condensing, which should be easy enough with good dry wood and adequate draft. If the draft is excessive, or if too much air is entering the stove (whether controlled or not), you might observe flue temps well in excess of what would indicate optimal efficiency. My particular stove and flue setup has always tended to run hot, so I know to some degree I am wasting wood.

Perhaps some others can suggest what a good range of flue temps might be for you, but since it appears you have been monitoring them, what are your observations of how they run during the course of a burn?

 

[Simmo6108]

Hi - Thanks for all the replies which are very informative. Since getting the burner installed I have became fascinated with the intricacies of burning and fuel, which has prompted this foray into the t'interweb. A far cry from the days of my father burning anything he could lay his hands on to burn in our open fire (I have memories of telegraph poles being cut in the back garden, and the chimney needing swept every 4 months!)

I am burning wood, although this is at a ferocious rate. I am considering having nights on smokeless in-between nights on wood to try and stretch it out, although coal is much less satisfactory to burn. That said, I don't live in Scotland anymore so the weather is mild right now.

I tend to be running hot when I read the flue temps although some of this is down to the fact this years woodpile is an eclectic mixture of scavenged wood and each load can burn vey differently. I never have it below 160 degrees C once going but it can be hard to prevent it getting to 300-350 on occasions, which is when I wondered whether I was simply heating the air above my chimney. Next year I am almost solidly on a pine/oak mix so the burn should be easier to control, or at least predict.

As an aside, my flue thermometer is magnetic and at the very top of the exposed pipe (about 6 inches from the top of the stove). Is this too close to get a sensible temp reading?

FionaD, I noted your comment on dry wood in the UK and would wholeheartedly agree with it. Fortunately I have a large enough space in my garden to store plenty of wood so have about 2-3 years worth C/S/S that I got from a local tree surgeon who had plenty to get shot of. As you say, this is all a little bit new in the UK (where coal fires were king until the mid-eighties and then gas central heating took over) but I see a lot of people getting fires around my way now. I think they are considered more of a feature rather than a way of being independent in your heating, which means any serious advice tends to come from across the pond.

 

[Jags]

A tube system of reburner needs about 1100F tube temp to go "active". Not sure about a cat.