Efficiency, or lack thereof

[woodpile]

Stove manufacturers cite efficiencies of around 70% without explaining what this means or how it was measured. I will assume it refers to comustion efficiency, that being how completely the wood is converted to CO2 and water (plus small amounts of other stuff). It sure as hell does not refer to heat extraction efficiency, and that is what this post is about. I use a Lopi Answer with a bower, and have a thermometer on the top plate of the firebox, in front of the flue pipe, and another thermometer on the single-wall stove pipe, about 3" above the stove. The maximum difference in temperature that I see is 100F. Given that I typically burn at 550F, this means that I am letting gasses leave my house at 450F. The best heat extraction efficiency would mean a flue temperature equal to the room temperature. This is what high-efficiency natural gas furnaces do, and they require a forced draft to push out the exhaust. I understand that one needs to waste some heat to drive the draft, but I think 250 is more than enough, and will probably prevent most water condensation. Most of the stoves I have seen have only the top plate of the firebox as a direct heat exchanger - the walls are insulated to a degree with the firebrick. Why don't stove manufaturers strive for heat extraction efficiency?

By the time the gasses leave the firebox, combustion is as complete as it is going to get, so why not add a real heat exchanger above the stove? While I'm sure I won't get to it this winter, I would like to try welding a heat exchanger out of an oil drum (30 or 55 gal) with several 1 - 2" EMT tubes and a bi-metal spring controlled bypass for when the exhaust is cool. It would need a cleanout for creosote, and may help keep that stuff from depositing in the chimney.

Scott

 

[Todd]

Efficiencies in woodstoves will never have an exact number. My take is 60-75% is about all you will get as far as heat output efficiency. There are too many variables like wood type and moisture, chimneys, weather and others. The new EPA stoves are fine tuned to give more heat but still need enough heat in the chimney to sustain a proper draft. How can a wood stove get over 70% efficiency if it needs heat to produce draft. If you steal heat from the chimney you lose stove performance. There is a fine line in there and every installation can be different. Most stove manufactures will test for efficiencies in their own lab or an independent lab and either pick the highest efficiency result or take the average if they are more honest. There is no standard real world efficiency test.

 

[rickw]

I also have an answer insert; and think the delivered efficiency is not so good. Last summer I wrapped the whole thing in rock wool. I don't know if it reduced the heat loss to the chimney all that much, but the stove is much better operating. The operating temperature has been boosted up, and it burns clean and easy. The best indication of this is that the glass stays clean; where it wasn't before.

With higher firebox temps I'm probably losing more heat up the chimney. This little stove might really benefit from a thermostatically controlled air intake, I think.

Putting on the insualtion increases the risk of overfiring; but the risk is not too bad I think because of the convection shroud - air can move around the actual stove.

I think the real "answer" here is a bigger, better unit. I'm looking at the BK Princess insert as well as the Woostock as a hearth stove.

One further thing thing that might help a lot is to install a block-off plate.

Luck.

 

[Corie]

The efficiencies cited by the manufacturers (mandated by the EPA) are overall efficiencies. How good if this stove at turning "x" number of BTU wood input into heat for the room. Actually, you'd be very surprised if you saw the combustion efficiency numbers, they are VERY high.

 

[Martin Strand III]

Trouble seeing the forest because of the trees?

Forget all those numbers about "efficiency", a rather complex topic.

The best you can do is to:

1. Use a modern EPA approved phase II stove centrally located in your home

2. Burn hot fires; i.e., no smoldering (smaller ones vs larger ones depending on need)

3. Burn only dry cordwood properly seasoned

4. Be vigilant and safe

Aye,
Marty

 

[savageactor7]

^yeah what Marty S says...

...I'm more interested in heat...not seeing how long I can keep coals. When I'm in and about my house I ever throw in 1 or 2 punky chunks and burn 'em hot for an hours heat. What do I care I'm saving propane, the wood is free, and most importantly we're warm cause we're burning less but hotter.

I look out at my chimney and there's no smoke coming out of it...cause the stove re-burns the smoke...perfect.

just say'en mostly I burn good wood but even punky wood can heat a house if you're around to tend it...and burn it hot.

Edit to add.

to answer the OP's question on efficiency I'm thinking the newer stoves reburning of the smoke is the heart of the matter. And anywhere in MI is colder than CNY.

 

[webbie]

combustion efficiencies, as corie mentions, are typically in the high 90's in test setups - or at decent burns even in your house.

So heat transfer becomes the big deal. That is why we have always cautioned about the delivered efficiency of certain fireplace inserts - because the efficiency claims are not for the installed insert, but usually for the stove itself (as if it was freestanding, or at least not in an exterior masonry chimney).

It makes sense that real world stove and insert efficiencies do not meet the best case scenarios. A figure of 60-65% for free standers and 50% for inserts would be my best guess.

Remember, delivered efficiency is Heat Transfer X combustion.
So a stove with 98% combustion efficiency and 60% heat transfer efficiency ends up at about 59% total.

 

[tradergordo]

I think its kind of up to the installer and homeowner to extract whatever "marginal" heat they want once the gasses leave the actual stove. For safety reasons, you need the flue gasses exiting the chimney to be >270F to limit creosote formation. You also have to account for the particular draft you expect in your climate and installation (chimney height and bends are major factors). A real long run of single wall pipe can extract just about every last bit of heat that can safely be extracted. Adding fans can extract even more, etc. If you don't have room for a really long run of single wall, you could stick heat sinks on your pipe (if you can't buy them, you could make them out of aluminum).

Obviously you need to to be very careful not to excessively cool your flue gasses.
If there is actually a demand for flue heat sinks, I'm interested in building some. I don't think I need them personally, I just need some ideas of things to do with the new home foundry I'm building

 

[woodpile]

Yeah home foundry! Do tell. I made a forge out of an old water heater - I get bright orange heat from steel and it can also melt aluminum.
My dad used to make flue pipe heat sinks by bending aluminum strips over a 1/2" thick steel form, the result is a square wave looking wiggle of aluminum that can be wrapped around the flue pipe. I may do the same here.

 

[tradergordo]

Yeah home foundry! Do tell. I made a forge out of an old water heater - I get bright orange heat from steel and it can also melt aluminum.
My dad used to make flue pipe heat sinks by bending aluminum strips over a 1/2" thick steel form, the result is a square wave looking wiggle of aluminum that can be wrapped around the flue pipe. I may do the same here.

I'm actually going to use an old water heater too.
http://backyardmetalcasting.com/harriette01.html
I trash picked mine just like that guy

 

[woodpile]

That guy smashed a lathe for scrap iron. That's wrong, just plain wrong. Use brake drums or whatever - a lathe is for machining the stuff you cast.

 

[webbie]

I bought mine from the casting cowboy.....
was in a hurry to get casting......

 

[tradergordo]

Nice. Just curious - what have you done with it so far and how well has it worked out?
I was originally going to go with propane, but then I saw the waste oil version which can melt iron and heck just about anyone can find almost unlimited amounts of free waste oil, so I'm going to try that first, but maybe the temps will be too hot for practical use (don't want to damage the furnace every time I use it). We'll see how it goes.

p.s. Backyard metal casting guy has plenty of working lathes, he probably just had some damaged parts. He trash picked a 400 lb. cast iron tub so I know he has plenty of iron to play with. I see those listed for free on craigslist every now and then, never thought I'd want to go though the hassle of picking one up but maybe I will now. An old damaged iron woodstove would also make for decent scrap. I've also got an old iron pipe that's at least 100 lbs. that I can use.

 

[webbie]

Melting iron is, IMHO, something to tackle only after stepping up from aluminum and then brass or bronze. It is easy to come by scrap aluminum cheap - or free. In fact, I can sell you a defective extendaflue for a buck a LB.

The furnace works great, and I built a couple molds and flasks. Have not done too much, but made some nice gifts as well as a tool (for tamping the sand), etc. I even started to make a mold so I could make the flasks (mold boxes) out of cast aluminum. So far I only made one side!

I think it is a good way to get started for a reasonable price and build from there. Here is a little plaque I cast for my wife - you can find lots of wooden letters and things at craft stores for making patterns.

As you can see my my mistakes, the process is definitely an art!

 

[woodpile]

That looks great. What do you use for molding sand? I've heard that sand with enough damp clay to make it clump when squeezed, but I question the use of water in metal casting. My home foundry experince centers on reducing cans to ingots and a house number. I've forged a cant hook for logs and a spanner for MacPherson strut collars.

 

[webbie]

You can buy molding sand....but it is expensive to ship.
Many companies sell the clay component, which you can then mix with locally obtained fine sand and get your molding material. Yes, you do have to dampen it, but that is how it is done in most places. Fancy founderies use a mixture which may have some oil in it instead of water.

http://en.wikipedia.org/wiki/Molding_sand
(broken link removed to http://www.gizmology.net/greensand.htm)

 

[TMonter]

if a gas condensing furnace can acheive 90% efficiency why are magic heat exchangers so dissed for doing the same thing. ive had mine glowing orange many times to clean it as per ac disconnect

Gas is a clean fuel that doesn't create creosote and safety issues when condensed. Wood isn't a clean spec fuel like gas.