Draft setting for most efficient heat

Englander 30-NC - I normally close the draft about halfway when the wood is burning nicely. But yesterday after coming home to the cold house, I just left the thing wide open, wanting to see how much heat it would give. I was surprised by how well-behaved the stove was, it did not overheat, the STT never exceeded 600. (This was from a coldstart with a fairly tight grouping of the wood, obviously hot start or loose stacking would change that.)

That got me to thinking - to get the best wood efficiency, does one leave the air wide open for shorter hot burns, or do the usual practice of closing the damper halfway when the fire is burning good and go for a longer burn? IOW, is hot and fast better than a moderate longer burn, for efficiency?

I have seen from time to time folks say that you need to reduce he air, "or else all the heat goes up the pipe." Is this true, or a fable? It seems that yes, the exhaust temperature goes up, resulting in more lost heat, but then again you are getting a large amount of heat in the house as well, and the high internal temperatures would result in really clean burns, so it might be more efficient to burn hot anyway?

Another way or looking at it - If the stove is 60% efficient at 40,000 BTU output, and 65% efficient at 60,000 BTU output, the higher output will result some more heat going up the pipe, but lots more heat in the room, for a net gain. (These numbers are purely imaginary.)

Thoughts?

 

I can't wait to see where this goes. I love draft talk and more air/less air and efficiency talk.

 

The claimed efficiencies of rocket stoves and masonry heaters is from high temp fires. I am thinking the woodstove should be similar, and the hotter it is run, the more efficient ...

 

This evening when I can home, the house was at 61F. I lit another roaring fire but stacked the wood in there a little looser. I eventually felt the need to dial it back, as it was a little over 600 and climbing. At halfway on the air damper, it pretty much stabilized until the gassing finished.

Now (3 hours later) the house is aperfect 70 degrees, and I am pulling the pile of coals forward and tossed a small split in at a time to wake it up, and the draft is again full open. This is from pine and douglas fir, I don't need to hear anything from you oak burners!

Lots of draft, lots of heat!

 

**a bit anecdotal but widely accepted by most burners**

In an EPA secondary stove, achieving a high firebox temp that starts secondary combustion is probably running the stove in the most efficient manner. In my case the air stays open til I know the secondary action will take off when I close the air. Depending on wood and loading style, keeping the air wide open may not cause extreme stt since the heat can pass the baffle and go up the chimney. There's a balance that every burner needs to learn.

 

To further the confusion, most agree that shorter, hotter fires result in nice heat transfer to the room and less coaling. This requires babysitting the stove, so it's not always practical for the average burner.
In fact, my 1750 manual mentions the smaller, hotter fires vs. the larger loads.

 

To add to that with a tube type stove we're not really closeing the air. Just changing where the air is fed into the fire.

 

This is what I have heard, but really wonder if it is true. Yes more will go up the chimney, but proportionally speaking perhaps more goes into the room? Leading to improved efficiency?

That is a mighty big hole that is being reduced, compared to the inlet to the tubes. I have suspect that total airflow is reduced significantly.

 

My instincts tell me ~500F stt is the best balance of complete combustion without flushing the heat out the chimney, transferring it to the house. YMMV.

 

The chimney will still be drawing the same amount of air. The velocity through the other inlets will increase.

 

Thinking on this ,,,

I think not. The draw "suction" will be the same as long as the temperature remains the same, but the inlet to the stove will be restricted, so the pressure in the stove will go down ...

Hey, this is actually a little complicated. There are some wierd pressure changes going on to force the air into the stove and then up the chimney!

 

You're right. How much? I would say depends upon the setup. But the velocity will increase though no? & add turbulence through the secondaries. Which will aid in the combustion of the smoke.

Very well thought out these things are.

 

I'd agree that the most efficient way to burn the wood is to get the secondaries hoppin'. If you leave the primary open, you'll still get some of that, but a lot more heat and combustion byproducts are going up the flue. They aren't staying in the box long enough to be consumed.
Oh, and if the Maple and Oak are burning (sorry, bushpilot ) with wide open air, I'd expect a stove meltdown. Probably similar with softwood as well, depending on amount of fuel.
Where the air control rod is set is also dependent on how far the spring handle has been "screwed" onto the rod. If one of us says we push it in to just even with the front lip of the ash tray, another of us could have it an inch further in and have the same actual setting. I had the spring screwed on more than midway into the spring, but recently unscrewed it about an inch. Changes where the spring sweet spot is, so keep that in mind.

 

But doesn't the "more heat in the room" occur over a shorter period of time? So you are increasing your rate of output, but not the total useful output over time, per cord of wood.

It seems to me that if a load of wood has a certain BTU potential, higher flue temps from having the primary air wide open indicate that you are losing some of that potential. While the high internal temperatures of wide-open-air would result in really clean burns, are they really cleaner than the burns you get with somewhat less air? Even if they are, the heat generated from a cleaner burn is only useful if it exceeds whatever addition heat is lost up the flue.

The idea of my cat-hybrid is that you can get a very low-emission burn with lower flue temps (far lower than my Oakwood!). In theory, if the greatest BTU potential of the wood is extracted during combustion, and it isn't going up the flue, it must be going into the living space. But without a doubt, if you need a blast of intense heat when it's really cold out, you have to crank up the rate of output with more air... and you also need to do that to burn down all the coals that remain from a longer, low-air burn!

 

Running with the air open after the firebox is up to temp absolutely is wasting heat up the stack. People that do this do tend to have clean chimneys though.
If left wide open too long though you will end up with a overfired stove. The way mine works is if you close the air when the STT is 500, it may stay at 500, or it may slowly drift up after a while, especially if you loose stack some real dry stuff. Now if you shut down when the STT is 700, then it seems to stay around 700 or a bit more...which obviously will put more heat to the house since the delta T is greater (assuming the same air setting)
One thing that is neglected by stove manufactures and owners is how much chimney draft can vary from house to house. This is very directly addressed by wood furnace manufactures, they tell you the draft reading that you MUST have (often in the -0.04" WC to -0.06" WC range) I always see people discussing various stove issues and nobody has ever taken a draft reading ( rarely even the stove installer/dealer) Since the chimney is the engine that drives the stove I think it only makes sense to have a gauge reading to see if you have a 1984 Ford Escort chimney or a 2014 Ford Shelby GT500 Mustang chimney...big difference in "HP" there. (like about 400) I have a manometer permanently hooked up to my wood furnace chimney ( I have also used it on my upstairs stove chimney just long enough to get a reading)
Being on the wood furnace forums a lot, I have seen lackluster performance cured by solving a chimney issue many times...
Am I still on topic...?

 

Not to mention how much it varies with extremely low outside temps!

 

Thanks for the input gentlemen, all real good thinking.

 

I've noticed in some manuals, that it's recommended to have a particular amount of draft (usually in the same range you mentioned).