Flue temps, low burns, dirty glass...

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Jim27

New Member
Dec 13, 2013
4
Illinois
I know this subject has be beat to death but I have read the recommended posts, learned a few things, and still have some questions.

We have a very well insulated home and the Harman is in the walkout basement where we spend a lot of time. Because of the good insulation and the fact that we keep the wood stove going nearly constantly at time we have to keep it dampered down. Wife worries about creosote in the flue even though we get it cleaned regularly. After the first year we hardly had anything in the pipe so have gone 2 years between cleanings sometimes and still very little to clean. The pipe goes from the stove up to the ceiling (standard 8 ft ceiling height) as single wall and continues straight up as insulated pipe about 30 feet farther. The draft is very good.

Recently I was told that the wood lasts just as long with the draft 1/2 open and blazing fire (flue temp about 450f?) as it does with a small fire (pipe temp about half that). Wife said she did some tests and it was true although I seriously doubt it. In any case the room temp was close to 80 and too hot for me.

The flue pipe gauge is a Rutland and is about 28" up the single wall pipe. Just read some posts about the difference between stove top temp and pipe temp and distance up the pipe, etc. I suppose a partial solution is to move the gauge down the pipe.

One issue we do have is the glass door getting dirty sometimes and it is hard to clean. The high fires will minimize that I think.

Other posts seemed to indicate 250 - 300 pipe temps were ok.

How do we have a low fire and keep the glass clean? Do low fires make the wood last longer? Are the low temps a problem with pipe build up or otherwise?
 

EatenByLimestone

Minister of Fire
Jul 12, 2006
7,330
Schenectady, NY
How do we have a low fire and keep the glass clean?

Smaller, hotter fires. This shouldn't be a problem with a well insulated house.

Do low fires make the wood last longer?

Sort of, the fire burns them up slower, but there is only so much energy in a piece of wood. You either release it fast or slow. You won't get more heat (energy) out of a log if you let it smolder.

Are the low temps a problem with pipe build up or otherwise?

They certainly can be. If combustion isn't complete and the temperature of the flue is low, the smoke can condense on the walls of your flue.
 

EatenByLimestone

Minister of Fire
Jul 12, 2006
7,330
Schenectady, NY
Welcome to the forum Jim!
 

Jim27

New Member
Dec 13, 2013
4
Illinois
Smaller, hotter fires. This shouldn't be a problem with a well insulated house.
Smaller, hotter fires - those are relative terms so I am trying to get to what is good in absolute terms. I know smoldering fires aren't good. But it is possible to have a small fire that doesn't smolder or smoke with clean (but small) flames but doesn't get the flue temperature very high on the gauge. The house if very well insulated (2x6 walls, extra attention to air sealing, good attic insulation, good windows, etc...) I am puzzled by why this is being mentioned. It seems to me if the house weren't well insulated I would be burning a hotter fire to keep it warm and wouldn't have as much of an issue with trying to have small burns. Am I missing something? Are you saying the house won't cool off much when the fire goes out so I should have short, hot fires and then let the fire go out for a bit before having another fire?

I have found that once I get the fire going I can wait for the several pieces used to start the fire to burn down to coals (but while they are still good and hot). Then I can put in one piece on top of the coals and it starts burning almost immediately. This way I am able to keep a small but flaming fire with no obvious smoke. I do have to feed it a piece at a time but this seems to be plenty of output for the heat needed. This seems like it works well but this keeps the flue pipe thermometer at about 150F. Pipe buildup hasn't been an issue.

Moving the gauge down the pipe made it read slightly higher but not a big difference.

To get hotter fires multiple pieces could be used (with the air inlet damper about half open, same as for the one piece at a time method) and get the temp up to the middle of the marked "burn zone" (about 400 - 450F). But it overheats the large room (and somewhat the rest of the house) so burning has to be sporadic.

When going to bed for the night I put in several of the largest pieces I have handy and damper it down. It smokes a bit more but not much. If I open the damper to get it all burning well it burns up the wood faster after the damper closed and it doesn't last the night.

Does anyone have links to articles that talk about burning techniques. Most I have found simply say general things like burn a hot fire to reduce buildup in the pipe and get good efficiency.
 

Enzo's Dad

Feeling the Heat
Dec 16, 2013
328
Canton, CT
I think the point is you can have a smaller hot fire, clean the glass and not produce enough heat to drive you out of the room. Every house and every stove set up is different. I think its all trial and error. Experiment with split size, and amout as well as damper control.
 

EatenByLimestone

Minister of Fire
Jul 12, 2006
7,330
Schenectady, NY
If I want to build a fire for a nice cold night, I'll place 8-10 good sized splits into the stove. This produces a long burning, hot fire. If the temperature is going up into the 40s or so and I want to keep the stove going, I'll place 4 to 6 splits in and make sure it burns hot. The stove will burn down, even out and since your house is well insulated it should hold the heat and you should be fine.
 

BobUrban

Minister of Fire
Jul 24, 2010
1,933
Central Michigan
small hot = less wood/more air play around with this plan and find your sweet spot
 

bluedogz

Minister of Fire
Oct 9, 2011
1,245
NE Maryland
Surprised nobody has asked... but how's your wood? Species? How long C/S/S?
 

DBoon

Minister of Fire
Jan 14, 2009
1,176
Central NY
Moving the gauge down the pipe made it read slightly higher but not a big difference.
It sounds like you are using a magnetic pipe temp sensor on the outside of the pipe. Many here have commented that these read a lot lower than a sensor in the pipe.
 

Beer Belly

Minister of Fire
Oct 26, 2011
2,186
Connecticut

Jim27

New Member
Dec 13, 2013
4
Illinois
Yes. Good link! Although I don't agree with "Never add just one or two pieces of wood to a fire." I have found that adding one piece of wood to a good, hot bed of coals just as the previous pieces are reduced to coals works very well for me. My wood is dry enough that the new piece catches fire very quickly and more pieces put out more heat than I need. I also question the part about setting up an extended burn by getting all the wood covered with charcoal by burning a blazing fire for a few minutes and then turning it down. I could be wrong and would be interested in what others think of this. I find loading big pieces helps as the article states and load several pieces but keep the draft low. The wood in contact with the coals burns and has minimal smoke and seems to work. Guess I should try the charcoal coating method a time or two...

Wood here is mix of oak, cherry, walnut, elm, hedge, a bit of sassafras (good for kindling)...

It sounds like you are using a magnetic pipe temp sensor on the outside of the pipe. Many here have commented that these read a lot lower than a sensor in the pipe.
Yes. I have seen comments about that and it is obvious that would be the case. But the gauge is marked below 275F as "creosote" and above 275 as "burn zone". I have also read some sites that recommend against having these gauges, especially if you have a glass door and can see the fire (I do).
http://woodheat.org/thermometers.html

Have any references that say lower temps are ok (as long as it isn't smoking and flames are present or it's a bed of coals)?
 

begreen

Mooderator
Staff member
Nov 18, 2005
83,007
South Puget Sound, WA
With 38 ft of chimney I think you would be surprised at how cool the flue gases are exiting the cap. Beyond the connector, is this all interior or exterior chimney?

When you say you are reading 250F is that the actual surface reading? Just for a test, take the thermometer and put it on the connector about a foot below the ceiling. What is the reading there?
 

Jim27

New Member
Dec 13, 2013
4
Illinois
With 38 ft of chimney I think you would be surprised at how cool the flue gases are exiting the cap. Beyond the connector, is this all interior or exterior chimney?

The insulated pipe goes through about 12 feet of conditioned space (floor above + dropped ceiling). The rest is enclosed in a chase to the top through the unheated attic.


When you say you are reading 250F is that the actual surface reading? Just for a test, take the thermometer and put it on the connector about a foot below the ceiling. What is the reading there?
A few inches below the ceiling connector the temp is about 50F lower (according to the stove pipe gauge which may not be that accurate...) and near the stove it is about 50F hotter. So at 225F in the middle of the pipe it is about 275F about a foot above the stove and about 175F near the ceiling. Clearly it will cool as it goes up the rest of the pipe. But I am not sure what you are getting at...
 
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