Insert thermal dynamics question

[Wood Scrounge]

Over the last week I had noticed that my inserts heat out put had been dropping to where I had to have it cranking to get the 12 X 22 room over 70 degrees. I even loaded it up with good dry mulberry and ran her wide open still could not get over 70. I noticed last night that the door latch was loose causing a gap between the door and the stove. I tightened up and in 2 hours the room was 75 before I throttled the stove back.
My question is why? It seems with the gap more air would have gotten in and made the fire hotter?

 

[starter]

I'll give it a go. When the door seal was leaking the fire burned hotter because more air was flowing through your insert but that meant more heat was being carried up and lost out of the chimney. Adding to that was more make up air was being brought into your house to feed the fire and this cold "in leakage" makes it colder in the house.

...so it was you causing the global warming yesterday!!!...
:jawdrop:
don't worry, I think I caused it the day before...

 

[Wood Scrounge]

...so it was you causing the global warming yesterday!!!...
:jawdrop:
don't worry, I think I caused it the day before...

LOL Nice,
your theory sounds plausible to me.

 

[habanero]

...When the door seal was leaking the fire burned hotter because more air was flowing through your insert but that meant more heat was being carried up and lost out of the chimney...

If the fire is burning hotter, then you should be getting more heat output. Of course more is likely being lost up the chimney, but the net output into the room should still be greater than if the fire was burning cooler. Otherwise, opening the draft of any stove would cool down a room (which obviously isn't the case).

Now as to the reason for the seemingly contradictory evidence (more air=less heat), that is a really good question. Wood Scrounge, were all other parameters the same before and after fixing the door seal (outside air temp., wind conditions, doors open/closed in the house, wood amount/type/moisture, etc.)? I can't see how a hotter fire would lead to less room heat, unless there are other factors in play.

 

[Wood Scrounge]

If the fire is burning hotter, then you should be getting more heat output. Of course more is likely being lost up the chimney, but the net output into the room should still be greater than if the fire was burning cooler. Otherwise, opening the draft of any stove would cool down a room (which obviously isn't the case).

Now as to the reason for the seemingly contradictory evidence (more air=less heat), that is a really good question. Wood Scrounge, were all other parameters the same before and after fixing the door seal (outside air temp., wind conditions, doors open/closed in the house, wood amount/type/moisture, etc.)? I can't see how a hotter fire would lead to less room heat, unless there are other factors in play.

for the most part outside temp was within 5 degrees of the nights before, the wood was the same mix as the past few weeks, and a few more days will tell for sure if the output into the room is truly more.

 

[RaisedByWolves]

Both starter and Dan hit the nail on the head.


WS, if you want to get the max output from a load of wood, try damping your stove back a bit more than you normally would. The heat wont come out of the stove as fast, but youll get more BUT's inside your house over the length of the burn.


.

 

[habanero]

I don't know what insert you have but almost all of the EPA approved stoves have long built in channels that the input air travels through and is pre heated before entering the fire box.

Air enter the fire box from other openings would not get this 'pre heating'. The purpose of preheating is to maintain high fire box temperatures to improve combustion efficiency.

Heat output = Energy in fuel X Combustion efficiency X Heat transfer efficiency.

That's a good point that I hadn't thought of. In essence the firebox was staying cooler even though more air was being supplied to the fire. That makes sense.

RBW, I agree with your damper comment to a certain extent, as long as you aren't starving your fire for oxygen. If you damp it too much you're going to get a rich-burn condition and subsequently smoke-which is just unburnt fuel going up the chimney.

 

[starter]

All true, I believe... although I have been occasionally guilty of overfiring with air to get a good blaze to feel the radiant heat on me bones, usually after being outside playing with water/ice. I think the extra smoke you see when under firing also deposits on the chimney as creosote.

Regarding dampering down to get the most heat our of the wood, absolutely true. We have a glass door and can see the fire within so I have played a lot with dampering down while maintaining a clean burning fire. By now I know where the fine points are on the damper and usually have clean glass and some coals left in the morning. As the guy who sold us the stove said, "what you see on the glass is what is going up and onto your chimney".

Last week I was working on our roof with the tech installing our new high speed internet aerial and the stove was on a slow burn (dampered down and happy). The wind pushed the chimney gases around us a number of times and we did not choke or have burning eyes, etc. just a good clean warm draft. If you starve your fire you can smell it, see it, and will be at risk for a chimney fire.

 

[starter]

Interesting. I guess the 50% left after pyrolytics is basically the carbon, like when making charcoal by starving the combustion process of air (this probably levaes more carbon intact by keeping the temperature to a minimum, but I'm rambling).

I usually damper down once I feel the new logs have caught sufficiently (IMHO) to sustain a clean burn when dampered down to the "sweet spot". I guess that the sweet spot I have found is still supplying enough air to burn the pyrolytics, cuz the glass is pretty clean come morning. Maybe I should wait a little longer than usual and then try dampering down a little more. I do love to tinker and understand...

 

[habanero]

I've played with O2 sensors before trying to get one set up for use in one of my homemade boilers, but haven't found anything that worked for long. That is what you'd need to accurately control the conditions inside the combustion chamber.

At a former lab I worked at we were pyrolyzing peanut and pecan shells to make activated carbon, and used a remote Near Infrared sensor to monitor the output gas. I wish I had something like that for tinkering with, but it was several thousand dollars (and was still only relatively accurate).

 

[mga]

i've got simple glass doors on mine that i've worked on to make them more "air-tight" than original. there is a vent on the bottom that slides open or shut. if i leave that vent open, my fire doesn't produce heat. go figure. the only thing i can guess is that the air is seeping under the grate and up the back and then up the chimney.

when that vent is closed, that fireplace heats up my 2,000 sq ft home without a sweat.

 

[windthrown]

I'll give it a go. When the door seal was leaking the fire burned hotter because more air was flowing through your insert but that meant more heat was being carried up and lost out of the chimney. Adding to that was more make up air was being brought into your house to feed the fire and this cold "in leakage" makes it colder in the house.

+1

You need an outside air supply to make any inside fireplace or stove efficient. Else you are sucking all the warm air out of the house to feed the fire.

 

[Wood Scrounge]

With the insert running all day until now (8 pm est) my house is warm, there is no doubt the door was the issue. I think the point about the extra air pulling the heat up the chimney is accurate.

 

[dkron]

I have thought about how much more efficient the inside/outside air source issue could be many times. The way the outside air venting (and air leaks) in my house is set up now from the original boiler/hot water heater and leaky window, I get some bad airflow at high burn and am sucking cold air into the house nowhere near the insert. One end of the house cools down. This is obviously much more noticable when it is colder out. A somewhat dampered down situation helps the heat rise from the basement because there is less cool air mixing on first and second floors. I know it may be different for different homes, but I wonder if anyone has feedback on how much efficiency is lost by sucking that cold air into the house and warm air out of the house when compared to an outside air source? I wonder what it would usually equate to in % of wood use. Also, I may be doing some masonry work on our chimny/box in the future and was wondering if and how I could get air from outside into a Lopi Freedom insert? I don't really want to poke a hole in the wall because it's in a daylight basement.

I have a friend that installed a high efficienty fireplace unit in his house with the outside (from his attached garage) air draw. I can feel that he doesn't deal with this issue.

 

[dkron]

TreeCo,
I'm not as confused as you may think. I realize an outside air source would not make the insert any more efficient. I would reduce the vacuum in my house, thus make my house less leaky, thus more efficient. Maybe my original post didn't make that clear.

Here is a quote from the link you provided.
"Passive air supplies don't make wood burners work better, they just make the house leakier."

So I wonder, would it be more cost effective to seal the leaky house or suck air from outside the house into the stove? I guess that is for me to figure out. The most likely scenario is that I won't do anything with the house or the insert. The article does shed some light on precautions about outside air sources that I was unaware of, thanks for sharing.

Home efficiency would also be a factor in how many chords of wood I use along with the wood burning heat transfer efficiency. That is where I was wondering if reducing the vacuum in my home where cool air has to travel across the first floor to the wood stove would create efficiency. Boilers and furnaces usually have an external air source nearby so cold air isn't sucked in via a chimney on the other side of the house, so why not a wood burner?

I appreciate your comment about HVAC people..... I guess my thermodynamic thoughts were just as much about the house as the insert.

Dkron,

Dkron,

If your house is leaking air like you say you have zero to gain from an outside air source. It will just cost money to install and to purchase the materials. It's sad that there is so much confusion on this issue. I've sent this same link in many times but it just doesn't seem to be sinking in.

http://woodheat.org/outdoorair/outdoorair.htm


Be cautious about taking advice from people who make money installing and selling the materials for outdoor air supplies. Also a lot of HVAC people are badly informed on this subject.

 

[starter]

I agree with almost all of what is stated here but there are exceptions to every situation so I'm throwing this in to ensure others don't get the wrong idea.

Most older houses are not tight enough to cause safety issues or problems with combustion but newer houses (R2000 standard, etc) can be very tight and these owners may experience troubles getting good draw on their fireplaces/wood stoves without additional air supply (opening a window, etc.).

My in-laws built their house about 10 years ago had a fireplace installed. The fireplace does not get enough air unless a window is opened nearby, the smoke and odour slowly accumulate until bery noticeable. If someone leaves a bathroom fan running then it is even worse. They could have benefited by having an external air supply installed.

PS> IMHO woodheat.org is the best web site for resources and information on heating with wood.