Understanding Burn Times

Can I please have some help understanding what is meant by burn time? I always thought I knew what it meant, but, if I think about it, I don't know exactly what it means. Specifically, what is considered burn time? Is it simply the amount of hours from when you load a stove to when you can then again reload the stove on a bed of coals and the fire reignites? Is it the amount of hours that a fire is actively burning? Is it the amount of hours that you get a certain minimal temperature (say 250 degrees) from the stove top?

 

There is no set definition. Some manufacturers are conservative with their numbers. Some tend to exaggerate.

 

My personal definition is: The length of time between loads, easily started off coals and the house is still warm.
This also varies, with the outside temperature.

 

Ask about specific stoves, describe your house and climate. Others using that stove, in a similar climate, with similar heat loads can give you their results.

 

I have a Woodstock Absolute Steel. I wasn't necessarily interested in determining what my burn time is for the stove, but rather what is meant by the term "burn time" in general. My goal was just to have a clearer understanding of the term. I can easily get 12 hours burn time (and more) if we're talking about being able to easily reload the stove and load reigniting on a bed of coals. I live in central Massachusetts. My house is 3200 square feet with cathedral ceilings upstairs and in the great room where the stove is, post & beam with SIPS, generally open into the dining room and kitchen and into the upstairs loft. Temps in the house are 68-75.

 

So, I'm asking about the term "burn time" because I've been emailing back and forth with Woodstock a little bit, and one of the things that I was asked about was what kind of burn times I am getting, and I wanted to make sure we were both coming from the same place when we were talking about burn time. They suggested shutting the air damper down right away to achieve a longer burn time, but if I do that, the pipe temp drops to and stays at about 175 and the stove temp stays at about 250 for quite a while (two hours maybe?) and once when I did that, I got creosote dripping a little bit from the one of the joints in the stove pipe, so my reaction to that was, well that can't be right. Some of the things I wonder about with my set up is lining the chimney with a 6 inch liner (it's an outside chimney about 25 feet tall from ground to top and a clay round 8 inch flue all the way up), as well as, adding a fresh air intake.

 

Cant wait for the answers you receive, last time i posted about flue temps i was told i was full of crap and had an inefficient stove so sitting this one out and making popcorn.
I think telling you to turn your stove down low is poor advice, flue temp need to be in the safe range.

 

A perfectly reasonable question without any specific answer. The term burn time is just not defined in any way and so invites all kinds of differences and frankly, it tends to invite lots of very misleading information.

My own use of burn time is the time the stove is putting out sufficient heat to heat my house. And of course using it that way brings in even more variables because burn time when it is 10F outside is a lot shorter than when it is 50F outside.

A more useful measurement I think is the firebox size, which is a pretty useful value for a woodstove. It also has the huge advantage of being something that can be reasonably compared with other stoves. Other things such as efficiency come into play so it is not an absolute measurement across all stoves and stove types but at least it is a certainty that a 4 cubit foot woodstove (a very large stove by the way) will absolutely burn longer, putting out a decent amount of heat than a 2 cubic foot woodstove will.

Given the two examples you mention, how long a fire can be 'caught' off of hot coals vs. how long a stove will hold a relatively warm stove surface, the second one is a more useful measurement, at least it is to me.

 

You may have already done this, but it might be best to ask woodstock what their definition is, since yall are the ones that need to be on the same page.

 

Good point. I’ll ask.

 

Unless you have a large hot coal bed I'd never shut down the air right away. Always get that fresh load going and at least wait for the flue temps to start rising before shutting down the air. I'm surprised Woodstock suggested this.

 

In the past, when I reloaded the stove, I always waited for the stove pipe temp to get back up to at least 250 before engaging the cat. They're telling me that if the stove top temp is at least 250, there's no reason to wait for the pipe temp to get to 250. There's enough heat inside the stove to ignite the cat. So, I've modified my practice. When I do that and keep the air damper at a setting of 3 or a few ticks below 3, the pipe temp stays at about 200 and the stove top temp stays at maybe 300 for about an hour or so before the temps start to climb, and then I shut the air damper down in steps until I eventually reach a setting on the air damper where i'm going to leave it. The temps staying steady at 200 (pipe) and maybe 300 (stove top) for a while were described to me as a result of the catalyst being an an in line pressure drop. This all makes sense to me, in that, what's the difference if I wait 30-45 minutes for a load of wood to reach at least 250 on the pipe before engaging the cat and the pipe temp during that time is probably 200 anyway versus engaging the cat right away and having the pipe temp being at about 200 for a period of time. It's probably better to engage the cat right away if the cat is working and eating some of the smoke to make heat, right? But yeah, it was suggested to me with experimenting with closing the air intake sooner (maybe he meant when the temps started to rise, but I didn't take it that way and I don't think that's what he meant). But when I close the air intake to just below 2 right away or soon after engaging the cat, the pipe temp drops to about 150-175 and stays there for quite a while and, yes, that does not seem right to me or make sense to me. Especially, if there was creosote dripping from a joint in the stove pipe when I tried that once. So, I think I'd stick with keeping the air intake to a setting of 3 or a few ticks below and waiting until the temps start to climb before lowering the air intake setting.

 

So you want to know if you should put an insulated liner in an 8” round clay lined chimney that is 25ft tall and is an outside chimney?

One of the few times I will answer...unequivocally...yes.

How tall is your stove pipe from stove to chimney? Single wall or double? I thought I read you mentioned having a cathedral ceiling. I assume your stove pipe goes through an outside wall?

 

It's single wall stove pipe that runs about 2 1/2 feet up from the top of the stove and about 18 inches to the thimble, passes through to an outside chimney. I want to put a liner in. I'm assuming it would be safer and I imagine the stove would burn more efficiently. My pipe temps do not run half of what I'm getting on the top of the stove like Woodstock says people typically get, and I'm wondering if I would be able to achieve that with a 6 inch liner all the way up the chimney. My temps are usually 300 on the stove pipe and 500 on the top of the stove at peak burn.

The other thing I've always wondered is if I need a fresh air intake, and I don't think I do but I don't know for sure. Tight house but the stove seems to be getting enough air, I think. I've tried opening a window and seeing if there's a difference, but I've never noticed a difference in the burn. And I have an air exchange/heat recovery system (taking air out of the house, bringing air into the house) that I generally run at night to prevent condensation on the windows, but I don't notice a difference in the way the stove burns when the air exchange system is running versus when it's turned off. I do get a little back puffing once in a while when the stove is coming out of cat mode, but not that often.

 

2 things, are these pipe temps taken externally, or internally?
And if you have creosote dripping out of a pipe joint, the pipe must be assembled backwards...

 

Super cold air coming in from outside has to be heated up, so remember that. A few windows just barely cracked open, open to the point you can’t even tell they’re open, is enough to allow cold air to become warmed up some from the house temp before getting to the stove. Some manufacturers are getting away from recommending the OAC. Windows are easier than o adjust in my opinion.

Also, remember your surface temps on pipe and stove top only represent about half of the internal temps.

Do you have any pictures of your chimney?
I don’t like to recommend people tear out a good clay liner if you don’t have to because that liner could be used for another stove at a different time for an 8” outlet stove. However, for your particular stove it sounds as if an insulated 6” liner would give you optimum draft, that is your best draft. Just remember optimum draft is best, but not always necessary so long as you have adequate draft. I don’t like to see people spend money tearing something out that is good.
Lots of good people here to give you different opinions and advice. Ultimately though, it’s up to you to gather as much info as you can and decide what is best for you.

 

Good points and things to check for.

 

The temps are taken externally. The stove pipe is assembled so that going in the direction from the chimney toward the stove, the pipe section fits inside the next section (so if pipe is going vertically, the one above fits into the one below) if this makes sense, which I believe is the correct installation.

As for a fresh air intake, I’d rather not install one if I didn’t need to. It would involve drilling through the floor of my hearth and into the basement, and an ugly hose going into the back of the stove.

The chimney is maybe 12 years old and in perfectly fine shape. I used to have a Vermont Castings Defiant 1975 model with an 8 inch outlet. My certified chimney sweep said I will need a liner eventually because he says eventually the joints between the clay tiles will wear out and cause cold air to get into the chimney and cause problems.

I don’t overly care about getting slightly higher pipe temps than Woodstock says I should. The only complaint I have about venting the 6 inch into the 8 inch chimney is that on a cold start I seem to get quite a bit of creosote from condensation dripping down the inside of the chimney during start up due to hot gases hitting a cold flue, which is evident on the bricks below the clean out. And I somewhat worry about that. My chimney guy recommends keeping a fire going, keeping the flue warm, but that doesn’t bode well for shoulder season.

And I thought putting in a chimney liner would just involve shoving the liner down the existing clay flue, and not tearing the 8 inch flue out?

I’ll try and send pictures of the chimney tomorrow.

 

That’s my set up inside.

 

Oh man. Sharp!!!

However, don’t mind me if I say that hearth deserves a WoodStock Progress Hybrid sitting there.