Burn Time Calculations

[StackedLumber]

How do manufacturers calculate burn time estimates for fireboxes? Is there any chart for burn time estimation based on cu. ft. size and type of wood burning? I know a lot of the estimations out there are a bunch of bull anyways-but is there any good way to figure out what the length of burn time might be for a given stove/furnace/boiler?

 

[flyingcow]

Bunch of wide open questions. Only thing i can suggest is if you got a few choices on boilers, put 'em out here. You'll get our real time experiences, but will it be the same as what you're going to put the boiler into? Good question. My Innova-30 will burn up a load of wood in about 3 1/2 hours, maybe 4hrs. This is using rock maple/beech, at about 22% mc. But i have to let my storage get down to 115ish. I'm sure this isn't what you were looking for, but it's not a easy type of question to answer.

 

[StackedLumber]

I guess I didn't realize that burn times were specific to brand of furnace or boiler. for instance take fire chief furnace that has, a 9 cu. ft fire box that's lined w/ fire brick. (29" deep) Burning red oak and maple splits and logs seasoned for 1 1/2 yr (don't know the mc) What's the burn time on that. I see Central Boiler, Tarm (non-gasific.) or some of these outdoor furnaces and boilers that claim-8, 10, 12 hr burn times . . . what do they use to calculate that? (or are burn time est. a bunch of baloney, like BTU outputs)

 

[Gooserider]

Burn times are hard to figure, as there are quite a few variables... With a wood stove, the key question is usually what setting do you have the air control at - The hotter you burn, the shorter the fire...

There is some correlation with firebox size, as any given volume of the same kind of wood will have a certain number of "potential BTU's of heat" - and the bigger the volume, the more potential BTU's - the question is how do you let those BTU's out? Do you burn fast to let them out a lot at a time, and get a short burn time, or do you let them dribble out slowly, and take a long time?

A few years back, Elk, one of our former members, and I got to take a VIP tour of the VC plant, escorted by one of their senior engineers (they don't usually do tours) - one of the things we were shown was a spread sheet of heat settings and burn times vs. BTU outputs for a particular stove running a specified load of wood. If the control setting was graphed you got a curve of BTU output vs. burn time. For any point on the curve, the area under the lines was the same product of BTU/hr and burn time - They could quite honestly state any given combination as valid, the question was what the marketing types wanted to push - burn time or heat output...

With a furnace or boiler, it gets a bit trickier, as you also have to figure in "idle mode" time, when the house isn't calling for heat, but the clock is still running... A person with storage will run flat out, with as little idle as possible, and burn all of a load in a very short time... Another guy running the same boiler w/o storage during shoulder season may spend most of the burn idling, as he has no heat demand, and get several hours longer "burn time" - yet the first person probably has gotten more usable BTU's from the load of wood...

Even taking the same guy, same boiler, same wood load, no storage, on a "demand day" where the boiler is getting a lot of demand will have a much shorter burn time than he did in shoulder season w/ no demand - how does he figure burn time?

Gooserider

 

[adenowski]

I have a Central Boiler 5036. They say 12 hour burn time, but that doesn't mean you fill it up and that load will burn start to finish and last 12 hours, different wood different performance. They say 12 hours, because the unit can sit at idle and smolder because the damper is shut, then relight when there is a demand for heat. This is why these type of OWB only get up to 40 - 45% efficiant., lot of heat up the pipe.

Take white oak. One full cord contains 25.7 MBtu, and weighs approx. 4012# per full cord. This translates to ruffly 200,000 btu per cu. ft. Since the central OWB is about 45% efficient, you only get 90,000btu out of the cu. ft. of oak (nice and dry stuff, not green).

This is a big reason I'm thinking of a EKO or some other gasser that has a lot better % efficient rating.

 

[markpee]

No matter what, you're going to burn through a lot of wood. This is my only disappointment with my EKO - but still better than using oil.

 

[ISeeDeadBTUs]

I guess I didn't realize that . . . ( . . . burn time est. a bunch of baloney, like BTU outputs)

Amen, Shorty!! Any salesmen and/or company that sells their units based on burntime are using buyers' ignorance to make profits. The heat load and BTU/Moisture of the wood will determine 'burntime'.

If burn time is your criteria for a hydronic unit's performance, you need to educate yourself much better Before buying.

I once sustained a fire 40 hours in my GW100. But it was pointless. For such occasions rely on your fossil backup. I do not like to see more than an 8-hour burntime if I can help it. Why waste the fuel??

Jimbo