Any opinions on Firebox Size and burn times

[rschoensta]

I made the chart below recently to help me get a handle on burn times between loadings.
(Note all computations subject to operator error.)
This chart sort of has me leaning toward the EKO 80 since burn time is one of my main concerns.
However I am not sure I have calculated the EKO firebox sizes correctly.
The firebox for the EKO's are described in gallons and I had to convert that to cu feet.
To come up with the Burn times I used the burn calculator on this site.
(Figuring 498,750 total btu's per 5 cu feet using hard wood)
I used it to figure total Btu's and divided that by the maximum rated output.
This is rough in part because it's not always clear if the stove rating is input or output.
For the EKO 80 I compute a firebox with 16.44 cu feet. (US liquid gallons to cu ft. The figure is 19 cu ft or so for dry gallons.)
That results in total BTU potential of 1.64 million or an almost 6 hour burn at maximum rated output.
Also I noted that for hardwood the estimated output is about 100,000 btu's per cu foot of firebox.
Which makes it easy to quickly calculate an estimated output for any size fire box.

The fourth column below indicates firebox size in cu ft and the last column indicates hours of burn time.

I take all this with a grain of salt but you have to start somewhere.


Btu output dia length Firebox Total Heat Output Weight Water Estimated Burn Time
eko's
25-85 7 20 4.14 412,965 1080 20 4.9
40-137 7 20 6.55 653,363 1212 25 4.8
60-205 7 25 10.96 1,093,260 1808 47 5.3
80-275 7 25 16.44 1,639,890 2315 54 6.0
Econburn
100 21 5.14 512,715 1440 28 5.1
150 21 6.19 617,453 1550 33 4.1
200 21 8.05 802,988 1860 44 4.0
300 23 11.94 1,191,015 2395 80 4.0
500 33 22.06 2,200,485 3220 118 4.4
Tarm
100 20 4.01 399,998 1060 41 4.0
140 20 5.35 533,663 1160 54 3.8
198 20 6 598,500 1210 60 3.0

Central Boiler
500 35 25.625 2,556,094 2900 450 5.1

Garn
1500-350 (275) 24-32 11.64 1,161,090 3550 4.2
2000-425 (320) 24-32 11.64 1,161,090 3980 3.6
3200-950 (720) 32-48 36.34 3,624,915 7500 5.0

Greenfire
90 16 4.44 443,333 2400 4.9
130 24 7.78 775,833 2800 6.0
130 16 4.81 480,278 2800
220 24 10.11 1,008,583 3200 4.6

Seton
90 18 10 997,500 2400 11.1
130 28 15 1,496,250 2800 11.5
180 38 20 1,995,000 3200 11.1

 

[webbie]

I think a more important dynamic is the heat load of the home and whether or not you intend to add storage. The ideal way to burn one of these boilers is at full boat, dumping all the heat into storage and then to the house. That being the case, burn time usually does not matter as much.

Many people would also be limited by their wood pile! For instance, if you fed a 16 cf firebox twice in 24 hours, you could burn a cord in 4 days.

So I think you have to start at the end - the heat load of the house and the amount of wood you have or intend to burn - and then work backwards.

 

[Nofossil]

I have to agree with Craig. Your numbers are about right - I get about 5 hours out of a load in my EKO 25. However, it's probably more important to realize that I need an average of about 7 hours of burn time per day. That works out to an initial partial load to start the fire, and a reload a couple of hours later. If my firebox were bigger, it wouldn't help except on really cold days.

There are a few ingrained beliefs that we all have if we started with conventional wood boilers, stoves, or furnaces. These aren't always valid with gasifiers, especially if you have storage.

1) "Big logs are good because they'll burn for a long time." Not true. The BTU value is the same if you split it smaller, and it will dry better that way. You need enough surface area to get good gas generation, so a load of big logs will not be as efficient as the same logs split smaller.

2) "It's good to have a fire going all the time." This is true from a comfort point of view if you don't have storage, but it's definitely not the way to go with a gasifier.

3) "A slow, steady, long fire is good." In a gasifier, short and hot is good. You'll get more heat out of the same wood in a short hot burn.

I'm sure there are more.

Your calculations will help folks in their planning, but it's important to recognize that firebox capacity in excess of heat load and storage capacity is not helpful.

 

[Eric Johnson]

An EKO 80 is a really, really big boiler. I think that without storage, you'd be idling it most of the time, thereby negating all of the advantages inherent in gasifiers. Many OWBs are rated for 275 K btu/hour (or whatever 80 KW works out to), but they're not gasifiers. Completely different animal, as nofossil suggests.

 

[rschoensta]

The use I am working on is for a highway garage.
I believe that time between loadings is important in order to get maximum savings.
On nights and weekends if the average time between loads is too short it will either require someone to come in more frequently than preferable
to load the boiler or else the oil burner will kick in. This will reduce the savings.
Also if the time between loadings is too short I am not certain how well the system will be accepted.
I would like to keep the operation time to a minimum.

I am definitely considering heat storage for all the reasons that have been discussed on this forum.
Plus it seems to me that if operated properly it should extend times between loadings to a degree.

As for amount of wood burned, the garage currently burns 5000 gallons of oil a year.
I have worked out the estimated equivalent cords of wood needed to replace that.

Assuming oil at 140k btu per gallon and 78% efficiency, I get the following amounts of wood needed assuming 22 million btu per cord.
49 cords at 50%
41 cords at 60%
35 cords at 70%
31 cords at 80%

So on cold days 1 cord every 4 days might be about right.

The real problem I have is that I am competing with OWB's.
The current highway commissioner definitely wants to go with wood but likes the load it and forget convenience of OWB's.
We have a number of locals who have them installed and are perfectly happy with them.
He knows this is at the price of additional wood and pollution but thinks the convenience factor over rides that.

I would like to find the best possible combination of convenience with all the benefits of high efficiency low emission wood boilers.

I think NYS is going to pass some regulations regarding OWB's.
Since the current proposed regs do not grandfather OWB's and as I read them wouldn't permit any of the current EPA approved boilers, it seems to me that an OWB is out of the question - although I have read here that econoburn is coming out with an efficient one this month.
Also one of the new central boilers is right on the line.

Even if OWB's are out of the questions the ease of operation of the new system compared to the experience people have had with OWB's will be important.
Most on these forums believe that operating a gasifier takes more effort and commitment than an OWB.

I want to make sure that a new system is as easy as possible to operate so that a lack of commitment doesn't lead to over reliance on the backup oil boiler.

I agree re oversizing the boiler relative to heat demand and storage.
The key as noted here is to burn a load hot continuously.

However my thought is that if you have 2 stoves rated with equal or similar output why not get the one with the bigger fire box.
(All else being equal.)
If you decide that 275k is what you need, then I am thinking I would rather get a stove that outputs 275k for 6 hours rather than 4 hours.

 

[Eric Johnson]

I don't think gasifiers are any more difficult or complex to operate than an OWB. They might be more complex to install, but operating a gasifier is really just a matter of keeping the fire going and/or starting a new one if it goes out. Neither process is difficult.

Even without storage, I think you could get by without too much problem. One approach would be to fill the thing to the gills before the last person leaves for the night and run the temp in your building up to 85 degrees at some point during the night, and you'd still have a nice warm garage when people come back the next morning. Since you're not going to be cooking out the residents in the middle of the night, that's a good (if not particularly efficient) approach.

Lots of different possible scenarios. I think that with the right sized unit, you'd be able to figure out a very workable routine that everyone would be able to follow.

 

[koch]

Have you looked at a Blue Forge? They are both an OWB and a gasification. To big for my use, but I've seen them at the fairs and they look like a nice design. FYI, saw one on e-bay just last week for about half price....

 

[rschoensta]

I have looked at the Blue Forge.
It looks nice.
However one problem is they provide almost no information on it at the web site.
Another is I live in NYS and I am anticipating regs which regulate OWB's.
The current proposed regs are very strict.
As I read them no OWB would qualify including the current batch of EPA approved owb's.
Although there is one that is borderline.
I am not sure that the blue forge would pass and as noted there is not enough info on their site to decide.

Also I would like to buy a brand that people have had experience with.
I don't think anyone in this forum has one.

Also I don't like the lack of info on their website.

 

[Nofossil]

Well, a highway garage is a whole different kettle of fish than a residential application. The Garn seems to get good marks in terms of ease of use, and I think there are folks who put them on a timer to ensure that there are coals left quite a while later.