Gasifier re-fill frequency?

[Andrew L.]

Hello all,

I have a locally made wood boiler with 200 gallons of storage within the unit itself. It's basically a OWB, but with secondary air tubes that supposedly create better combustion. It's supposedly rated at 76% efficiency, which I don't believe for a second, as it goes through a fair bit of wood. When it got cold this fall, I was dissatisfied with the initial 200 gallons of storage, and found that a large fire in the box would do two things 1.) overheat the system, causing my heat dump to nicely heat the garage (wasting heat), and 2.) not keep up to temperature for anything approaching 12 hours. The one thing I'll say for the system is that it was relatively inexpensive, simple, and robust. Natural draft, etc.

So, I got on this site and started looking at heat storages. I found and installed a 500 gal propane tank and it has made a tremendous difference in duration. I can not get away with at least 12 hours between fills, and on warm days, could go 20. Now that that issue is solved, I'm back to picking on my boiler.

Am I wasting my wood with this unit? I don't want to spend 10K on a gasifier, but I'm also a huge fan of efficiency. What sort of time can folks get between burns with units such as Tarm, EKO, Econoburn, etc, with a 500 gal storage? The nice thing about this boiler is that I can open it after 16 hours and there are still enough coals to rekindle. Would that be the case with a gasifier?

This is a great site. Thanks for all the opinions and expertise!

Drew

 

[stee6043]

I don't think you'll find anyone with a gasser, with storage, that is getting any "long" burn times as you described above. My max burn time with mixed hardwoods is 4 hours on a full firebox. For me it typically takes two full loads to bring 1000 gallons of water back up to temp after a day of heating from storage. So one may assume that running flat out for 4 hours, depending on the boiler output and wood type, could potentially bring 500 gallons up to temp.

The idea behind storage is to enable a boiler to run full-bore and not idle. Without idling you don't get the super long burn times. More folks will chime in I'm sure...

 

[Nofossil]

Chiming in.....

I've been thinking about this. I get 3 or 4 hours out of a full load. That's about half a day's worth of heat, so I need to put two loads in my gasifier each day. To be fair, I also have to start it.

What that means is that there's a 4 hour period during the day when I have to tend it three times - build the fire, load 1, and load 2. Because I have storage, I have a good deal of latitude about when that 4 hour block is - I generally choose about 6:00 to 10:00 in the evening.

I suspect that people with 12 hour burns don't spend any less time feeding their boilers - my loads are 40 pounds or so, and I can load it during a 60 second commercial. Starting it takes about ten minutes.

My take is that gasifiers with storage need a different mindset about burn times.

 

[leaddog]

Chiming in.....

I've been thinking about this. I get 3 or 4 hours out of a full load. That's about half a day's worth of heat, so I need to put two loads in my gasifier each day. To be fair, I also have to start it.

What that means is that there's a 4 hour period during the day when I have to tend it three times - build the fire, load 1, and load 2. Because I have storage, I have a good deal of latitude about when that 4 hour block is - I generally choose about 6:00 to 10:00 in the evening.

I suspect that people with 12 hour burns don't spend any less time feeding their boilers - my loads are 40 pounds or so, and I can load it during a 60 second commercial. Starting it takes about ten minutes.

My take is that gasifiers with storage need a different mindset about burn times.

I also fill mine a couple times a day and since there is usually coals it only takes 3 or 4 mins. BUT then I spend the next hr watching the thing. I like to watch the temp go up and the modulating valve operate and how the stack temp goes up or down and look in and watch it gasifiy and see what color flame I have. There is something about these things that makes me want to watch. My wife will ask "what took so long and are those that hard to get heat from?" And all I can say was I was just watching it burn. It's better than going out and looking at the guage on the propane tank move.
leaddog

 

[timberr]

Andrew,

I have and Eko 25 w/ 500 gal. of storage. I am heating 1,800 sq. ft., DHW and a hot tub. I find I do 2 burns/day to keep everything happy. The burns last about 5 hrs. If it is really cold (day temps 10 & below 0 at night) about half way through the burn I will add wood.

Good luck

 

[Duetech]

Andrew L.,
It's hard to tell if we are comparing apples to apples or oranges. That is how much wood is it you are burning per day and for what size structure/s? I don't have storage. I have 1700 sq.ft. I am heating and get around 20 (+/-) days from a cord (4'x4'x8') of good hard wood. Way less with soft or low density woods and run a fire 24-7. But as you can see guys with storage do better.

 

[sparke]

To paraphrase others - My Greenfire gets a 12 hour burn with no heat storage. Once I added my tank I now get 4-5 hours burn time. This is because the BTU's go into the tank instead of the boiler idling. I think the real question should be: How much wood do you burn in a typical season and can you reduce that amount? Once your unit is running efficiently, there is still work to be done to gain maximum efficiency. This will come from several areas including: Learning to run your boiler properly (sounds simple but it can take a while), properly seasoned wood, tank insulation, pumps, etc... The list goes on and on. I notice Aqua-Them has this add-on: (broken link removed to http://www.aqua-therm.com/pages/index.php?pID=82) If I owned an outside non-epa boiler. I would try to design something similar. To answer your original question: A gasifying boiler will reduce wood consumption 20 to 50% compared to a traditional OWB. So it is not hard to figure out the payback based on what you pay for wood...

 

[sparke]

Woops, your unit already has secondary combustion?

 

[Nofossil]

Separate point - 200 gallons is way too little storage to accomplish any useful purpose. Quick calculation:

Heat storage capacity in BTU = gallons x delta_t x 8.3

where delta_t is the difference between the average temp of storage fully charged and the lowest temp where storage is useful for heating your loads.

Assume that you can heat your storage to an average temperature of 210 degrees, and you can get useful heat out of storage all the way down to 140 degrees. That gives a delta_t of 210 - 140, or 70 degrees. 70 x 200 x 8.3 = 116,200 BTU. That's with a MUCH wider than normal operating range - kind of a best case for storage. My operating range is 170 to 120 for comparison, with an 880 gallon tank. Most people figure that 500 gallons is a minimum, and that only makes sense for a house with a very small heat load.

I don't know the output of your boiler or the heat load of your house, but as a general rule you'd like your storage to carry your house for 12 to 18 hours.

 

[flyingcow]

about the same as all the others here. I've got 820gals of storage with a tarm Innova 30. Just came home(1800sq/ft 2 story) and house is 68(nice day outside, 30 degrees).Thats where the t-stat was set. I filled the unit(for the second time) up at 9pm last night. Went to work about 130 am tank was 181/178/165. That's about as high as I want it. 3 kids + wife took showers this morning. My domestic water is running thru my boilermate, that will change Wed. I'll have the Dom coil hooked up, I think this will help me alot on my life of tank temp. Still have more controls to finalize also.
Tank temp when I came home at 4pm was 116/117/121. My wife was home all day, working hard at cleaning the house. I suspect that I will have to reload the unit at about 7pm. This final loading should bring tank up to temp. I only get about 3 1/2 hours before I reload, than the second load lasts about 4 hours.
Doesn't take a couple of minutes to start kindlin', than fill unit and leave. In about 35/40 minutes I got 180 degree water entering house. Second trip is just to fill unit. So that 2 trips to the garage. When we had mornings of -38 and a high of +3 at noon wood usage was way up.

The smallest tank Tarm recommend for my 100,000 btu unit was 600 gals. The 140/170,000 btu unit was 800 gals and the next size up was 1000gal.

 

[ISeeDeadBTUs]

No storage for me . . . though I have contemplated a 'buffer tank' to smooth out the demand during those times when 6 zones come on, then 5 of those zones go off.

As everyone has said, you can't compare burn times with no storage to burn times with storage. At best you can try to compare wood consumption. Storage has to be at least a little bit better in that department. But you also have hassle-factor . . . I don't restart fires from November to early March.

I think the biggest mistake buyers make is buying based on advertised 'burn rates'. If you are home for half the day you should be able to get by on 8 hour burn times. The trick for those of us without storage is know how to size the load so as to get just enough fuel for the required time. More of an art than a science.

Jimbo

 

[Nofossil]

As everyone has said, you can't compare burn times with no storage to burn times with storage. At best you can try to compare wood consumption.
Jimbo

I think it's also fair to compare the total amount of time spent feeding and tending, as well as looking at how the feeding/tending schedule fits the needs of the user.

Some of the attraction of OWBs is the idea that you only have to feed them two or three times in a day. Yes, it may be a lot of wood, but you're not tied to it with a short cord. Without storage, a long slow burn is better at matching the heat load of the house. Maybe not efficient, but comfort and convenience are valuable too.

If you're trying to heat efficiently using a small gasifier without storage, you might well drive yourself crazy. Maximum efficiency requires short hot fires, but the output during a fire is too much. The best you can do is make really short fires and let them go almmost all the way out before refilling. That means tending every three or four hours. Not convenient or even possible for many people.

Storage makes a big difference in convenience. It allows you to do tending on your own schedule. For instance:

Sunday we went to a Super Bowl party (big surprise). We had to leave at half time to pick up our daughter. I built a fire at the end of half time, and refilled it after the end of the game. That was enough to carry us through last night with no problems at all. Last night I built a fire at 8:00 and refilled it at 9:00. That will carry us through some time tomorrow, though I'll build a fire tonight whenever it's convenient.

In this weather, I only have to build a fire and do a single refill about an hour later. It doesn't get much easier than that. I never have to build two fires in a day, and very rarely have to do more than two refills.

If I had more storage and a larger boiler, I could get by on a single refill almost all the time, and I could skip days more often.