# wood/oil boilers



## ssupercoolss (Jan 28, 2008)

i just found this very informative site, and hoping i could get my choices somewhat "boiled" down for me.  my long range plan is to replace my oil boiler with a wood/oil unit.  right now i use a pellet stove for about 85% of my heating needs.  but due to some pipes running through an unheated crawlspace, and up an exterior wall, when the temps get down to the low 20's i have to run the boiler.  what i am looking for is the brand names of units that have wood/oil in the same unit.  my research thus far has come up with AHS(which so far i am partial too) and tarm.  if anyone knows of some other wood/oil units i should be looking at, i would greatly appreciate the help.  for me right now, water storage is not in the budget, but definetly does have some appeal to it.  with a wood boiler i would probably be running the pellet stove about 50% of the time.

thanks in advance


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## Mainewood (Jan 28, 2008)

http://benjaminboilers.com/combination-series.htm


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## Eric Johnson (Jan 29, 2008)

Welcome to the Boiler Room, futureboiler and Mainewood.

The AHS is a wood gasification boiler. I believe the Tarm Excel is not. The Benjamin in Mainewood's link isn't either. The AHS Wood Gun is the only combination oil/gasifier that I am aware of. On the wood side, gasifiers are about twice as efficient as most non-gasifiers, or what we call conventional wood-fired boilers around here.

I had a Marathon Logwood combination oil/wood boiler for about 9 years. 

Feel free to join in any threads or start some of your own to get the information you're looking for. The webmaster, Crag Issod, used to own Tarm USA, so hopefully he will provide some information about its offerings. I don't believe we have any AHS Wood Gun owners around here---yet.


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## Mainewood (Jan 29, 2008)

Eric Johnson said:
			
		

> Welcome to the Boiler Room, futureboiler and Mainewood.
> 
> The AHS is a wood gasification boiler. I believe the Tarm Excel is not. The Benjamin in Mainewood's link isn't either. The AHS Wood Gun is the only combination oil/gasifier that I am aware of. On the wood side, gasifiers are about twice as efficient as most non-gasifiers, or what we call conventional wood-fired boilers around here.
> I had a Marathon Logwood combination oil/wood boiler for about 9 years.
> ...



Eric, How do you define efficiency? 

thanks, Mainewood


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## Eric Johnson (Jan 29, 2008)

As I understand it, overall efficiency is the combination of combustion efficiency, which is the percentage of btus actually recovered from the fuel compared to its potential, and heat transfer efficiency, which is a measure of how many of those btus actually get transferred through the heat exchanger and into the water.

Smoke and high stack temps, then, are good indicators of inefficiency. My old boiler smoked and produced creosote, so I had to keep my stack temps up to keep it from forming in the chimney. Not a very efficient setup.

It may be more complicated, and I'm sure guys like Master of Sparks and nofossil can provide more complete descriptions, but I think those are the basics.


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## webbie (Jan 29, 2008)

Our man nofossil here is getting a handle on total system efficiency, and has some pretty good stats available. He has enough instrumentation in his house to run the Space Shuttle, and also seem truly inspired to find out "the rest of the story".

The Tarm Excel is a high efficiency downdraft (fan forced) model....I should know since I introduced it to the market. It is basically the same technology as the Tarm Solo models, but with the addition of another combustion chamber and more firetubes for the oil side. I can say without reservation that this is a superior product. 

I don't keep up on all the other technologies, but there is a lot of factors to consider. The best oil/wood units will have a completely separate chamber and heat exchange for the oil burner. The reasons are simple:
1. It allows full tuning of the oil chamber
2. The oil chamber will not get soot and ash in it
3. You can actually add the output of both the oil and wood chamber together (in some cases), because they use a different set of firetubes.

Further to the efficiency question, there are a lot of factors at play.....
1. As Eric says, the combustion efficiency is VERY important, because that means the fuel is being used - cleaner chimney, air, etc...most BTU's per lb of wood.
2. The Heat Transfer to the water of the boiler - this relates to the boiler firetubes and the "sq ft of heating surface" - meaning boiler walls surrounded by water.
3. Insulation of the unit - so that heat does not soak out through the jacket.

Then, after those criteria, there are other factors such as the efficiency of the heat delivery system. But I suppose these are somewhat the same no matter what fuel is used. Storage is a great thing, but some heat may be lost there (standby loses).

So far we are only guessing at total system efficiencies.


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## Nofossil (Jan 29, 2008)

Webmaster said:
			
		

> So far we are only guessing at total system efficiencies....



...but we are getting some pretty good guesses. I've calculated overall system efficiency two different ways, and I've come up with numbers that line up pretty well.

First, at top-down estimate. I know what my oil consumption was in gallons per degree day back before I had wood, and I know the approximate efficiency of my oil burner (from manufacturer's specs). I also have two full seasons of heating solely with wood. From that, I can calculate that I'm getting about 15,000,000 BTU per cord.

Second, a bottoms-up estimate. This involves measuring the moisture content of the wood, weighing it, burning it, and measuring the heat delivered into each heat load (baseboards, hot tub, hot water, and storage). From that I calculate that my system efficiency is about 55% based on a theoretical fuel value of 8600 BTU/lb of bone dry wood. Given my mix of species, that works out to around 14,000,000 BTU/cord. I'd expect this number to be lower than the first, because there are heat losses from the system that end up heating the house anyway - heat radiated from the boiler, stovepipe, and chimney, for instance. The first number in effect gives me credit for those losses, while the second number does not.

These numbers tally very well with my brother's gasifier experience. He had a very clean burning conventional boiler, and switched to a gasifier. He saw a 40% reduction in the amount of wood required to generate the same amount of heat. He's even more obsessed than I am - weighs all his wood and keeps a journal.

Based on a fair amount of hard data, I think a properly operated gasifier uses about half the wood of the best non-gasifier designs. I also expect that there's a lot of variation, and we're still working on exactly what 'properly operated' means.


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## Eric Johnson (Jan 30, 2008)

I gather that you guys got ahold of EKO's efficiency tests on the 80. Did you find that useful in your analysis of your system, nofossil? Can you comment on the test conditions vs. your real-world operating conditions?

I'm wondering how many other manufacturers would be willing to make their testing procedures available.


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## Mainewood (Jan 30, 2008)

nofossil said:
			
		

> Webmaster said:
> 
> 
> 
> ...




nofossil, What "clean burning" conventional boiler did your brother have and did he have any water storage with it?


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## Nofossil (Jan 30, 2008)

Eric Johnson said:
			
		

> I gather that you guys got ahold of EKO's efficiency tests on the 80. Did you find that useful in your analysis of your system, nofossil? Can you comment on the test conditions vs. your real-world operating conditions?
> 
> I'm wondering how many other manufacturers would be willing to make their testing procedures available.



In some ways, the test of the EKO 80 was not realistic, but it's instructive for those of us fascinated with efficiency. Here's a summary:


```
-                      EKO 80 test      My EKO 25
Pump gpm                34                 8
Water in temp          130               145
Water out temp         145               170
Flue temp              248               550
Efficiency              90%               ??%
```


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## Eric Johnson (Jan 30, 2008)

Any thoughts on what your efficiency numbers would look like with turbulators and resulting lower exhaust temps?


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## Nofossil (Jan 30, 2008)

Eric Johnson said:
			
		

> Any thoughts on what your efficiency numbers would look like with turbulators and resulting lower exhaust temps?



Dunno. I'm picking up a turbulator prototype this week. I'll publish test results as soon as I can get data.


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