# Multi-fuel furnaces and boilers



## Eric Johnson (Oct 19, 2007)

"Multi-fuel" boilers and furnaces are an interesting phenomenon and the beneficiaries of a huge loophole in the code structure.

First off, the loophole: Central heating appliances are already exempt from all EPA air quality regulations, but that's not what I'm talking about. It's normally against code to vent more than one appliance into the same chimney, especially if one is oil and the other one is wood. There are a number of reasons for this, but the one that's most compelling to me is related to safety. A heavily-creosoted chimney that is suddenly used to vent an oil or gas appliance will have a lot less draft than a clean chimney. And if it's plugged up bad enough, you're going to get carbon monoxide and other deadly gasses venting back into your house. Even a chimney with a thin creosote glaze can "popcorn" up under some conditions, creating a blockage that wasn't visible when you switched over. (You did clean and inspect the chimney before switching from wood to oil, right?).

But if you have a "multi-fuel" boiler or furnace, i.e., one that burns both oil and wood, then you can hook it up to the same chimney. In most cases you have no choice because there's only one exhaust outlet on the thing. It's dangerous, but completely legal. Anyone who sells you one of these things without completely explaining the risks and verifying that you understand what is being said, is in my judgment, putting you at unnecessary risk.

There are two ways to build a multi-fuel appliance. One way is to leave an opening in the wood firebox for an oil gun to be mounted. This gun will shoot its flame into the firebox and warm the air or water surrounding it. When you switch back to wood, it remains in place. This approach has several distinct disadvantages. First, a wood-burning firebox is terribly inefficient for burning oil because it doesn't have nearly the surface area enjoyed by a dedicated oil burner, and much of the heat therefore goes right up the stack. So why don't they have a more efficient design? Well, surface area in a fossil-fuel firebox means lots of nooks and crannies, and those get jammed up pretty quick with creosote, ash and other junk when you try to burn wood around them. So you design the thing to be good at burning wood or oil--not both. Secondly, an oil nozzle is a pretty delicate, precision piece of equipment. They're not expensive and relatively easy to replace, but they won't hold up to the kind of abuse they'll see if lodged in a working wood firebox. So if you switch over to oil a couple of hours before leaving for vacation in the winter, you're probably going to come home to a cold, frozen up house. The oil burner might sort of work for awhile, but it won't do it for long. If you're smart, you'll either remove the oil gun when burning wood, or replace the nozzle before firing up the oil. The oil gun also is a source of uncontrollable air incursion into the firebox. It's hard to get the most out of your air-tight boiler when you can't control all the air.

A much better way to design a multi-fuel appliance is the approach taken by Tarm and probably some of the other better brands. What they do is have separate fireboxes for wood and oil. This costs more, but they can exploit the strengths of each, resulting in better efficiency and more reliable operation. I believe they still vent into the same flue, though it would be easy enough to build one with separate exhaust vents. But with this design, you've essentially got two boilers in one.

In my opinion (and I've only owned the shared-firebox kind), the best arrangement is actually two boilers or furnaces. In the case of boilers, you can pipe a wood boiler in parallel with a conventional oil burner without too much trouble, and then replace or repair each independently as needed, without losing the ability to heat your house in the process. If you only have one boiler and it springs a leak in the dead of winter (when else?), having the multi-fuel option doesn't do you much good--it just costs a lot more to replace, and you're probably going to have to quickly drain your system and move into a motel while you figure out what to do. If either a parallel wood- or oil-fired unit craps out on you anytime, you can fix or replace it at your leisure.


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## Nofossil (Oct 19, 2007)

I'm very interested in this for another reason. As you know, my brother and I are mad scientists and built a 'from scratch' gasification boiler for his house. When mine dies, I'd like to do the same thing, but with a critical difference: I want an oil burner that fires into the upper chamber. I'm thinking that you would mount it so that the oil boiler tube is your air inlet - oil burners have fans already, so this is not a huge stretch. 

If you could pull this off, you would get two benefits:

1) You could fire the oil for a couple of minutes to get the wood started and the gasification chamber hot. Much less external smoke, no fussing around.
2) You would have backup heat when you're away.

You would need to have some protection of the oil nozzle so that it didn't get creosote condensation on it. Perhaps you could do it with airflow, or perhaps you'd need a little lever-operated cover.

Your comment about woodstoves not having as much HX surface area as oil boiler is valid for traditional wood boilers, but not necessarily for wood gasification boilers. The one my brother built has way more surface area than my oil furnace. The EKO is pretty close - I haven't done the calculation.


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## leaddog (Oct 19, 2007)

nofossil said:
			
		

> I'm very interested in this for another reason. As you know, my brother and I are mad scientists and built a 'from scratch' gasification boiler for his house. When mine dies, I'd like to do the same thing, but with a critical difference: I want an oil burner that fires into the upper chamber. I'm thinking that you would mount it so that the oil boiler tube is your air inlet - oil burners have fans already, so this is not a huge stretch.
> 
> If you could pull this off, you would get two benefits:
> 
> ...



MY step son has a combination furnace that works that way I think it is called a Grizzly. You use the oil to light the wood . It works real well but being and older furnace its not real effecient. The oil has a ceramic chamber next to the wood and then it exausts to the wood. I'll try and get more info.
leaddog


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## Eric Johnson (Oct 19, 2007)

I should have made clear that I was referring to conventional, air-tight wood-fired boilers, not firetube gasifiers. Tarm, and many others, still make and sell them. But maybe not for long if the laws change. A gasifier would certainly work pretty well if the hot gas was blown into a conventional oil furnace heat exchanger. I'm familiar with some old chip gasifiers that did just that.

I like the idea of putting an oil gun in a gasifier. I know you can burn soft coal in an EKO. I wonder if there's a way to completely burn the oil smoke in the gasification chamber. I wonder if you run the risk of blowing the thing up while you're trying.

After trying to comingle the gas and wood on my old multi-fuel boiler for about a season and a half, I finally decided that the oil was strictly a spring/fall/vacation feature, and I would just remove the gun when I started burning wood for the season and plug up the tube with a 4" rubber and steel pipe plug. Come spring I'd switch back to oil. Trying to run the boiler on wood without plugging the oil gun tube led to "puffing" whenever the draft damper would close. A big series of small explosions in the ash drawer would eventually fill the basement with smoke. Wife not crazy about that.


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## Shak (Oct 19, 2007)

Eric,
I've got to agree with you on the two boiler point of view. I installed my Necchi and Campiglio boiler along side my existing oil boiler. When I'm burning a fire I simlpy circulate water between the boilers. As long as the water temp stays above 150 the oil burner never fires. If i go away for the weekend and the fire goes out NBD. And I get the best efficiency from both fuels. Domestic hot water is also supplied from this system.


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## Eric Johnson (Oct 19, 2007)

Another thing I forgot to mention is potential damage to the chimney. Fuel oil contains sulfur (though less now than before, and a lot less in the future). And wood contains water vapor. Mix the two in the same chimney, especially if it's a masonry chimney, and you run the risk of creating sulfuric acid. And other bad stuff. I'm no chemist, but that can't be good for the chimney. Probably not good for the steel that your firebox is made of, either.


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## Nofossil (Oct 19, 2007)

Eric Johnson said:
			
		

> Another thing I forgot to mention is potential damage to the chimney. Fuel oil contains sulfur (though less now than before, and a lot less in the future). And wood contains water vapor. Mix the two in the same chimney, especially if it's a masonry chimney, and you run the risk of creating sulfuric acid. And other bad stuff. I'm no chemist, but that can't be good for the chimney. Probably not good for the steel that your firebox is made of, either.



Oil burner flue gas contains water vapor, too - a byproduct of combustion. While there's certainly sulfuric acid in the chimney, it's low enough in concentration that it doesn't cause problems. I wouldn't get freaked about it, but I also wouldn't drink any condensate from the chimney ;-)


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## jklingel (Oct 23, 2007)

The sulfuric acid concentration mentioned above is not ALWAYS low. A plumber friend has seen several oil boiler stacks corrode disastrously in a few years; one happened in the first winter. He and the owner took that one to the petrol plant, and they confessed that "sometimes we get a little extra sulfur by-product." I guess so.... FWIW, that is all I know... about anything. j


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## slowzuki (Oct 23, 2007)

The corrosion if fairly common in high efficiency appliances in well insulated houses.  The chimney ends up spending a lot of time below the dewpoint and you get plenty of condensate.


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## jklingel (Oct 23, 2007)

That is what the plumber said, and we get plenty of time for the temps to produce condensate, for sure. My son installed a Side Shot (I think it is called) that blows the exhaust out about a mile from the house. It is SS and there is no insulated pipe between the boiler and it (about 4'). The only nuisance is ice build up, which has to be chipped off periodically. We are looking into insulating it somehow. If it ain't one thing.....


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