"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.
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.
