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Any downside to wood gaser/oil combo unit?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by mark123, Jan 9, 2010.

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  1. mark123

    mark123 Member

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    I am considering replacing my oil/conventional boiler combo (Benjamin CC 500) unit with a gasifier but would still need oil back-up. To save space I was thinking of a WoodGun with built in oil back-up and domestic coil. Is there anyone here with one of these and is there any advantage of having 2 seperate units?

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  2. DaveBP

    DaveBP Minister of Fire

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    If you can compare the price of the combo boiler with the price of the same boiler without the oil option there might be a very large difference.

    I found in my research that this difference could more than pay for a separate very high efficiency boiler unit that would be a lot cheaper to run. I can't believe the oil burner options available in the wood boilers are very efficient.
  3. mark123

    mark123 Member

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    The wood gun's option is around $2000.00 and around here that is about the price for a common oil fired boiler. I do not plan on using oil very much, maybe 1% of the time so the efficiency does't matter, plus a cold start high efficiency boiler around here is around 6000. My biggest reason is the much smaller footprint.
  4. DaveBP

    DaveBP Minister of Fire

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    At those comparative prices and that little actual use I don't think you can lose.
  5. Gooserider

    Gooserider Mod Emeritus

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    The two biggest downsides to a combo unit is lowered efficiency and lack of redundancy, plus in some designs, the potential for increased problems with the fossil burner.

    1. Efficiency - the two fuels have very different burning properties, so it is very hard to build a combo unit that burns both with high efficiency - usually one side or the other will suffer (if not both)

    2. Lack of redundancy - If you have two separate units, and something breaks, then your odds of being able to run off the other and take your time getting the repair made are pretty good - with a combo, you may have more of an urgent situation - and since wood boilers aren't the most common things in the world, your fossil service guy might not be equipped to solve the problem... In addition if one side of a combo unit dies horribly, you have to replace the entire thing, rather than just the separate unit that failed...

    3. Some (NOT ALL) of the combo units put the fossil burner so that it shares the combustion chamber or parts of the smoke path with the wood side... Fossil burners are fairly delicate, and it is not at all unusual for them to have problems because they got hit with stuff from the wood side, or were contaminated by wood combustion byproducts... Look for a combo unit that puts the two fuels in different areas, and only combines the exhausts...

    Not to say that any of these issues are deal breakers, but just something to keep in mind...

    Gooserider
  6. sbleiweiss

    sbleiweiss New Member

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    I have a Woodgun and separate oil boiler. I decided to go that way for a few reasons. First, as Gooserider points out, there are potential issues with the soot from the wood burning fouling the electric eye in the oil burner. AHS has a refractory plug that will allow the oil burner path to be sealed off, but you have to manually remove the plug before firing the oil boiler. It makes it a manual switch-over (not wife friendly).

    I also believe that the "variable speed draft fan" option is important with oil. I have read here that the draft fan running full speed makes the oil burner difficult to get dialed in.

    There are other attributes of this boiler that are in stark contrast to a modern high efficiency oil boiler. Thermal mass is vastly different for one. My Buderus holds 10 gallons of water and heats up from a dead cold start in a few minutes. The woodgun (E140) holds 80 gallons and it's thermal mass is equivalent to over 100 gallons. It won't warm up quickly on oil, you would need to keep it hot all the time. The wood boiler needs this large thermal mass to allow it to cycle on and off without storage, but for oil, it's not optimal. Insulation is another issue. The front of the woodgun has no insulation and more than heats my basement when running wood. I don't think it would get as hot when running oil since this does not warm the firebox area, but the overall heat loss would be about the same. My boiler will drop about 20 degrees in about 12 hours (120 degrees F down to 100) due to heat loss to the basement.

    In the end, if you plan to run wood year round and oil is a fall-back, I think it's a fine solution. If you plan to run on oil 4-6 months per year, you might want to consider separate units. It was an easy choice for me because I had an extra flue in the basement for the wood boiler. If you have only a single flue now, running a dedicated vent pipe for the wood boiler can be quite expensive and may dominate the decision making process.

    Tarm (Bioheat) has a dual fuel boiler (Excel) also. Not sure if there are others.
    Good luck,
    Scott B.
  7. muncybob

    muncybob Minister of Fire

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    I have the WG combo boiler. A combo boiler was not my preferred choice but having only 1 chimney and the cost of replacing that chimney with a dual flu was prohibitive. 2 units would be ideal in my mind for reasons already stated. I also thought about going with a seperate lp unit but didn't want an outside storage tank.
    I have had issues in getting the oil burner to run without creating condensation due to the high draft caused by the WG draft fan. This has been resolved by defeating the fan during the oil burn stage. I plan to use the oil burner primarliy as back up just as you seem to intend. I'm not sure on the efficiency of the oil side but it has to be better than my old boiler which was around 50%! The wood side is very efficient for me as thus far I've burned less than 2 cords since October. Going into this type of heat I wanted to burn 4 cords or less per year and it seems I may be on target for that.
    As mentioned, it will take a fair amount of oil to get to temp from scratch but not too bad if you use it as an auto backup in the event the wood goes out as the temp will only be done to 150 or so. Overall I am pleased at this point. So far though I am only running the oil just to be sure the burner is working properly when doing the regular cleaning.
    scottb, what plug are you referring to when running oil? I have not done anything to burn oil expect turn the switch to oil or auto.
  8. sbleiweiss

    sbleiweiss New Member

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    When I talked with AHS about the oil burner, they suggested 2 things to keep the oil burner from being fouled by wood combustion by-products. The first was simply to fire the oil burner once a week to help burn off any crud and make sure it was in working order. It sounds like this is working for you. The second, more extreme method, was to install a refractory cement plug in the oil burner pathway. This would block all wood exhaust from getting to the burner, but the plug needs to be removed before using oil. AHS can supply this if you needed it. I have never seen it, but I would guess that it installs where the oil burner tunnel comes out behind the lower front door.
  9. Eric Johnson

    Eric Johnson Mod Emeritus

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    When I had a combo unit (not a gasifier) I just put a rubber pipe plug into the oil burner tube, after taking the gun out. Since I only used it as a backup when I went on vacation and during the shoulder seasons, it's wasn't a big hassle, and I was always confident that the oil burner would work. Simply firing it up once a week isn't really a very good option. You'll wind up replacing a lot of nozzles.

    What Goose said is right on. IMO, you're way ahead to buy a cheap oil burner. You can power-vent it out the side of the building if you don't have the chimney capacity. As he said, when you spring a leak on a combo unit, you're SOL--and that usually happens in the dead of winter.
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