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Oil Boiler and Wood Boiler Connection

Ravenwoods Member Posts: 12
I've been using a wood burning boiler for the past three winters to heat my oil burning boiler so it doesn't use oil. The current connection works well. During the winter our primary source of heat is the wood boiler. We use the oil boiler to keep the house warm if we are away, run out of firewood, etc. During spring, summer and fall when there is not enough demand to justify using the wood boiler, we heat with the oil boiler.

 The supply from the wood boiler connects to the pipe before the circulator, which is currently in the traditional location of before the boiler with the expansion tank after the oil boiler. The return to the wood burning boiler is after the boiler and  expanion tank.

I'm in the process of adding on rooms to my house and so decided to redo the boiler piping as it currently looks very unprofessional and totally unorganized. It was never setup for easy rational add ons.

I  plan on putting the expansion tank after the boiler and before the circulator. Then I'll have a supply manifold for the various zones.

Dan Holohan has diagrams for two boiler systems. But I don't think they apply in my case since the job of the wood burning boiler is only to keep the oil burning boiler hot. It does not supplement the oil burning boiler.

The wood burning boiler is connected by 1.25" pex and it is located in my garage about 80 feet away from the oil burning boiler. This loop has its own circulator.

I was thinking I should connect the supply to the oil burning boiler from the wood boiler just before the oil boiler (oil boiler return line).. Should the return to the wood boiler be before the oil boilers circulator and after the air separator/expansion tank? Or before the air separator/expansion tank? Or after the circulator?

Another scenario might be to connection the supply from the wood burner just before the supply manifold for all the house zones. But this might not be the best, since most zones will now be radiant floor heat. It would be better it seems for the hottest glycol to circulate through the oil boiler first before so it is cooler before heading out to the zones. Less mixing would be required before hitting the radiant floor zones.

I'm also adding a 119 gallon heat storage tank to store hot water in. This will give the wood boiler something to do during warmer weather when demand for heat is lower. There is be some sort of differential thermostat that will open the zone valve  to circulate heat back to the oil boiler when the temperature differential between the two indicates the wood boiler is low on wood.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    a few questions...

    It's a little hard to follow your post, so bear with me.

    What is the oil boiler doing?  Are you just piping thru it because you didn't do any re-piping, or are you keeping it as a back up?  Do you plan to ever re-use it?  Does it run at all?

    What size/type is the oil boiler, capacity, etc.  How much water does it hold?

    Same questions for the wood boiler.

    Does the wood boiler have glycol in it only? Or does the whole system have glycol?

    How are you controlling the output of the wood boiler?

    What are you doing for domestic hot water.

    What kind of controls are you using for boilers.

    If you could post some pics of what's going on near the oil boiler, that would help.

    If you don't need, or don't want the oil boiler, it may be better to remove it, and go with something that will store alot of water (buffer tank). Then pipe all the zones traditionally off of this.  But you may wish to keep it, so you get some heat overnite, if you go away for the weekend, etc.

    Provide more of the picture and I (we) will offer our suggestions.

    There was an error rendering this rich post.

  • Ironman
    Ironman Member Posts: 7,376
    Wood Boiler Integration

    As Steve requested, more info would be helpful.

    We install a lot of wood boilers and our standard method to connect to an existing boiler is to pipe into the return of the existing boiler with two closely spaced Tee's (like with primary/secondary) and let the wood boiler inject heat there. It doesn't matter if it's done before or after the circ if that's on the return since the Tee's provide hydraulic separation. I would recommend putting the circ on the supply, down stream of the expansion tank connection, though.

     Follow the same rules as for primary/secondary: straight pipe for 8 pipe diameters minimum on the inlet side of the Tee's; straight pipe for 4 pipe diameters minimum on the outlet side of the Tee's; keep the Tee's as close together as possible, not more than 12", center to center; injection coming in on the upstream Tee, going out on the downstream Tee.

    Use a heat exchanger if the wood boiler is unpressurized or if you don't want to glycol the entire system.

    Set the aquastat on the oil boiler about 15 - 20* below the temp that the wood boiler can supply (but not below 160*). Or, you can use a double throw aquastat that senses water temp from the wood boiler and switches off the oil burner when the wood boiler reaches sufficient temp (170 - 180*).

    Again, more info would be helpful.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669

    I usually install the Tee's with the return water from the house going to the first tee that goes to the wood boiler and then the second tee injects the hot water from the wood boiler into the return of the fossel fuel boiler. This way the coldest water goes to the wood boiler for best heat transfer and then the hot water gets injected downstream of the first tee so there won't be any heat from the wood boiler getting recycled back to the wood boiler.
  • Ravenwoods
    Ravenwoods Member Posts: 12
    More Information

    I've edited my original post and answered some of your questions there. But I've added more information below.

    Oil boiler heats our domestic hot water in the summer, and provides heating for the house when there is warm weather. The wood burning boiler works most efficiently when there is demand placed on it. I'm in Fairbanks, Alaska and we have long winters and at times it gets 45 below.  We are not at the lower elevations where it gets 55 below. Usually this is only for a few days. But we do get weeks of weather 20-30 below where the high daytime temperature never gets above minus 20.

    Oil boiler: Weil/Mclain 100,000 btu. Not sure of liquid capacity.

    Wood boiler: Greenwood 100,000 btu. 7.5 gallons liquid capacity. It is connected to a plate heat exchanger. Wood boiler does have glycol as well as everything else.There is a differential thermostat the measure the temperature at the oil boiler and the Greenwood. So the circulator is turned on and off according to that thermostat. An aquastat also controls the circulator to prevent cold temperature returns in the Greenwood. A zone valve also prevents unwanted circulation from the oil boiler. Previously the circulator at the oil boiler did circulate out to the greenwood even when the greenwood circulator was off. So I put in a zone valve to control that.

    The oil boiler is controlled by a tekmar control unit. When I'm running the greenwood, I just turn the oil boiler off. I understand it is not good to burn oil in a cold boiler, but there is no harm in a boiler if the temperature drops below 140 as long as you are not burning oil. Often the wood burns low in the greenwood before I get home from work and the temperature in the oil boiler will be 120 or less. So to prevent the oil boiler from coming on I just turn it off.

    Domestic hot water is heated in the oil boiler.

  • Ironman
    Ironman Member Posts: 7,376


    You are correct in your description: the injection should be on the down stream Tee. I must have been having another senior moment. These come about the same time as  grand children. :)

    Thanks for pointing out my error and setting things right.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • trevb
    trevb Member Posts: 2
    how do i hook a woodstove to my oil boiler

    I have a wood stove with piping in it that can be hooked to the base board heating. But the wood stove makes so much heat the system "blows off" often. What I want to do is stop burning oil and just use this wood stove for my winter heat and hot water. To do this I know I have to pipe the wood stove to the oil boiler. What I do not know is what parts I need and where the parts go in the system. If someone can tell me how to do this or where to print of some plans to make this system work I would be very happy. And I could stop burning so much oil.

    Thank you

  • Zman
    Zman Member Posts: 7,569
    Storage and control

    Check out the attached file.

    The key to whole thing is to create a buffer and control it correctly.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • trevb
    trevb Member Posts: 2
    Oil Boiler and Wood Boiler Connection

    thank you very much! this is very helpful. I will be doing a lot of reading today as it is snowing.
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