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Combustion Air - Forno Bravo Forum: The Wood-Fired Oven Community

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Combustion Air

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  • Combustion Air

    Well, I went and put a wood-fired oven inside of a tight, energy-efficient home and now I'm beginning to wonder how this will work.

    I have a wood-burning EPA fireplace, but that is sealed and has a duct from the outside to supply combustion air.

    And, there is a very large kitchen range hood that exhausts a huge volume of air, but there is also a mechanical vent wired to open and allow some air into the house when the hood turns on. Besides, the hood really only runs for a few minutes while you are using the range, but the wood oven will need to burn for hours.

    I'm wondering what I should do to get some air to the oven fire. I haven't built the hearth yet and I thought about building an outside air duct right into the hearth with a grate of some kind for an outlet just outside the oven opening.

    Thoughts from any of you experienced indoor oven users?

  • #2
    Re: Combustion Air

    I just did a google maps of where you live, Ive never even given this cold air in a thought.
    Maybe some sort of a manual vent near the oven opening to allow air in?
    The English language was invented by people who couldnt spell.

    My Build.

    Books.

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    • #3
      Re: Combustion Air

      Yeah, we are in a heating climate, so a "green" build will be tight with controlled and heat-recovered ventilation. How much air does an oven fire draw?

      I've been enamoured of the "blast doors" that several posters have displayed. The actual vents on those can't be more than a few square inches in area. I could probably fit a 3" PVC pipe into the hearth and have a grille just outside the oven opening. I'm wondering if that would pass enough air. Because it is a residential installation, the flue pipe is quite high (about 14') so the draw should be stronger than most.

      Brickie, I like the way your opening has tapered bricks around the edge. Buy them like that or just cut them on the masonry saw?

      Comment


      • #4
        Re: Combustion Air

        Originally posted by GreenBldr View Post

        Brickie, I like the way your opening has tapered bricks around the edge. Buy them like that or just cut them on the masonry saw?
        I bought what they call squint bricks, they are made for bay window returns, I then cut the taper on the saw.
        The English language was invented by people who couldnt spell.

        My Build.

        Books.

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        • #5
          Re: Combustion Air

          A tightly built modern house should already be bringing in outside air as part of it's HVAC system. It will generally look like a second dryer vent that is hooked into the return ductwork. If you are certain you don't have that give me the diameter of your 14' flue and I can calculate the draught it will create and therefore tell you how much make-up air you need and hence how big of a duct you will need.

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          • #6
            Re: Combustion Air

            Technically speaking, the tightly built houses bring in the same amount of air as they exhaust, using a heat or energy recovery ventilator. So, in other words, the air exchange is already balanced. Functionally speaking, we can't make them perfectly tight, so there is still a small amount of leakage, but it shouldn't be enough to support appliances which depend on combustion such as gas fired heaters, fireplaces, or, in this case, wood-fired ovens. Each one of them has to have a specific source of outside combustion air.

            Small fires, like the burners on a regular kitchen range aren't an issue, but the bigger the fire, the more one needs to provide a source of combustion air if you don't want to depressurize the house envelope. Indoor wood-fired ovens are still relatively rare, so I haven't seen this issue come up before in conjunction with modern tight houses.

            Thank you for your kind offer to supply calculations. The actual parameters would be:

            INDOOR TEMP: 68F
            OUTDOOR TEMP: 50F (make up air)

            It is 6" double wall stainless flue and the actual height of the pipe opening from the combustion area or cooking surface due to the oven height is 10'.

            The most reasonable source of combustion air would be a run of about 6 feet in 4" round metal duct with two 90 degree elbows, taken from a point 2 feet below the combustion level. Workable?

            Comment


            • #7
              Re: Combustion Air

              Guess I'm on my own for the combustion air supply venting.

              I'll take lots of pictures, in case it actually works. :-)

              Comment


              • #8
                Re: Combustion Air

                If you already have a vent, why put in another?
                Kindled with zeal and fired with passion.

                Comment


                • #9
                  Re: Combustion Air

                  Not sure which vent you are referring to, David. I am talking about a vent to supply air (oxygen) to the fire in the wood oven.

                  Comment


                  • #10
                    Re: Combustion Air

                    It is an open field, for sure. Unless you are providing outside air to the point source (for example to the lower entry area of your oven), then the device does not require a combustion air kit. As an example, a double sided fireplace in the center of a room or between 2 rooms does not require outside air provision.

                    If your oven is already built, then you need to increase the outside air available through your HVAC system, not a point source.

                    Comment


                    • #11
                      Re: Combustion Air

                      Unless you are providing outside air to the point source (for example to the lower entry area of your oven)
                      That's what I'm thinking needs to be done in this case.
                      the device does not require a combustion air kit.
                      What is that based on? Volume of air burned? Ventilation Code?
                      As an example, a double sided fireplace in the center of a room or between 2 rooms does not require outside air provision.
                      I think that may depend on the state you live in and whether or not your house is "tight" or not.
                      then you need to increase the outside air available through your HVAC system
                      This home has a Heat Recovery Ventilator (HRV) in which the same volume of air is brought in as is exhausted, i.e. it is a balanced system.

                      Comment


                      • #12
                        Re: Combustion Air

                        I have a wood-burning EPA fireplace, but that is sealed and has a duct from the outside to supply combustion air.


                        As you already have a vent to the outside in your fireplace, you could simply open the fireplace door and your WFO will draw air from it too. Only trouble would be that you'd need to leave the door open if the WFO were being used or if both were being used simultaneously.
                        Maybe you can rig the vent in your hood so that you can open it manually to allow air in.
                        Last edited by david s; 01-25-2012, 08:49 PM.
                        Kindled with zeal and fired with passion.

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                        • #13
                          Re: Combustion Air

                          I think there is a pretty good chance that we will want to use the two simultaneously.

                          I would also be worried about the cross draft from the EPA fireplace all the way over to the wood oven - it pretty much cuts through main living area and could be a cold stream during certain months.

                          The hood vent is more in the kitchen near the wood oven, but it is only something that needs to be used for a few minutes at a time - when there is smoke or a large amount of things going on the range, while the wood oven might need to burn for hours.

                          Both of those things are what got me thinking about having a source nearer the point of combustion.

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