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Makeup air for a wood-burning fireplace in a tight home

Chaubenee | Posted in General Questions on

I am building a tight home with air sealing, R75 cellulose in cold attic floor, excellent Marvin windows, southern exposure and min-splits for heat and cooling. No gas appliances. We intend on using an ERV (Ultimateair) but we have one major extravagance: a Rumford masonry fireplace.

Is the ERV enough make up air or does anyone suggest using an added duct or HRV through the wall next to fireplace? Maybe something a a pair of these Lunos on each side or something? The masonry fireplace will be inside the envelope and we capture quite a bit of sunlight on bright sunny winter days as it is positioned properly to do that. Please don’t try to talk me out of it, I have wanted one all my life and have gone to great lengths in other area of the building design to make up for it. Any thoughts on this? Thanks!

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  1. Expert Member
    Dana Dorsett | | #1

    An HRV or ERV are nowhere big enough for make-up air for an open hearth fireplace and aren't designed for it. In a tight house your only option for running an open hearth fireplace is to open a window.

    In some areas open hearth fireplaces have very restricted use due to the extremely high PM2.5 and other pollutants compared to EPA rated wood burning appliances. A tight EPS rated wood stove with a big viewing window is both much friendlier to the neighbors and yourself, and it's a heating appliance with a high enough efficiency that it's at least possible to operate it in a sustainable way. The sub-20% efficiency of open hearth woodburning is so low and emission per BTU deliver are so high that it's worse than burning coal. With most temperate zone forests burning wood needs thermal efficiency north of 60% to make substantially better than burning fossils , when all the carbon and forest-carbon sequestration aspects are factored in. That's aside from the local air pollution issue.

    A Rumsford fireplace isn't a luxury- it's a crime against the planet, and possibly against your neighbors. There is no place for an open hearth wood burning in an otherwise high-performance home, and the notion that there is ANYTHING you can do " other area of the building design to make up for it..." needs some really careful enviro-accounting, a dubious assertion at best.

    If you're committed to doing that crime, nobody is going to stop you (except on restricted burning days in some neighborhoods.)

    But they look pretty kewl, in an antiquey-homey-historical sort of way, eh? ;-)

  2. Chaubenee | | #2

    Well, then I will open the window! Thanks!

  3. Expert Member

    I live opposite a provincial campground with open fire pits at each site. There is also a YMCA camp just down the road. I'm curious as to whether they are guilty of crimes against the planet too? Is it also a crime for the campers to have driven out here form the city?

  4. propeller | | #4

    Joe have a look at this Rumford fireplace.
    It offers supply air and decent efficiency.

  5. GBA Editor
    Martin Holladay | | #5

    I'm not qualified to act as a "crime against the planet" prosecutor, so I'm going to avoid legal language. (I suspect, when we all have to plead our fate with St. Peter in front of the heavenly gates, we may discover that almost every resident of North America has been guilty of crimes against our planet. But I digress.)

    Dana is correct that neither an ERV nor a pair of Lunos fans is a makeup air appliance. Those ventilating appliances are part of a balanced ventilation system, and the manufacturers specifically note that they are not intended to provide makeup air.

    Your question mentioned that you are planning to install a "wood burning fireplace in a tight home." It's fair to say that every energy expert that you care to consult will advise against your plan.

    Back in the mid-1970s, when that slim paperback book extolling the Rumford fireplace design could be seen on the shelves of every bookstore with a "back to the land" section, I helped my friend build a Rumford fireplace. He used a stone-and-mortar design, using expensive firebricks to line the fireplace itself, and it had the narrow, high design of a classic Rumford. It drew well, but he only used it once a year. I'm not sure whether, if he had to build his house again, he would have included a wood-burning fireplace.

  6. JTyler | | #6

    Joe - I grew up in a home with a rumford fireplace and completely understand your refusal to compromise this feature. I think that with some dedicated duct work and a floor register near the fireplace, you could probably have a fire without opening the window, and I doubt anyone would be chaining themself to the trees outside your house with a megaphone while you enjoyed the open hearth.

    I am also about to start a super insulated home build. My initial plan included a rumford and a not-so-tight envelope. I won't try to talk you out of your choice, but I will say spend some time reading about and looking at masonry heaters. I have wanted a rumford fireplace in my house since I was a kid, and now that my plan is a tight home with a masonry heater, I don't feel like I am compromising.

    Good luck with your build!

  7. Chaubenee | | #7

    I am not concerned since I own 26 acres and my nearest neighbor is currently 2000 feet to the west. To the East I own the wetlands and uplands which I had deed protected with the cooperation of the ACOE in such a way that they could never be developed, and in doing so helped to eliminate the key way to a contiguous tract of over 3000 acres both wet and dry. I went to extraordinary expense purchasing this tract so that developers could not ever cram a few hundred homes into the area. I think if I am ever on trial for the two or three dozen fires a winter I might burn, I will produce the canceled checks and the deed covenants I had filed. Nonetheless I needn't really have to explain my desire to have a wood burning fire with some of the blow down hardwoods I can collect off my large wood lot. I respect Dana very much for his knowledge. So as to setting up a duct next to the fireplace, how would you temper the air that comes in? Anyone have an idea as to the design I might employ with a damper or something?

    1. HuntCountry | | #35

      Good for you! We're adding two back-to-back (54"x60") Rumfords in our new addition to compliment the 4 fireplaces in the existing house - six total. The whole point of a Rumford design is that it burns very clean (and generates a lot of radiant heat) so I'm not sure why everyone is so bent out of shape with your plans. Like you, we also have woods on the property that create an endless supply of wind downed trees which are either going into a burn pile or our new Rumfords, else they'd require a gas powered chipper. We're adding a dedicated make-up air system with a backdraft damper on the exterior wall vent to prevent incoming air when not needed.

  8. GBA Editor
    Martin Holladay | | #8

    It's hard to give advice on air sealing this type of makeup air duct, because you clearly aren't planning to build an energy-efficient house. (By the way, what's your plan for preventing exfiltrating air from rushing up your chimney flue when the fireplace is not in use?)

    My usual advice for makeup air ducts near wood stoves is that these ducts should usually be between 4 inches in diameter and 6 inches in diameter. If you care about air sealing, the best possible way to shut these ducts is with a plumbing ball valve. There are two disadvantages to this approach: large-diameter ball valves are expensive, and this type of ball valve looks goofy in your living room. However, if you are an energy nerd, the ball valve approach is certainly the best.

    However, if I had a Rumford fireplace with a leaky flue, I doubt if I would care very much if my makeup air duct also leaked.

    As far as tempering your makeup air is concerned, the usual approach is to install a high-wattage electric resistance heater in your makeup air duct. These heaters are expensive to purchase and crazy expensive to operate. For more information on tempered makeup air, see Makeup Air for Range Hoods.

  9. JTyler | | #9

    Ball valve below the floor with a nice looking iron lever coming up through a slot in a grate?

  10. GBA Editor
    Martin Holladay | | #10

    Or a ball valve in the basement. Or a ball valve in a wall-mounted cabinet in your living room. There are lots of variations on this idea.

  11. Dana1 | | #11

    It would take a MONSTER sized ball valve to supply the make-up air necessary for an open hearth fireplace. A proximity vent has some limited value for air-tight wood burning appliances, but almost none for an open hearth fireplace. Make-up air volumes for open hearth burners are an order of magnitude higher than those needed wood stoves.

  12. GBA Editor
    Martin Holladay | | #12

    Dana is probably right. My advice was aimed at those with wood-burning stoves. In your case, just open a window.

  13. Chaubenee | | #13

    I have a door next to heart in plan, that leads to a south exposed screen room with concrete floor. I will see what type of apparatus I can rig up that collects sunshine and prewarms somewhat, the air that comes. Nonetheless, my plan is a chain operated chimney cap damper, and a hearth box cover- I am planning a foam lined wooden box cover, nicely trimmed out by a master carpenter, with an artist commissioned oil painted front to cover the opening when not in use. On the night I have a fire, I would remove it and place it in the screen porch. I imagine this cover will be on about 97% of the time or better. I am hoping I can get foam own the back to R-16 or better and have some hooks which keep the gasket tightened down. Here in upstate NY as you know, the shoulder seasons are long, and those are the times when no heating or cooling is happening. The winter is the time we are most concerned with energy loss. My Mason will build a smooth lip around the edge. I have looked at masonry heaters too, and I understand they offer more efficiency.

  14. GBA Editor
    Martin Holladay | | #14

    I'm glad to hear that you have a plan to air seal the fireplace opening. Good luck with your project.

  15. Chaubenee | | #15

    Well, for the huge amount of time one isn't burning, there has to be a plan. What about using schedule 40 PVC with a ball valve? A 4" runs about $80 bucks if you look for a good price. I wonder if I can feed the pvc to a brass grate right on the top surface of the raised hearth, and put a small door into the front of the hearth that opens and accesses the valve. Perhaps burying the pipe in the foundation wall or under the basement slab will temper the air, but if it is that close to the firebox, the tempering gets less important it would seem.

  16. Expert Member
    MALCOLM TAYLOR | | #16

    Some codes (like ours) preclude using combustable ducting like PVC for wood burning appliances. It's probably worth finding out if yours does.

  17. woodreader | | #17

    Instead of using a $80 ball valve, consider a 4-6" PVC coupling that is threaded on one end. Use a matching PVC cap with a soft foam on the inside. When you thread your cap the threads will push putting pressure on your soft foam. It should tighten up your air leakage.

  18. Chaubenee | | #18

    Richard, perhaps code says no to plastic, so I was thinking of three 3" steel or galvanized pipes coming up through a little trap door with a custom gasketed cover as you similarly describe. These could be buried into the foundation of the house and fireplace and cone up directly in front of the hearth opening. I will discuss what the old Italian mason says once we have a meeting regarding it. I bet he has dealt with this before I some respect. He is highly regarded and competent in not wanting to do any "cobbled" work. I will also reach out to Superior Clay in Ohio this week on it, as they build the throats and Rumford kits. I suspect this is achievable without going into the ERV as you have warned against.

  19. Dana1 | | #19

    An 8" or 10" duct valve is closer to what you would need for make up air for an open hearth fireplace- something comparable or larger in size to the cross sectional area to the flue. This is not the same scenario as an air-tight wood stove and more comparable to an atmospheric drafted furnace, only bigger, with greater volumes of dilution air than is used to control flue condensation with fossil burners.

    For enhanced air-tightness, the gasketed top closing flue damper is a far better choice than the typical bottom flue flaps or the non-gasketed flue top dampers.

    A metal flue liner would also help, as would a sheet-steel layer within the masonry build around the fire box. Mortared masonry may LOOK air tight, but it can be surprisingly leaky even when new, and only becomes leakier over time.

    In scanning the responses it appears I was a bit emoticon-deficient when describing open hearth fireplaces a "crime". (It's easier to see the wink in person, eh? ;-) ) While the sustainability and pollution points are true, it's understood that this isn't intended to be a heating appliance, and will have very intermittent use- primarily decoration. I DO think they're a bad idea even as decoration, in that they drive outdoor air infiltration and even with air tight dampers are prone to becoming something of a cold spot in the house when not actively burning, due to convection within the flue.

  20. Chaubenee | | #20

    Yes, Dana: while my 1969 GTO convertible gives off more emissions at idle than a new car gives off going 55mph, it too is a decoration. But seeing as I drive it 300 miles a year and I bought it when I was 19, restored it when I was a kid, sold it to build my business, bought it back later for triple what I sold it for, and had some of the happiest memories of my life in it- it also stays. What do I do to make it justify it? Well, I drive my other cars less, recycle, pick up street trash when I walk my dog, use LED bulbs every where, keep my thermostat at 62 in the winter and never fly anywhere on a jumbo jet. But some things we just find a way to manage as they are frills and life would be boring and gray without them. Your points are well noted on that post. The top damper is far better, I agree. I designed it to be almost all in the envelope. I plan on sealing as best as possible the front for the 97-99% of time when it is just sitting there. It is a remnant of a bygone ere to be. No doubt. I never intended it to be a heating appliance. But for esthetic and enjoyment reasons there is little I could do to replace it, just like my GTO.

  21. Expert Member
    MALCOLM TAYLOR | | #21

    Apologies to Dana. There aren't many things more annoying than an ernest reply to something posted in jest.

  22. Expert Member
    Dana Dorsett | | #22

    Malcolm- no apologies necessary, it was truly my fault for including too much pointy-nosed information with insufficient emoticon. The IS the internet, after all- can't be too careful when posting direct allegations of planetary criminal conspiracy! :-)

    Joe: Thinking about it a bit more carefully, even I was undersizing the proximity air duct. Codes vary on make up air requirements for atmospheric drafted natural gas burners, but typically run between 0.5-1 square inch for every 1000 BTU/hr, and an open hearth fireplace probably needs more. A fire in a Rumsford fireplace could easily hit 100,000 BTU/hr, maybe even 200,000 BTU/hr), so 100 square inches of make-up air would just be getting STARTED for a tight house. That's a 12" round, or eight 4" rounds. Anything less than that is mere frosting on the cake, and it probably takes 2x that much.

    Which all goes back to opening a window as the "right" solution here, especially given that you're only using it a dozen or so times a year.

  23. Chaubenee | | #23

    Well, after careful deliberation I have decided that for a number of reasons, the Rumford will be going! It will be replaced with a sealed LP gas-fired unit. So there will be lots of happy people, including my wife who will now be able to click a remote control to light a fire when I am working late. We like this a lot. Does anyone have feedback on the quality of these units from thus brand?

  24. augustine_lee | | #24

    Does anyone have experience with the Thermalec NER make up air system? It supposedly can being in up to 2500 CFM of pre-heated air into the house, which if connected to the whole house duct system, might be able to distribute warm air evenly. I'm perfectly aware that we are expending energy to burn energy, but if the goal is to have an open fire in a tight house, this may potentially be a solution. Any thoughts?

  25. Chaubenee | | #25

    I am nt doing the Rumford but I am interested in make up air for range hood and for clothes dryer which no one ever seems to ask about. I did find this item for a reasonable cost...

  26. augustine_lee | | #26

    Hey Joe, I have to say I was a little disappointed to read that you gave up on the Rumford. It boggles my mind that of all the complex engineering problems out there that have been solved, we can't figure this thing out. I am figuring on having a nice piece of glass I will bolt on to the face of the fireplace when not in use, sealed with silicone gaskets against a marble surround. Those boost can be decorative like the head of a lion or something, I dunno. Then when in use in the winter, have make up air supplied into the whole house using this thermolec device. I'll tell you how it works out.

  27. Expert Member
    MALCOLM TAYLOR | | #27

    Augustine wrote: "It boggles my mind that of all the complex engineering problems out there that have been solved, we can't figure this thing out."

    But surely it has been solved. The problem was how to incorporate a wood burning appliance in an energy efficient manner. The engineered solution is an air tight wood stove. You are asking a different question outside the parameters of engineering to elegantly solve: How can I incorporate a method of combustion that is incompatible with efficiency in a tight house? There will be all sorts of solutions, like the one you have settled on, but this simply isn't the type of problem you can engineer your way around.

  28. StoneCircle | | #28

    I'm intrigued that the open fireplace came back in another Q&A! We haven't built our house yet, and the solution for our fireplace is still up in the air. Interesting that Joe has opted against the Rumford. We're still hopeful.

  29. Chaubenee | | #29

    Here is the place for the Rumford- in an outdoor living space like a timber framed gazebo, or BBQ grilling pavilion with seating, or around an open hearth in the yard. A place to gather in front of the fireplace and toast marshmallows, drink wine and tell ghost stories in the autumn or to relax with your loved ones on chilly summer nights. I imagine, depending on where you live, you will get as many as 9 or 10 months a year to really enjoy it. It is just too damn hard to pull off, plus it is a ton of money if you pay someone to do it. If you want to experiment in masonry, you sure don't want it to be in the fireplace in your new home's living room. But outside is another matter and one can afford to take more risks.

  30. bettyrubble | | #30

    I live in a 1960s "raised ranch" with a fireplace on the top living level, and another on the lower level. When we purchased the house five years ago, we worked with the Energy Star program to have the attic insulated and sealed, as well as the exterior: caulk around the windows, doors, etc.

    We have made no more than three fires in our top level fireplace, but every time, smoke fills our lower level. We could not figure this out. The flues for both fireplaces are separate.
    They are also separate from the boiler for the heat and hot water. All three flues were inspected after purchase--they all passed with high marks.

    We had a chimney sweep look at both fireplace flues. Both were in excellent condition, and clean. His evaluation was that the work we did to insulate and tighten the house when we moved in was a factor.

    Because we had made the house much tighter, when we lit a fire in the top level fireplace, the house needed to literally suck air into the house to equalize the pressure. The largest and nearest leak of air into the house is the flue for the lower level fireplace.

    So, we need to do two things: 1, figure out a way to tightly seal the lower level fireplace, and 2, figure out a way to bring air into the house so the fireplace has enough oxygen.

    1 is not that hard. 2 is more difficult. The house has all casement type windows. We would need to build an elaborate and perhaps not very attractive makeup air system **just** to use the fireplace. Or, open a window near the fireplace.

    We have not made any decisions yet. Dry wood stored under a tarp in the back yard awaits.

    1. DennisWood | | #31

      Opening a window for your three fires makes sense to me. Open hearth fireplaces make about zero sense to me, when air tight units (we have one in our home) can output 60K BTU using about 1/10th the wood with zero smoke exiting the chimney. It has a large glass front/door which the cats (and my kids) gravitate towards due to the radiant heat that fills the room during use.

      I grew up in a house with an open fireplace and even at a young age questioned using it as we would be freezing 8 feet away from it. The new generation of high efficiency wood burning fireplaces are pretty awesome.

    2. nynick | | #34

      Let's see if I understand this. You're saying that while the upper level chimney is spewing smoke, that smoke is then sucked down the lower level chimney that's next to it and into the house as the only source of make up air to feed the upper level fire place? If that's true, you're house is seriously tight.
      A operable chimney cap should be installed on both fireplace chimneys anyway as part of your energy efficient house, opening the upper level cap only when lighting a fire. This should solve the smoke problem. If you're fire doesn't burn at that point, I'd buy one of those outside air intake kits or just crack a window 3 times a year. I'm betting there's enough air leakage elsewhere to have a fire though. Doesn't take much.

  31. DoUpstateNY | | #32

    I have a historic home with 3 Rumsfords.
    We burn only on special days during the year and sometimes at Christmas, but being a second home that's it. I also own a home that I live in with a Rumsford that we placed an insert in. At the time we're weren't concerned with make up air, but 7 years into this home we are thinking otherwise.

    The Rumsford was retrofitted with a stainless flue. That's good. But in purchasing an insert without a make up air option we are tossing energy away.

    In this house we also have a freestanding wood stove with makeup air and a pellet stove with makeup air. We also have an oil burner that we realized would suck the warm air in the basement protecting the water pipes from freezing in about 2.5 minutes when it kicked on. That's 0
    Zero degree or less air replacing 25 to 40 Degree air while the furnace is running.

    Sooooo... we shut down the boiler, spent $600 on antifreeze, and now we run the pellet stove 100% of the time along with the two wood stoves (one of which has no makeup air.

    I have seriously thought of giving the non makeup unit a 4 to 6" vent in the wall next to the stove, but after reading these posts I have another idea...

    The air on the floor around the fireplace is relatively warm. If I take the flue for the makeup air to the outside of the basement via a hole in the stone foundation, that air will be cold. Coming into the house. Much colder than the warmer air in the house for sure... possibly 80 degrees difference.

    With that, I'm wondering if the cold will actually enter the home freely if I place the grates for that make up air directly in the floor in front of the fireplace?

    The warm air should hang out... and the cold air only entering as the stove pulls air out. And with the grates for the MUA next to the base of the stove within 1 ft of the intake for the makeup air, I'm thinking that there could be some sort of self balancing act going on?

    What are your thoughts?


    1. CarsonZone5B | | #33

      look up outside air kits (OAK) and you will find kits and manuals for this type of installation. In general, these are most effective for sealed units that can draw almost all their air from outside and radiate heat to the home. is a good forum for these as well.

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