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Community and Q&A

Fireplace vs energy-efficiency

user-1106403 | Posted in Green Building Techniques on

I’m striving for a net zero house in northern CA zone 11, which I think is a zone 3B on the US map. 6 inch blown cellulose walls with 1 ” outside foam, R50 blown ceiling; mini-splits on each level. Here’s the issue; my wife wants a fireplace for aesthetic reasons; I would prefer a wood stove with glass front as a supplemental heat source. . She’s ok with an insert in the fireplace.
Is there technology that will make the insert a supplemental heater and not blow half my heat budget up the chimney when it’s not on? Blowers to get more heat output? Your thoughts appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Just to be sure we are all on the same page:

    1. I understand that you want a wood stove.

    2. What does your wife want -- a wood-burning fireplace or a gas-burning fireplace?

    3. What do you mean by an "insert"? Do you mean a metal fireplace unit with a fake surround so that it looks a little like an old fireplace? Or do you mean a wood stove that can be retrofit into a masonry opening?

  2. user-1106403 | | #2

    wife wants a wood burner, but with the wood stove inserted into the fireplace opening. I think her objections are to the metal stove pipe of a woodstove, and a big box sitting in front of the surround.

  3. GBA Editor
    Martin Holladay | | #3

    And will this be a real fireplace (in other words, one made of bricks, or concrete blocks faced with bricks, or concrete blocks faced with stone)?

    Or will it instead be a fake opening made out of framing lumber and covered with thin tile, with a stainless-steel chimney running up a vertical wooden chase?

  4. Expert Member
    Dana Dorsett | | #4

    Air tight glass-front wood burning fireplace inserts that take piped-in outdoor air for combustion exist, as do sealed-combustion gas units. Both can hit the 70s or even low 80s for efficiency, but none hit the condensing gas 90%+ range.

    By contrast open hearth fireplaces (or inserts with the door wide open) run ~7-10% efficiency when burning. Air-tight versions don't suck air 24/365 the way open-hearth fireplaces do as long as the doors are closed & latched. There is no place in a Net Zero house for an open hearth wood burning fireplace IMHO- it's an inefficient fuel (ab)user the primary feature of which is a local air pollution problem. Wood stoves and inserts generate a lot of sooty particulates too, but EPA rated versions burn VERY cleanly compared to open hearths once they're up to temp, the bulk of the pollution occurring while getting up to temp on cold-starts.

    Blowers don't necessarily improve the thermal efficiency of inserts, but it does improve the capacity ( total BTU output ), and helps distribute heat around the space, but in your house getting more heat out of it isn't likely to be as big an issue as much as overheating the space would be. Glass-front woodstoves have most of the same appeal as open hearth fireplaces, and radiates heat in all directions, making them preferable to an insert-solution- they're a more comfortable heat source, an offer most of the visual appeal.

    With any heating appliance it's good to start out with the heat load of the house at the 99% outside design temp, which will be pretty low in a Net Zero house. Most fireplaces & inserts (and woodstoves) would likely have a peak output several times your peak heat load number, and if you oversize it by more than ~2x you risk turning the place into a sauna by the time it's up to temp and burning cleanly. A high-mass house (concrete or masonry floors/walls etc) takes the edge off some of the flash-heat sauna effect, or a high-mass woodburner like a ceramic or soapstone wood stove. The latter take more time to get up to temp, but distributes heat over a wider time span. That make them great for continuous heating, but not-so-great for short quick fires.

    So, first things first- do a Manual-J or I=B=R type heat load calculation on both a room-by-room & whole-house basis, see if you can't find a solution that isn't grossly oversized for the room(s) that would be heated by the fireplace/insert/stove.

    There are a few decent but tiny soapstone stoves out there now suitable for PassiveHouse or Net Zero type houses (in either a Euro-modern or old-school 'merican style.)

    A moderately high-R very tight house in a zone-3 climate will have a fairly short and shallow heating season- probably shorter than you're used to if you've been living in a so-so not so tight house with 2x4 construction and R19 batts in the attic.

  5. Expert Member
    Dana Dorsett | | #5

    Massive masonry heaters get around the circular stove-pipe look and add a lot of thermal mass to the house, (you'll never get the "hot flash" sauna effect) but they're expensive (about 4-5 grand for a smallest no-name versions, $10K + for a Tulikivi soapstoner, not including installation), which may be hard to rationalize if it's primarily for aesthetics.


  6. user-1106403 | | #6

    Dana; thanks, your points reinforce all the discussions I have had with my wife. "Air tight glass-front wood burning fireplace inserts that take piped-in outdoor air for combustion exist," this is what I need to know more about. Do you have any additional info, or just do a Google search? I'm aware of the need to keep it small; HEED and BeOpt 1.4 suggest 20MBtu/year heating need ; I'm not sure how to translate that to BTu/hour for a stove.

  7. Expert Member
    Dana Dorsett | | #7

    The smaller Euro-modern round stoves tend to look OK even with round stovepipes. (Hearthstone Bari/Tula , Morso 6100 or 8100 series, etc.) and there are variations on venting options that would avoid the vertical stove-pipe look. (google 'em.) They don't have to come with a boxy-cube look.

    The smaller Morso units run 20-25KBTU/hr, but they don't have as much thermal mass as the 35KBTU/hr soapstone-clad Hearthstone Bari.

    [edited to add, after reading your cross-post]

    With a zip code (for heating degree-day and 99% outside design temp data) we can zone in on it, but you're probably looking at a whole-house heat load under 20,000BTU/hr, and maybe even under 15,000BTU/hr. If you're really close to one of the cities with a 99% design temp listed in the Manual-J we can work it from there:

    As an example, take a climate like Red Bluff or Redding which run about 2500-3000 HDD (base 65F), with design temps around 30-32F, or about (65F-30F=) 35F heating-degrees. For a very rough cut on the heat load, assume:

    20MBTU over 3000 HDD, which is ~6700 BTU per HDD.

    With 24 hours in a heating degree day, that's about (6700 BTU/24 hours =) 280 BTU per degree-hour.

    With 30 heating degrees, the peak heat load is going to be (35F x 280 =) 9800BTU/hr.

    With a design temp in the low 30s Fahrenheit the stoves you should be looking at would be 20-25,000BTU/hr, maybe as high as 35,000BTU/hr if the unit has some thermal mass, but any bigger than that has to be fairly high in thermal mass (stone or ceramic clad, not iron) to keep it from roasting you by the time you got it up to a clean-burning temp.

  8. Expert Member
  9. user-1106403 | | #9

    Malcolm, that's the look, but it puts out about 2 to 3 times the heat I need. Dana, I believe I saw one statement that Auburn, my location, is very similar to Red Bluff or Redding. We are a little higher elevation, so lower summer, lower winter but close. I like the soapstone stuff, and Malcolms suggestion works if I can size it better. Thanks to everyone for your help.

  10. KHWillets | | #10

    Keep in mind that the peak output is not the btu/hour to use for design calcs. For instance that R90 lists a minimum output of around 11k btu/hr. The higher output may be useful for raising the temperature quickly (say in the morning after leaving the heat off at night), but you should be able to throttle down to a low level which is closer to your steady-state.

    Catalytic stoves/fireplaces are especially good at the slow burn; they can operate more efficiently at low temperatures.

    You can pencil out how many btu a fully loaded stove will produce, for instance one cubic foot of softwood, at 60% efficiency, works out to about 100k btu, which you might need if you leave the heat off for 10 hours at 10k btu/hr. heat loss.

  11. Katy_Hollbacher | | #11

    I realize this thread is dead but still want to make everyone aware there IS a high-efficiency condensing gas fireplace on the market that's been out for 2 or 3 years- it's the Mantis by Empire Comfort Systems and has EnerChoice efficiency ratings of 88.2 - 90.5%:

    EnerChoice .xls file:

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