Green & Passive design
As an architectural designer, I have long been interested in green and passive home design, having dabbled and dreamed. I now have a project that will encompass both, as well as the use of SIPs for roof and walls.
Simple questions with elusive answers:
1. Where can I find the angles of the sun at June 21 & Dec. 21 for a specific latitude?
2. How do a loft area and high vaulted ceilings fit in? It seems that both the Window to Wall ratios, and mechanical load numbers are all calculated using square footage. Surely ceiling height and volume have an impact.
3. Is there ANY wood-burning fireplace or stove that doesn’t cause a net negative?
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Replies
Becky,
Q. "Where can I find the angles of the sun at June 21 & Dec. 21 for a specific latitude?"
A. Sustainable by Design: Design Tools.
-- Martin Holladay
Becky,
Q. "How do a loft area and high vaulted ceilings fit in? It seems that both the Window to Wall ratios, and mechanical load numbers are all calculated using square footage. Surely ceiling height and volume have an impact."
A. A house with 16-foot ceilings will have walls that are twice as high -- and therefore have greater area -- than a house with 8-foot ceilings. So wall height will definitely affect the window-to-wall ratio.
There are lots of ways to calculate a design heating load and design cooling load, but all accurate methods will include inputs for wall height.
-- Martin Holladay
Becky,
Q. "Is there ANY wood-burning fireplace or stove that doesn't cause a net negative?"
A. What do you mean by a "net negative"? When I light a fire in my wood stove, the interior air temperature rises. There is a direct linear correlation between firewood burned and indoor air temperature.
A wood stove may have a negative environmental impact, of course, and we've discussed that fact a lot on GBA.
Perhaps you meant to write, "Does an operating wood stove cause the indoor air pressure to be negative with respect to the outdoor air pressure?" The answer is "sometimes." If you are worried about this, you should include a duct that supplies outdoor combustion air to your wood stove.
For more information on combustion air for wood stoves, see:
All About Wood Stoves
How to Provide Makeup Air for a Wood Stove
-- Martin Holladay
Thanks for your help, Martin!
1. I was able to use the Sustainable by Design tools. Great Help. :-)
2. I will have to look more carefully. Thanks.
3. All of the discussions (here and other sources) seem to bash the use of Fireplaces because they draw the heat from other rooms and out the chimney, seemingly assuming that there is no door on the unit. I haven't found any discussion of the benefits or drawbacks of a a sealed door fireplace/ wood stove. Also, is there any advantage of a wood stove over a wood fireplace?
Becky,
Q. "Is there any advantage of a wood stove over a wood fireplace?"
A. Yes. A wood stove is better than a wood-burning fireplace in all respects. Wood-burning fireplaces leak enormous amounts of air, even when fitted with doors. No green home should have a wood-burning fireplace.
-- Martin Holladay
Thanks, Martin, for the info about the stove. That had been my suspicion. I had a salesperson tell me there was no difference. :-/
Aside from the huge difference in efficiency as a heating appliance, the particulate emissions characteristics of EPA certified wood stoves are ENORMOUSLY cleaner than open hearth fireplaces- on the order of 5 grams per hour vs. 150 grams per hours, and delivering 4-5x the amount of heat into the room as well. An old-school pre-EPA wood stove is on the order of 50 grams/hour.
An EPA certified wood burning INSERT (which is different from a factory-built fireplace) is essentially a wood stove designed to be retrofitted into an open hearth fireplace, and is about as efficient & clean as a wood stove. It's only as efficient if the it's fully within the thermal boundary of the house (not in an uninsulated masonry chimney on an exterior wall, which is a bad thing to have on multiple accounts.) EPA compliant factory built fireplace emissions are rated in terms of grams per kilogram, not grams per hour, but are understood to be at least an order of magnitude higher emissions in terms of grams of emissions per MMBTU of heat delivered into the house, but it's also generally used much fewer hours, since it is not considered to be a space heating appliance, but rather something for ambience.
The forestry practices of the source fuel wood and efficiency of the wood stove both make a difference as to whether the woodstove is carbon net-neutral or lower carb than a natural-gas burner in a meaningful amount of time, but an open hearth fireplace is such low efficiency that the "payback" of the carbon debt is effectively forever (> 100 years, even with the best forestry practices.) With reasonable (not necessarily best-case) forestry and a 60%+ efficiency stove the carbon footprint is less than natural gas in less than a decade, and carbon neutral in about 20 years.
I do believe that some newer high efficiency "insert" versions might be an option for a fireplace. They are, in a way, like building a wood stove into your fireplace enclosure.
Another alternative is to utilize a masonry heater. Better efficiency both from heating and combustion perspectives. Two downsides with masonry heaters though: Typically the home needs to be constructed with additional structural elements to support the masonry/stone/etc. & That masonry and substructure doesn't come cheap.
All great info! Thanks, Dana Dorset & Andrew Bater.
We would not be building an open fireplace. Whichever is used, a wood-burning insert or wood-stove, the structure below would be a slab-on-grade, so we would need to pour a thicker pad at that location. Also, it would be completely contained within the structure.
The property is rural, with about 50 acres of trees and lots of deadfall. The wood-burner would be for heat and for ambience, but primarily as a back-up if the electricity is out for an extended period. There is no access to gas supply unless they install a large volume tank on the property. This is not an attractive option, due to cost and safety. With a nearly unlimited supply of free wood, the client feels that is the best heating back-up.
Photo voltaic panels will likely be added at some point, so then electricity would be more reliable.
Lots for them to consider. I appreciate all your help.
Becky, in response to your comment that the wood-burner "would be ... primarily as a back-up ..."
If that is the scenario, then I definitely would steer you towards a high efficiency wood stove or high efficiency insert, and ignore my thought that a masonry heater option could be viable; my reasoning follows. (Dana has given you good advice on how to shop for high efficiency inserts etc.)
Masonry heaters are not well suited towards intermittent, as needed, operation. They have long warm up (and discharge) times, which of course is part of the appeal. If the power goes out at 4 PM, and your house starts to get cold at 6 PM and you start your first fire then, well you won't be warm again until 4 AM.
There is actually even somewhat of a safety reason to avoid intermittent use of a masonry heater, particularly early season. The typical recommendation is to start out with a very small fire and then build over a sequence of days to larger fires. This is to prevent any moisture trapped in the bricks or masonry from rapidly expanding causing cracking etc.
BTW, I do burn wood (with a masonry heater) and have one additional thought about using wood only as an emergency source. The trick is keeping any stored wood dry. Wood stored outdoors just tends to go to pot after awhile. The outer layers become mushy, mushrooms grow on it, insects live under the bark, rodents and snakes get into the pile, etc. For example I once split open a log and a whole live mouse nest fell out!
There always seems to be an inflection point between wood that is well seasoned, and wood starting to go punk. Rotating your stock through continual use is the most typical way to avoid that.
A wood stove with a metal chimney will not need additional slab thickness (even a soapstone model), but a masonry chimney would.
It's important to size the wood stove for the design heat load too, not more than 2x if you can help it. An oversized wood stove would have to be throttled back to a less efficient burn rate (with higher emissions) to keep from turning a high-R/low-load house into a sauna, which is probably not the ambience most people are looking for. Think of a heating appliance with a ~3:1 turn down ratio. Most begin to suffer the efficiency & emissions hit if constantly fired at less than 1/3 it's BTU rating. The only way around it is to burn intermittently with small but hot fires that are allowed to burn to nearly-out, then rekindled with another hot fire. Loading it full up then throttling back to smolder mode to make it through a cold night with a 5x oversized wood stove isn't the cleanest or most efficient way to go.
There are some pretty good pipsqueak sized cast iron & soapstone wood-burners out there heating low load houses (even PassiveHouses), as well as some fancy & pretty (usually pricey) ceramic wood stoves with sufficiently low firing rates.
Thanks so much for the amazing information.
Andrew, we will have to plan for a good way to store the wood. hmmm....not indoors (too easy for bugs to hitch-hike in), not outdoors (issues as mentioned).
Dana, I had to laugh at your description of a pipsqueak sized unit. Do you have any brand names and models you could recommend?
There are actually 2 houses being built. One will be about 1000 sq.ft. w a ceiling that vaults to 20' and an open loft. The second house will be a one-storey of about 1700 sq.ft. with a ceiling vault of about 16'.
Both homes will be super-insulated and sealed, so there will need to be a good focus on air exchange.