UPDATED: 1/3/11 with expert opinions from Mark Sevier and Peter Yost
Chris Koehn will be building a 1,600-sq.-ft. home in British Columbia for owners who want to heat primarily with wood. They envision a wood-burning cookstove and a fireplace, and they’d also like to incorporate some solar capability.
Because of its island location, the house will be off the electricity grid.
Among their concerns is that some parts of their house, including the bathroom and an upstairs bedroom, may be too far from a heat source to be comfortable. Koehn is considering heating the tile floors in the bathroom, but he’s concerned with how much electricity it would take to run the pumps.
There’s also the question of what to use as a source of backup heat to keep the house from freezing in winter, as well as help it meet local building codes.
“I’m looking for potential solutions from folks who have faced similar challenges,” he says in a Q&A post. The replies are the subject of this week’s Q&A Spotlight.
Keep it simple
Simple mechanical systems are best, suggests GBA senior editor Martin Holladay, who has lived off-grid for 35 years.
“Anyone who chooses to live off-grid should be willing to be flexible about indoor air temperatures in remote rooms on the coldest days of the year,” he says. “You can minimize these temperature differences, however, by paying scrupulous attention to air sealing and by providing very high levels of insulation in your floors, walls, and ceilings.”
Unlike grid-connected homes, which can always draw on utility electricity as required, an off-grid house must be completely self-sufficient. Electricity, whether it’s generated by a wind turbine, a photovoltaic array, or a fuel-fired generator, is precious.
“Whatever you do,” Holladay tells him, “don’t choose any heating equipment that will require electricity to operate.”
He suggests one or more propane space heaters with through-the-wall venting and air intake. Empire makes several models that will run without electricity.
Robert Riversong would steer the owners away from a fireplace unless it was a Rumford design with a gasketed chimney cap. But he suggests that a wood cookstove with a water coil could thermosiphon hot water to a radiant floor in the bathroom, a system that does not rely on electrically driven pumps.
Can he use solar hot water for space heat?
Koehn appears to be leaning toward a backup or supplemental heating system in which solar hot water is used for heat. The water would be circulated by pumps running on PV-generated electricity.
The idea is appealing, but not without flaws, as Holladay points out.
“When it’s cold and cloudy, a hydronic space heating system requires electricity to operate the circulators,” he writes. “This really is a deal-killer — in the middle of winter, when you are cold, where is the electricity coming from? I have friends who built a new off-grid house with a hydronic heating system. They had to run their gasoline-powered generator all winter. The noise drove them nuts and the fuel cost nearly bankrupted them.”
And, he adds, there’s a serious chance for a freeze if the house is unoccupied for a weekend. Draining the water lines is a “real pain.”
Making the cookstove do double duty
Robert Riversong suggests that a wood cookstove with a water coil could thermosiphon hot water to a radiant floor in the bathroom, a system that does not rely on electrically driven pumps. But stay away from a stove with a coil in the flue, which interferes with exhaust flow and also increases the formation of creosote.
Mr. Greenguy also likes the idea: “One can always run hydronic heat off a wood stove with a liquid heat exchange,” he says. “There is more than enough heat from wood combustion if you can move it where you need and being off-grid, a hybrid system (wood/hydronic) with a small pump is all you need to move heat, and with the addition of a tank, could store a days worth of heat, too.”
Lucas Durand, however, isn’t so sure. “Getting some heat for [domestic hot water] is one thing, but a wood stove is not the right appliance for running a hydronic space heating system — unless it’s been especially designed for that purpose.”
Which brings us back to theme of simplicity: “If the wood stove is running, you’ve got heat,” Holladay says. “No need to complicate a perfectly good wood stove by trying to connect it with a hydonic heating system with circulators.”
The fine points of wood heat
Wood heat is a little more complicated than it may seem.
For example, Gord Schiller points out, stay away from beach wood. Because of the salt it contains, it will corrode both the stove and the flue, even a stainless steel flue, and it will void the manufacturer’s warranty.
Among the other suggestions Schiller makes: Build a good sized wood shed next to the house where one year’s wood can be separated from the next, get a gas-powered wood splitter, make roof access to the flue as easy as possible to make cleaning easier, and make the flue as straight as possible for better efficiency and easier cleaning.
Schiller advises buying a high-quality wood stove. “Burn times of the stove and efficiency of the stove will reduce the amount of wood that you burn and therefore the trickle-down effect of not having to spend too much time cutting wood,” he says.
Build a tight, solar-oriented house
In addition to specific suggestions on how to provide backup heat, a number of contributors recommended a tight, well-insulated building that keeps heating loads low.
“Definitely incorporate all the passive solar possible in that locale,” writes Robert Riversong, “with sufficient direct-gain thermal mass for diurnal storage, as that will reduce fuel consumption.”
David Meiland, who lives not far from Koehn’s building site, adds this: “You can do quite well here if you have good southern exposure, design for passive solar, build a very well insulated shell, and generally minimize heating and electrical needs. “
“Keep the ceilings low; no wide open spaces to waste energy, and doors on every space to close when the room is not needed or when the room is being used and the other side isn’t,” says R from Sooke. “Look at designs from years ago when most houses only had wood heat. There were reasons they did things they way they did. You don’t have to reinvent the wheel; just use it all to your advantage.”
Our expert’s opinion
Here are comments from Peter Yost, GBA’s technical director:
It’s tough to figure just what to add to Martin’s experience of more than 30 years off the grid. But I did check in with my former colleague from Building Science Corp, Mark Sevier, a top engineer who was with Solar Design Associates until leaving his position there to finish up his own grid-tied zero energy home. Here are Mark’s key points:
1. Prep your client for the off-the-grid lifestyle. Martin would be the first to agree here, I am sure. You have to know and manage your loads with great care, or pay the back-up generator “piper.” And, make sure your client has some sense what they are getting into, off-grid living is a different experience than utility grid supported living. You take over the utility company’s responsibilities when you aren’t connected.
2. Worry about surface area, not volume. It’s the former that is the real driver for heat loss and air leakage, not the latter. And window square footage loses five to 10 times as much heat as wall/roof area, and costs five to 10 times more, suggesting a conservative approach to their use.
3. High performance airtight homes behave fundamentally differently than conventional ones. Stratification is much less of an issue in air-tight, well-insulated structures and closing off rooms won’t benefit as much as you would think unless you insulate and air seal interior walls and floors.
4. Locate the wood stove “point-source” centrally. In both the high performance homes Mark has built, a centrally-located wood stove with an open floor plan convectively connected first and second floor spaces. And while we are on wood stoves, they can be quite problematic if not airtight and with dedicated combustion air in really airtight homes.
5. Floor plans for passive design. As Mark says, “…bedrooms, bathrooms, TV room, and kitchen on north; living, hallways, stairs, dining, playroom, etc. on south. Essentially the private spaces or those that can’t tolerate glare or need wall space should be north-facing, and public spaces should be south facing, to avoid drawing shades on the passive solar design.”
Mark sums up this way:
“If I were given a clean sheet of paper, I’d recommend a well insulated enclosure (R-25 to 35), passive solar attributes in proportion to the client’s interest, propane thermocouple controlled centralized air-based back-up heating unit, ‘sealed’ wood stove with ducted air intake, small/right-sized solar hot water array with propane back-up, ‘right-sized’ PV array at ~50-60 degree tilt (to even winter/summer output), and propane back-up generator.
“It’s probably a good idea to use PEX piping everywhere (except the solar hot water loop due high temperatures), since it can tolerate freezing (any hydronic loops should have glycol). At some point, Murphy is likely to stop by, no matter how well you think you have things covered–a broken window in a cloudy stretch when they’re on vacation in winter, for example.
“I like interestingly complicated integrated mechanical stuff probably more than the next guy, but I’ve come to see that it isn’t for most people, so I agree with Martin’s simple-is-better perspective. Warm air and insulating enclosure, minimal freezable water system.”
Peter’s closing note: Mark’s zero energy home near Boston, MA, is a remarkable project, which I sincerely hope will become a GBA Green Homes case study sometime later this year.