Patricia Appelbaum is in the market for a new wood-burning stove, one without a catalytic element, to provide mostly supplemental heat for her 1,600-square-foot home. There are a lot of models to choose from, and that’s part of the problem.
“We understand that a non-cat stove needs a minimum temperature of around 500 degrees for an efficient secondary burn,” she says in a post in GreenBuildingAdvisor’s Q&A forum. “But we would like to have enough capacity to heat the whole house occasionally if necessary.”
Appelbaum’s family already has an older wood stove, a large model, that some people in the family like to run at a lower-than-optimum temperature. Before buying a new stove, she’d like to know whether having one with a smaller firebox would make any difference.
That’s the topic for this Q&A Spotlight.
A small, hot fire is best
A fire that burns quickly and intensely is the most efficient kind because it produces the most heat per unit of fuel, and also the least amount of air pollution, GBA senior editor Martin Holladay says. Hot fires also produce less creosote than low, smoldering fires that are starved for air.
“Going to a stove in the 50,000-55,000 Bth/h range can still work for a house with a ~25,000 Btu/h design heat load, but try not to go beyond that,” advises GBA reader Dana Dorsett. “Once the stove is up to temperature, it will still burn cleanly when throttled back to ~25% of its rated fire, which isn’t going to roast people out very quickly.”
The thermal mass of a stove becomes an important factor when the stove is used only intermittently but at high temperature, he adds. But there’s a caveat — when the output of the stove is three or four times more than the heating load, it won’t be as effective.
“A soapstone stove can be 2x oversized for the heat load and still work well with an intermittent high-temperature burn strategy,” Dorsett says, “but less well at 3-4x oversizing, since it takes time to get the stove up to temperature (due to the thermal mass that helps you coast between firings).”
Err on the side of small
Marc Labrie sees parallels between the size of a wood stove and the size of a house: “Do you prefer one that is 2% of the time too small or 98% of the time too big?”
Referring Appelbaum to Woodheat.org, Labrie argues there’s nothing green about a big wood stove with a smoldering fire. “Gone are the days of loading huge unspilt blocks and choking off the air supply before bed,” he writes, “a procedure that wastes much of the wood’s potential and coats the chimney with flammable creosote.”
For his money, a small firebox with a hot fire keeps both the chimney and the glass doors cleaner with less work.
Yes, replies Dorsett, but having a wood stove with a small firebox means someone will have to stay up to tend it. “Some amount of oversizing still works OK, and lets you get more sleep on the really cold nights,” Dorsett says. “A reasonable compromise is ~1.5x oversizing, which still means it has to hot enough that the glass will stay clean. (Some stove designs are better than others at keeping the glass clean too.) But until you’ve done the heat load calculations it’s hard to pick one.”
And, Holladay points out, there is the human variable, which makes the ideal 500 degree burn target a matter of wishful thinking rather than reality.
“A wood stove isn’t like a furnace. It is fueled by humans who load it with oddly-shaped chunks of wood, and who have a variety of needs: a desire to warm their cold hands, to dry their mittens, and to make a fire last all night, with coals to greet them in the morning,” Holladay says. “All of these factors mean that aiming for 500 degrees is probably not going to happen.”
The pellet stove option
One way of taking some of the variables out of the picture is by opting for a pellet stove, which would operate more automatically than a wood stove fed by hand, Charlie Sullivan suggests.
Pellet stoves incorporate electrically driven augers that move compressed wood pellets from a hopper to the firebox; the feed rate is determined by the setting the temperature on a thermostat. And that’s part of the problem: without electricity, the auger won’t work.
“The issue with pellet stoves is that we’d rather not have to depend on the electric grid (and we don’t have alternative power),” Applebaum says.
Granted, Sullivan says, but some pellet stoves can be run on a 12-volt battery as a backup, and the battery pack could be sized to run the stove for as long as she wanted. He points to four companies that offer than option on their pellet stoves: American Energy Systems, Sierra Products, Inc., Quadra-Fire, and Thelin Hearth Products.
Dorsett suggests another problem with pellet stoves: the cost and availability of fuel.
“A few years ago many people in my area got caught short during an exceptionally cool stretch of spring weather, and many of the local vendors don’t restock after early March, and there was a real run on retail pellet inventories,” he said. “It’s a more convenient fuel than cordwood in many ways, but it isn’t nearly as cheap. And (as a co-worker of mine recently discovered as she shivered while waiting for the UPS truck to arrive with the replacement board) the power controls will sometimes have reliability issues, too, just like any other electromechanical whatzit.”
Our expert’s opinion
GBA technical director Peter Yost adds these thoughts:
We often ask a lot — and I think too much — of our wood stoves: burn cleanly and efficiently, heat a space or room as well as a whole home, fire and heat up quickly yet accomplish an overnight burn. Their job is tough enough to begin with, being both the heating plant and the distribution system.
So I would just like to add a resource and another perspective:
The best resource on choosing a wood stove is “Buying, Installing, and Maintaining a Wood Stove.” This is actually great content and a collection of the best resources on specific aspects of the challenge. And note that the EPA has a list of approved products that includes both catalytic and non-catalytic stoves.
Gary Goodemote is the owner of Friends of the Sun here in Brattleboro, Vermont, as well as the chair of our local Sustainable Energy Outreach Network. Gary has been selling, installing, and maintaining wood stoves for more than 25 years. When I asked him about the Applebaum situation, here is what he said:
“It sounds like a medium to large stove would be a good choice. I try to get people to think of the future when they may find that a new, high-efficiency stove is more enjoyable to use than their old stove. In that case, they would do well to have a larger stove and get more use out of it. If they’re certain that they won’t use it that much, no matter how easy or enjoyable it is, than they should probably go with a medium stove. I wouldn’t recommend a large stove if the expectation is that people will use it just once in a while to heat the whole house.
“It’s hard to get more than four- or five-hour burn times from a small stove. A medium stove can give a user seven or eight hours. For many people, seven to eight hours is the minimum acceptable burn time and that’s often the determining factor. New, high-efficiency stoves can be slightly oversized and still be efficient, as long as they’re heated up sufficiently to start with. If a stove is drastically over-sized, it’s hard to get it up to temperature initially without overheating the space.
“One more factor that often comes into play is the setting. A small stove would look and feel okay in a room that is smaller and has a low ceiling. It may look and feel out of place in a room with a vaulted ceiling and a lot of wall space, not matter how well matched it is by the numbers.
“The most difficult sizing situation for us, in the store, is for people who have very well insulated, very tight construction, small homes. It’s very likely that anything we might recommend for a wood stove would be too large.”