Woodstoves: does efficient burning depend on size?
We are in the market for a new, non-catalytic woodstove. We understand that a non-cat stove needs a minimum temperature of around 500° for an efficient secondary burn. Question: Does attaining that 500° depend on the size of the stove at all? Or will the same quantity of wood produce the same burn temperature regardless of firebox size? (I know, dry, seasoned hardwood, etc.)
As background: most of the time, we use wood heat to supplement our main heat source (a reasonably efficient gas furnace). But we would like to have enough capacity to heat the whole house, around 1600 ft.²) occasionally if necessary. With our current large stove (an older model but also designed for a secondary burn), some members of the family like to keep the fire lower than the optimum temperature. Would a smaller stove make any difference?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Patricia,
A small, hot fire that burns quickly is the most efficient type of fire. Such a fire produces more heat per unit of fuel, and also produces the least amount of air pollution.
In theory, you can burn a small, hot fire in a small stove or a big stove. The key is not adding too much fuel, and making sure that the fuel is split into small pieces so that it burns quickly.
Only add as much fuel as you need to warm up the space you need to warm up. Then supply enough air to fire to allow it to burn hot. If the fire overheats the room where the stove is located, you probably added too much fuel. Experience will provide the feedback you need.
With a larger stove, the same principles apply. You only need to buy a larger stove if you are afraid that a small stove won't have the BTU/h output that you need.
Thanks, Martin. Any chance that the amount of fuel we need to warm up the space, in a good hot fire, would still produce less than 500°? Just curious.
With any heating appliance it's useful to size it based on the actual design heat load of the zone it's heating. You don't say where you are or the insulation levels of the house, etc. so it's hard to say for sure, but a code-min 1600' house in the colder half of the lower 48 of the US would have a heat load between 20,000-30,000 BTU/hr which is a the smaller end of woodstoves, small enough that you may not be able to deliver an overnight burn due to firebox size constraints.
Going to a stove in the 50,000-55,000 BTU/hr range can still work for a house with a ~25,000 BTU/hr design heat load, but try not to go beyond that. Once the stove is up to temp it will still burn cleanly when throttled back to ~25% of it's rated fire, which isn't going to roast people out very quickly.
If you are going with an intermittent-burn at high heat strategy, that works best if the stove itself has substantial thermal mass, to keep it from cycling the room temperature too much. High mass masonry heaters work pretty well, and are specifically designed for that type of use, but are pretty expensive (unless you build it yourself). A soapstone stove can be 2x oversized for the heat load and still work well with an intermittent high-temp burn strategy, but less well at 3-4x oversizing, since it takes time to get the stove up to temp (due to the thermal mass that helps you coast between firings.)
Several ways to skin the cat, but do a Manual-J or I=B=R type heat load calculation, and try to find something suitable that's less than 2x oversized for the zone it's heating, which means at the average winter heat load it can be firing above 25% of it's rated max most the time.
Patricia,
As Dana said, there are lots of variables. My guess is that most wood stove owners don't hit the ideal 500 degree burn target very often. The stove is often cooler or hotter than that, for a variety of reasons.
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. All of these factors mean that aiming for 500 degrees is probably not going to happen.
A completely different direction would be to substitute a pellet stove. That avoids the "some members of the family" issues, because it is more automatic.
Dana, thanks for the helpful details. Martin, thanks for the real-world perspective. But humans can at least try for optimal actions in an imperfect world, right? Charlie, thanks for the suggestion. 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). Further comments welcome …
Yes, I understand the disadvantage of pellet stoves depending on power and not being as good for backup as wood stoves are. A partial solution to that problem is to select a pellet stove that has a provision to hook up a 12 V battery as a backup. You can size the battery for as long a backup as you want. Some of the companies that offer that option are American Energy Systems, Sierra Products, Quadrafire, and Thelin. But of course a wood stove is much better for an extended power outage.
Patricia,
Another angle to consider is the stove's appearance since these appliances aren't generally tucked away in utility rooms.
With care, a good wood stove can out-live its owners so if you're in the market, its worth looking at well made stoves that will also suit the decor of your home.
Part of taking good care of your stove is to try and fire it to a high temperature once in a while (once a day if in daily use) - a hot morning fire following a slow over-night burn is not a bad way to go.
Read this site and clear up many myths and misinformation about wood stoves and why they work or fail to work; particularly the section on chimneys and location:
http://woodheat.org/
A stove is easily compared to a house where the question is, do you prefer one that is 2% of the time too small or 98% of the time too big?
I personally prefer a small firebox that burn hot and clean where the chimney and glass door stay cleaner with less work…Consider that you’ll only spend 2% of the heating season at the 98% outdoor design temperature!
A big stove that spend most of the time smoldering is not green. Gone are the days of loading huge unsplit blocks and choking off the air supply before bed, a procedure that wastes much of the wood's potential energy and coat the chimney with flammable creosote. Here’s a good piece of advice on "How to Buy the Right Wood Stove" http://woodheat.org/buy-right-stove.html
Have a cozy winter. Marc
Interesting about the 12V battery – I didn't know that. Woodheat.org is a good resource. Thanks, all.
Marc- the only issue with undersizing it is that somebody has to stay up and keep stoking the stove when it's at the 98th percentile temperature bin. Some amount of oversizing still works OK, and lets you get more sleep on the really cold nights. 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.
The other issue with pellet stoves is fuel cost & availability. In some locations it can be a real problem. 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. It's a more convenient fuel than cord wood in many ways, but it isn't nearly as cheap. And (as a co-worker of mine recently discovered as she shivered through several 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. But they can be more convenient and easy to use, less human interaction required than with a wood stove.