Wood Stove Thermal Mass
A lot of folks ask us about the materials of different wood stoves, and how that contributes to the stove retaining heat. People want their wood stoves to retain heat longer, so that they can stay warm even after the fire goes out.
The thermal mass of a wood stove is dictated by its materials of construction and its physical mass, i.e. its weight. Small stoves are typically not big or heavy, so the amount of heat they hold on to is fairly limited.
Let's examine the issue of thermal mass a little deeper, and discuss strategies for increasing your stove's thermal mass. If you don't care about the math and want to skip to augmenting your stove's thermal mass, click here to jump to the fun part.
Specific Heat of Wood Stove Materials
Thermal mass of a particular material is dictated by that material's Specific Heat. Specific heat is the amount of heat energy required to raise the temperature of a material by a certain amount. It also works in the other direction—specific heat is the amount of heat energy that a material can hold on to, and release into the room later.
Let's take a look at some specific heats of familiar materials.
|304 Stainless Steel||502|
The standard units for specific heat are in Joules per kilogram degree kelvin. To make these numbers a little more useful, let's switch to Freedom Units. There's 1055 Joules in a BTU, 1.8 degrees F in a degree K, and 2.2 pounds in a kilogram. Here's the same chart in BTU/lb°F.
|304 Stainless Steel||0.120|
Thermal Mass of Popular Small Wood Stoves
Our medium sized stove, the Dwarf 4kW, is made mostly of mild steel (with some cast iron, fire brick, and glass mixed in) and weighs about 100 lbs assembled. 1 kW is about 3,412 BTU, so the Dwarf 4kW is rated for about 13,648 BTU per hour. Assuming we get the stove body up to 450 degrees F in a 50 degree F room (and assuming the whole 100 lbs is steel for simplicity's sake) our 100 lbs of mild steel will have the potential to release:
0.122 BTU/lb°F * 400 degrees F * 100 lbs = 4,880 BTUs of energy stored (1.43 kW)
That's only about 20 minutes' worth of heat stored. It's probably not going to make a huge difference in the warmth of your space after the fire goes out.
Let's try it with another 4kW stove, the Salamander Hobbit. When asking about heat retention, this is the stove that most people compare against the Dwarf. It's cast iron, and a little heavier at 110 lbs.
0.110 BTU/lb°F * 400 degrees F * 110 lbs = 4,840 BTUs (1.42 kW) of energy stored
Most people naturally expect that cast iron would retain heat better than steel, but they're actually pretty similar. In fact, steel has a slightly higher specific heat than cast iron.
The difference in heat retention between steel and cast iron that people are used to is due to the different mass of familiar objects. Steel cooking pans tend to be lighter and thinner than cast iron ones, so the cast iron pans retain heat better. If you made a steel pan as thick and heavy as a cast iron one, you'll have similar heat retention properties. The same is true of wood stoves.
How about The Kimberly? Stainless steel and 56 lbs.
0.120 BTU/lb°F * 400 degrees F * 56 lbs = 2,688 BTUs (0.79 kW) of energy stored
Not as much that time. Stainless steel has a specific heat between carbon steel and cast iron, so you'd expect to get similar results. But since The Kimberly is so lightweight, it retains much less heat than the Dwarf 4kW or the Salamander Hobbit.
How about a slightly smaller stove, the Cubic Mini Grizzly, around 3kW in size. 34 pounds of mostly steel.
0.122 BTU/lb°F * 400 degrees F * 34 lbs = 1,659 BTUs (0.49 kW) of energy stored
Compare that to our Dwarf 3kW Standard with a similar output, but taller and beefier construction at 75 lbs:
0.122 BTU/lb°F * 400 degrees F * 75 lbs = 3,660 BTUs (1.07 kW) of energy stored
A heavier stove means significantly more energy stored. How about the Dwarf 5kW at 150 lbs?
0.122 BTU/lb°F * 400 degrees F * 150 lbs = 7,320 BTUs (2.15 kW) of energy stored
What happens if you add the oven attachment to make the Dwarf Cookstove Combo, 225 lbs total?
0.122 BTU/lb°F * 400 degrees F * 225 lbs = 10,980 BTUs (3.22 kW) of energy stored
Augmenting Thermal Mass
Regardless of the small stove you choose, there's generally not enough mass to radiate significant amounts of heat for much longer than an hour or so after the fire dies down. And in practice, some of that heat ends up going up the chimney. Unless you add additional mass, you need to keep the fire burning as long as possible to keep getting heat out of your stove.
Let's explore a few practical ways to augment the mass of a small wood stove for longer heat retention.
Soapstone is a popular material for wood stove construction because it's beautiful, resilient, and retains a lot of heat. The Dwarf line of wood stoves comes with factory mounting points for adding accessories to the sides of the stove. Soapstone panels aren't available as a factory accessory at the time of this writing, but could be custom made by a local stone supplier.
Let's add 4" thick soapstone panels to the sides of the Dwarf 4kW which are about 10" x 16". That's 640 cubic inches of soapstone on each side for 1,280 cubic inches total, at 1.4 ounce per cubic inch is about 112 pounds of stone.
0.187 BTU/lb°F * 400 degrees F * 112 lbs = 8,377 BTUs (2.46 kW) of energy stored
Now we're getting somewhere! That's 2.46 kW of energy stored in addition to the 1.43 kW stored by the stove itself, for a total of 3.89 kW. That's like running the stove for an extra hour.
112 pounds of soapstone might be an impractical addition for an RV where weight is at a premium, but soapstone could be an option for a tiny house or a cabin.
Burying Your Stove in Rocks
If your tiny space is stationary so weight is not an issue, burying your stove in rocks, sand, or clay could drastically increase your thermal mass.
If you were to build a cage around the top, sides, and back of your wood stove (keeping an opening for the door and air controls on the front of the stove) and fill that cage with landscaping stone, you could store quite a bit of energy in a small space. 1 yard of limestone is cheap in bulk from your local landscaping supply, and weighs about 2,500 lbs.
0.217 BTU/lb°F * 400 degrees F * 2,500 lbs = 217,000 BTUs (64 kW) of energy stored
That's a lot of stored heat! Now, in practice, you're probably not going to get the whole pile of rocks 400 degrees F higher than the room temperature. Remember, there's still the limitation of how much heat your 3-5 kW stove can actually produce. But there is still massive potential in this strategy for storing heat.
Adding a Water Boiler
If you take a look at the specific heat chart above, you'll notice that water is an outlier at 1.000 BTU/lb°F. Water has an absurdly high specific heat, so it's a fantastic material for adding thermal mass. Water can be used with radiators and even in-floor heat systems to store and distribute heat into your living space.
Water needs to be kept below 212 degrees F to prevent it from boiling (and potentially causing your water tank to explode), so let's assume we have a safety relief valve in place (or use atmospheric storage) and keep 50 gallons (415 lbs) of water at a cozy 150 degrees F versus our 50 degree F room.
1.000 BTU/lb°F * 100 degrees F * 415 lbs = 41,500 BTUs (12.2 kW) of energy stored
Not bad! And since water has such a high specific heat, it'll also release that heat gradually over the course of many hours rather than all at once.
There isn't a water boiler option for the Dwarf stove yet, but we're working on one. Drop us a line at firstname.lastname@example.org if you'd like an update when the water boiler is available, or if you'd like some suggestions on simple DIY water jacket designs that a few of our other customers have built.