How can I add thermal mass to a woodstove, without a lot of weight?

Question

It's well-documented that adding thermal mass to a woodstove will improve efficiency.

You can burn smaller splits of wood hotter, causing more complete combustion. The additional heat output won't over-heat the room, because it is buffered by the thermal mass. The thermal mass can keep the space warm long after the fire has gone out.

Many woodstove users will fill the firebox with wood in the evening, and then close the damper, leaving the wood to smolder overnight. This can keep the home warm for a long time, but it is wasteful of fuel, pollutes the air, and deposits creosote in the chimney. This practice becomes unnecessary with sufficient thermal mass.

High thermal mass designs I have seen require a dedicated footing in the ground. This is a significant additional expense and inconvenience vs. a conventional woodstove, which can rest on a conventional framed wooden floor.

How can I use thermal mass to increase woodstove efficiency without overloading the floor?

Answer 1

(Corrected)The "thermal mass" value you're interested in increasing is essentially the same (exceptions mentioned in this other answer) as the mass (essentially weight, for those not into learning physics!) of your material multiplied by its "specific heat capacity".

That doesn't help much when it comes to keeping mass down. However, there is a way around it. You can use liquid mass (water is cheap, and very heavy) and store it elsewhere. You just need a way to gently circulate it past the stove and store it in a tank somewhere (one that can handle the temperature increasing somewhat higher than your average water tank!).

If your tank is located outside of the space you want to heat, you'll need to insulate it heavily, along with the pipes leading to it. In that case you may also need panels for radiating the heat back into the building, which you'd circulate the hot water through.

Using a liquid to distribute heat is a common method of heating a building; sometimes oil is used and a furnace is running while the building needs heating though. Using oil almost certainly isn't going to be sustainable though, especially when you want to store a tank of it.

Using water, all parts of the system will need to be protected from corrosion (from the inside, especially). It will also need to be able to handle the changes in pressure as the water warms up (and a sudden increase if the water happened to boil by overheating).

Other than that, adding mass to any other part of your building that is reasonably warm will help, especially if the insulation is good. This may be an option too. It doesn't have to be right next to the stove (though the closer it is, the more effective it will be)

Answer 2

There are various ways to include thermal mass with a lower weight. Different substances have different specific specific heat. So, for a given weight, a material with twice the specific heat, will have twice the thermal mass.

Water has one of the highest specific heat values of any known material, so for a heat buffer with high thermal mass, low weight, and no phase changes, water is one of the places to start. The solid alternative would be to look at materials that are used in storage heaters (heat banks) - clay, ceramics, (maybe magnetite).

Phase-change materials are another way to do it. A high latent heat of fusion means that a lot of energy is needed for each kg of material, in order to turn it from solid to liquid. So, to get a good thermal buffer, you'd need to find a material that has a high latent heat of fusion, and has a melting point at standard pressure that is in a useful range for you - and that's probably around 30°-70°C. Such a phase change material will store lots of heat in relatively low weight when it melts; and release it again when it solidifies.

Answer 3

A simple incremental addition is to add a top cover to your stove. Presuming you don't cook on the stove this is a nice addition that most woodstoves would be better to have.

You will need an assessment of the load bearing capacity of the floor. That information should be available in the building specifications for your home. If you don't have them (most people don't) you can get a copy for a small fee from the local government office that issues building permits. If this information is unavailable a competent carpenter, structural engineer, or architect can tell you with sufficient accuracy after examining the building. Use this information to determine the weight of top cover your floor can support.

Contact companies in your area that build custom stone countertops. Tell them you want a slab cut to fit the top of your stove. Granite and soapstone are the common choices, but there may be other options available where you live. They can cut/form a slab to shape of a thickness that produces a final weight your floor can bear. The finished product just sits ontop of your stove.

Answer 4

Some years ago I rented a house that had a wood burning fire place in the rear of the home. It was a fairly smart layout as the fireplace was in the family room and there was a 3 step up area to the kitchen, then small raise in each ceiling as you went through the home, then the stairway going up to the bedrooms in the front of the home. This allowed heat to rise, then flow from room to room while new cooler air would sink and slowly move back to the room being heated. The rear where the fire place was would still get too hot when we tried to heat most of the home using the fire place, even with a fair amount of brick built-in around the fireplace.

My solution at the time was a cheap temporary fix. We had a couch and love seat along 2 of the walls in the family room and by moving them out less than 4 inches more from the wall I started to save round 3 liter bottled water bottles we bough a fair amount of for the water itself. They would stack on top of one another so I started lining the wall behind the love seat and couch with them stacked 2 high. By the time I was finished filling the area with refilled water bottles the temperature would stay reasonable an extra 4+ hours after the fire went out and we didn't feel overheated like we did before adding them to the room. So we could run the fireplace all day if we wanted, then let it go out and maintain a decent temperature most the night. All cheap and hidden out of sight and with the weight distributed. An added bonus was that the dogs could no longer hide behind the couch which we were trying to keep them from anyway.

Answer 5

I've designed my own stoves and outdoor boilers and met with considerable success with my boiler projects in particular.

My first outdoor boiler was a free "jack" boiler, very small, but with some mods, heated my thousand sq. Home and domestic hot water on less than half the wood my friends with $5k-$10k boilers used. The rocket stove mass heaters are essentially the result I achieved with my boiler mods. I first moved my outdoor boiler indoors, building a small, steel panel lined boiler shed 30ft (9m) from my home.

I built custom rigid foam boxed liners for my supply and return lines, insulated the shed with salvaged fiberglass to R36 in both walls and ceilings, and included a dedicated fresh air intake for combustion air...on the coldest days, -15F (-26C), that shed was never below 80F (27C). At first I kept the stock water jacket, and merely fabled up an 8ft (2.5m) water jacketed heat exchanger out of 100lb (45kg) LP cylinders with 6in (15cm) well casing as the exhaust pipe.

Burning absolutely dry, seasoned wood, plus always having a week's worth stacked in that hot dry shed to ensure it being both dry and warm, really helped ensure a clean hot burn. I only fired the stove up twice a day, two longish hot complete burns, and my exchanger pumped the 190F (88C) + water into a dozen more salvaged LP cylinders, modified to encourage good flow, stored in my cellar. I also used a non functioning gas water heater as my heat exchange from my boilers water supply to heat separate water for domestic hot water.

My biggest leap was when I decided to fill the water jacket with refractory cement, thus insulating my actual firebox, creating a much, much hotter fire temp, and put a stop gate with silicone seal in the top exhaust, routed the new exhaust out the bottom with in line combustion air, and realized I had me a downdraft gasifier! Essentially, the downdraft gasifier operates like the so called rocket stove, but with more controlled combustion/forced combustion, thus I could double my already awesome exchanger length built into the exhaust stream, and still never worry about gassing myself while in the shed. By this time, my tiny boiler was heating my home, endless hot water for family uses, and the in-floor heat I put in my two story (800sqft/74sqm each) shop... on even less wood for fuel than before.

Answer 6

There is really no safe practical way to modify a conventional wood stove to add thermal mass - best solution is a wood stove design that incorporates thermal mass.

Before people start comments about rocket mass heaters, PLEASE DO YOUR RESEARCH. Here is a web site for an educated answer: https://www.richsoil.com/rocket-stove-mass-heater.jsp Erica and Ernie Wisner are without a doubt two of the most knowledgeable people around when it comes to thermal mass and rocket stove mass heaters. I have NO interest with these individuals, I just believe in the correct information being available for all.

Let us be honest here for a moment - the definition of mass should answer this question up front... and PLEASE do not heat water with a wood stove in any type of a sealed container unless you are a competent engineer - overheated water = steam = BIG EXPLOSION. Please be safe when playing with fire. Life is too precious to waste.