Can I put thermal mass around my conventional wood stove to warm up the room

I have a Hearthstone wood stove.  It has soapstone body. It is  How can I retain more of the heat?  Can I put thermal mass around it?  If so what kinds?Here are some pictures of the area .  I am hoping for some brainstorming.

That is a funny little alcove for the heater.
If the sides were at an angle, like a Rumford design it would reflect more heat outwards.
I would stack fired clay solid bricks in front of the green tiles to a height at least the same as the tiles.
They could be laid loosley until you are happy with the outcome and then 'glued' together to prevent collapse.

I don't know if it will help at all. When designing masonry heaters, any surface that is closer than 15 cm to the wall is disregarded in heat radiation calculations. In this case all the heat will go back to the stacked bricks behind and after an hour or so they will start radiating forward, but also backwards towards the alcove. It would definitely help to move the stove forward and then surround it. In this case all accumulated heat would radiate to the room. In case of trying John's suggestion I would put insulation between surrounding bricks and the green tiles.

Because of the location in what appears to be a bumped out portion of the wall, I think some kind of reflective insulation would  be helpful.

Does the manufacturer make a water heating attachment?

Given your current setup, I think your options are rather limited.

In theory you could add a single stack of firebrick around the sides and back of the alcove. It would capture some of the radiant heat, at least.

I think it's important to avoid actual contact with the body of the stove and any improvised mass. The stove is engineered to stand alone; if you interfere with its natural radiation of heat it could cause serious internal damage.

William Bronson wrote: Because of the location in what appears to be a bumped out portion of the wall, I think some kind of reflective insulation would  be helpful.

Perhaps, but don't use anything flammable. The stove and chimney surfaces are too close. In an emergency you could cover the back wall with aluminum foil, I guess, and hold it on with thumb tacks. It would help a little with reflecting radiant heat, and certainly be a conversation piece.

Add a pot of beeswax to the top of it, beeswax will hold a surprising amount of heat.

If you do stack brick around the stove, give at least a 1 inch clearance.  They're designed to radiate heat away, so holding the heat against the stove could cause some serious damage.

Also, aim a small fan towards the stove.  It will push cooler, denser air towards the stove to be warmed up which will pull air warmer away from the stove and out into the room.  We use a computer fan - it's surprising how well it works.

I like Thomas' idea about using beeswax.  If you can't do the beeswax maybe a kettle of water would help add some heat to the room.

I also like the fan idea.  We use the small square computer fans to move warm air so I would suggest mounting a couple in the warmest spot behind the heater.

I also thought to suggest moving the stove forward though that would mean the stove pipe would need to be moved.

So I don't claim to be an energy engineer, but seems to me this situation presents a good case study for discussion.

What makes this stove different.......better or worse........than any one of the RMH options? It is vastly less efficient.

So wood represents a form of stored solar energy......a battery of sorts. Energy is released as wood is burnt. The trick is to capture then distribute this energy in the form of heat, which is the desired goal.

A wood stove of this type contains the hot fire, with the plate steel sides top and bottom and interior fire brick absorbing radiant and conducted energy from the fire......like a heat exchanger......then again radiating it into the room. Radiant heat is more or less line of sight. Let the sun hit you, you warm up. Move to the shade.....and you cool down. So by having this stove tucked back into an alcove, line of sight to the room is to the front. Line of sight to both sides and the back are to the tile lined walls. They may heat up, but then what? Where does that heat go? Into the room, or conducted to the wall behind them? So some form of material......brick slow release....or plate steel faster release......can absorb it to be released back into the room, provided there is no heat sink behind it. An air pocket or better yet, an insulation board like a foil faced polyiso sheet keeps heat from being conducted to the outside wall. So OP is on the right track. One other way to get more heat is to enhance convection off the stove. Fans are a way to do that. One I find fascinating is the heat powered wood stove fans placed on top of stove. A form of forced air distribution. I have no idea how such a fan works, but they do.

So how does this differ from the heatilator fireplace that is similar? Not much. A fireplace only radiates heat into the room line of sight to the fire with most heat going straight up the chimney. A heatilator is an improvement as it has an air channel below, behind and above the fire box. Fire box absorbs radiant and conducted heat from fire, heats up, then by convection......a flow under, behind and above and out into the room is setup as heat rises. Add a fan and it becomes a type of forced air heat. But most of heat from firewood still escapes up the chimney.

And in both cases......wood stove or fireplace, neither one burns wood at anything that approaches the temps needed to combust all the particulates in the smoke, so fuel is only partly consumed, plus most of the heat is sent up the chimney.

So advantage of the RMH concept is combustion of wood is at such high temps, all of it is burnt. Heat is then captured first in steel barrel or bell, to be released into the room by radiation and convection, then rather than remaining heat being expelled up the chimney, heat energy is further absorbed into thermal mass to be stored, then released over time in the room. Sit on the mass and you feel warmth by conduction. Be in the room and you feel warmth by radiation. But well over 99% of the energy stored in the wood is retained in the structure in the form of heat.

Some version of a RMH should be the only wood burning heat appliance used.

(As an aside, first time I witnessed the "rocket" affect was years ago when I placed a fireplace insert into a masonry fireplace. After months of use, the fire got pretty hot one day during startup. Hot enough it caught all the minor build up creosote in the chimney on fire. That was a runaway freight train. Mine was mild. Neighbors hot enough it caught their house on fire. Only minor damage, but the whole thing very educational as to what not to do.)

BTW, as an addendum to the flue fire story, the crew hired to fix the damage did so by first removing all existing masonry flue inside the chase, then built a brick chimney. Inside that, they dropped a vibrating bell. They then dropped a mortar mix that was forced into place by the vibrating bell,  which was slowly raised up by a hand cranked winch.  Back then, the mortar mix may have had asbestos in it. Can't say. But the end result was a one piece liner inside the brick framework, that they then came back to finish with a ceramic liner. Have often wondered if something similar could be used to make the heat riser core inside an RMH?

Eugene Howard wrote:And in both cases......wood stove or fireplace, neither one burns wood at anything that approaches the temps needed to combust all the particulates in the smoke, so fuel is only partly consumed, plus most of the heat is sent up the chimney.

I think that is an overstatement. That may apply to old dumb metal boxes, and yes they are out there. But in my experience, it does not apply to well engineered and well operated wood stoves. I had a very good one for 15 years, which recycled flue gas back into the combustion chamber, mixed it with heated air, and reburned all components. It kicked out a ton of heat with a few blocks of dry poplar, and I did not have to clean the chimney once in all that time.

I had an old pot bellied stove and one year I added a lot of mass around it to make it perform a little better. All I did was pile rock behind the stove laid up like a rock wall in lose, but organized fashion. It was cheap and easy and added probably a half ton of mass. Overall, I think it was an improvement, but it did not really help heat the room better, but really just tempered out the highs and lows of the burning cycle.

In that way mass does not really change anything. Physics states that "Energy can neither be created nor destroyed" so adding mass will not change that. A cord of wood burned produces x-amount of heat energy no matter what it is burned in. Some stoves do better at sending less heat up the chimney, but the cord of wood has the same btu content. This is the one component that people often leave out: the BTU is an equation with a time element to it. Like a rocket mass heater burns hot and is designed so little heat goes up the chimney, with the mass collecting a lot of the heat produced, but it also radiates that heat back out slowly over time since so much mass is involved. Unfortunately, that time allows cold air to infiltrate the building so a building either has to be super insulated, or the person has to be in close contact with the heated mass to really work well.

In my case, the mass worked because of my lifestyle: in the middle of the night when the stove died out, the rock which absorbed a lot of that radiant heat while it was well stoked, would release it back out into the room tempering the hold/cold differential of the room. But had I got up and stoked the fire, the room would have been a lot warmer than just having some rocks placed around the stove. Of course, I would have consumed more wood too though...

In looking at your photo, you could easily heat the room better by adding some elbows in the chimney pipe and bringing the stove out of the alcove. Since heat energy radiates in a straight path, the further you can get that stove out into the room, the more radiant heat will be sent out into the room instead of into the tiled walls of the alcove. The further out you can come, the better. You might have to buy some cement board or buy a heath rug so that the floor is adequately protected in front of the stove, but that will improve the heat output of your stove.

Jan Turner wrote:I have a Hearthstone wood stove.  It has soapstone body. It is  How can I retain more of the heat?  Can I put thermal mass around it?  If so what kinds?Here are some pictures of the area .  I am hoping for some brainstorming.

I had a very similar situation a few years ago, but the alcove was a stone chimney with the fireplace walls, into which a 5kW metal stove was placed. On a cold night after it had been running a few hours, the chimney base was warm to the touch outdoors, even when breezy and the temperatures were around freezing point. I reckoned something like a third of the stove's heat was being lost, through the chimney stone and the stainless steel closure plate which closed off the chimney void.

Pulled forwards a foot and a half and sending the flue out of the rear, I surrounded it with something like 200 bricks with a piece of old fire guard to bridge over the top of the stove. Perhaps 60 heavy firebricks from an old "night storage heater", the rest older engineering bricks - as heavy as I could find. Behind them I slid in 6 or so sheets of cooking foil to reflect heat back into the room and a couple of old computer fans were rigged so they moved the air a little, more so than just the thermal currents alone would provide.

Result? I knew there would be an improvement, but didn't know just how substantial it would be. Coming into a very cold house, it perhaps took 10 minutes (35 instead of 25) for the room to warm to comfortable from lighting the fire, followed by a period of 45 minutes to an hour where things felt little different, perhaps slightly less intense heat. From that point onwards, the the difference was increasingly massive!

There was more heat, to the point the whole house (two other larger rooms downstairs, two really big ones upstairs) would be comfortably warm even in the coldest of weather when previously it was a struggle to maintain this. There wasn't the rise and fall of heat as the stove cycled and nights were fantastically comfortable, with mornings warm enough not to race to light the stove when otherwise that was the case.

It felt as if the stove was one third larger when running but the heat was a more pleasant quality, it felt more like central heating through the night and into the next morning. The computer fans failed in time and without, the effect was almost as good - to the point I didn't replace them. I was initially concerned that placing bricks to within three inches or so of the stove could be a problem, but not a problem. If they'd been touching, I could forsee potential probs.

Had we been staying, I would have built something a little more permanent (while retaining air gaps between bricks) and used some French multifoil instead of the cooking foil sheets - it's many layers, each insulated.

I've done a similar thing, to good effect.  



Pebbles in a tile and brick box.  Mitigates immediate heat and slowly releases it over time.

Like Beau Davidson, I learned little things can add up to a lot.

In my shop, I had a barrel stove. I poured sand on the bottom, to isolate the metal from the hot embers. That added a bit of mass. Next, I stacked fire bricks up the inside, which was more mass.  The chamber was big, so I was able to be generous with the bricks.

In the end, I wanted both protection for the metal and a heat flywheel.

I did the same thing for the Franklin stove in the house, but added a piece of metal just short of being the width of the box and that had to be bent to get it in the chamber. That forced the fire around the side, rather than allowing it to go straight up the chimney.

Then, at the beach house I rented, I stacked bricks on the stove surface and up against the stove.  They acted like a flywheel, but the entire basement was too. It took about a week, but once the concrete walls and floors came up to steam, it was a super comfortable place to be. When the fire went out, it was a couple days before the cold took over again.

Insulating the outside of the walls would have really bumped the flywheel value up.

In every situation, I had to consider what was under the stove. The concrete floor could take a lot of weight, but not so much the wood floor in the shop with the hollow space under it.

I have been constructing an addition that holds a wood cook stove...one of the "code" issues had to do with windows behind/too close to stoves...major heat loss. Love the stained glass, but I would guess it is  a potential heat loss.

SIDE NOTE ON THE VALUE OF MASS FOR HEAT STORAGE:

Back about fifty years ago, a story was published about the demolition of a very old bakery. It's ovens weighed TONS (many of them).  Shortly after starting the demolition, they learned the oven was far too hot to tear into, even though it had been shut down weeks before.  Doing so would have destroyed equipment and hurt people.  They ended up having to wait weeks more for it to cool enough to tear down.

So, these are all valuable tidbits of info.  I just want to drop a quick note in here saying that it might be wise to consider replacing any conventional woodstove with a simple rocket mass heater, like the pebble-style.  I had a chimney-fire at my place due to creosote buildup that occured within 8 weeks of winter use.  8 weeks!  Freaking scary.  All is well, thankfully, but the experience lit a fire under me (well, over me and beside me, actually) to go ahead and spend two weekends building the RMH, or to just pick up a liberator and call it done.

With such a pretty, and presumably fairly new and efficient and expensive, woodstove, I could see being reluctant to replace it with another kind of heater. I would mention that for a regular metal stove, the clearances to the likely wood-framed walls are very close and may be unsafe. Tile is fireproof, but doesn't keep the framing behind it from getting hot and eventually charring. Maybe the soapstone surfaces allow much closer clearances while remaining safe. For safety in the current configuration I would advise adding a metal heat shield spaced an inch from the walls so air can circulate behind it.

Without changing the stove, you would get much better heating from it by pulling it out into the room a foot or two. Add floor protection to code, and put a good layer of noncombustible insulation against the back and half of the side walls of the alcove, then stack or mortar up a double layer of heavy brick or stone in a tapered "U" shape in front of the insulation. I know the tile is beautiful, but it is not really helping you as it is. Slope the stovepipe from stovetop back to the existing ceiling penetration so it stays as vertical as practical with gentle bends.

That alcove makes it tough. Marble is becoming such a popular countertop material maybe that instead of bricks for asthetics. Does the back have the soapstone tiles as well as the sides?

When I put my barrel stove up in my shop, I:

(1)  I allowed about 24" on each side of the stove.

(2) I covered all the exterior sheathing and 2x's with aluminum foil.

(3)  I pulled all the paper from the glass insulation, to avoid that potential combustion source, and installed it between studs.

(4) I installed rock. I'd test driven it using a propane torch.  Impressive stuff in that it takes a LOT of heat to char the paper, then a lot more to get past the rock to the next paper layer.

(5) About 12" in, I put up cement board.

Even with the stove cherry hot, you could put your hand between the cement board and the rock and it was merely warm, not uncomfortable.

Soapstone is preferred, price aside, for its ability to withstand higher temps than most fire bricks, and still store and give off heat.  IF memory serves, it's rated around 3,000 degrees Fahrenheit, so, if you get there outside the firebox, you may already have major problems.

Glenn Herbert wrote:With such a pretty, and presumably fairly new and efficient and expensive, woodstove, I could see being reluctant to replace it with another kind of heater. I would mention that for a regular metal stove, the clearances to the likely wood-framed walls are very close and may be unsafe. Tile is fireproof, but doesn't keep the framing behind it from getting hot and eventually charring. Maybe the soapstone surfaces allow much closer clearances while remaining safe. For safety in the current configuration I would advise adding a metal heat shield spaced an inch from the walls so air can circulate behind it.

Without changing the stove, you would get much better heating from it by pulling it out into the room a foot or two. Add floor protection to code, and put a good layer of noncombustible insulation against the back and half of the side walls of the alcove, then stack or mortar up a double layer of heavy brick or stone in a tapered "U" shape in front of the insulation. I know the tile is beautiful, but it is not really helping you as it is. Slope the stovepipe from stovetop back to the existing ceiling penetration so it stays as vertical as practical with gentle bends.

Another way to get heat moving, there are non-electric fans. I had one years ago, but it seemed the surface had to get gawd-aweful hot (like 400 degrees F) for it to start spinning.  I have a new one and the spin begins almost as soon as the stove top warms. I am very pleased with it, it moves a good bit of air around the room. You should look into heat shields. I am making one for my Amish cook stove out of old re-cycled metal roofing on steel studs, one inch gap from the floor and sides. This creates convection of cooler floor air up the wall behind the shielding. I believe you will find that clearances can be reduced and it is much safer.

Please don’t put any type of wax on your stove as overheated wax can burst into flames.

The limitation of space with the stove being in that alcove poses a problem. A few years ago I made a couple of water heaters for someone that we bolted onto the back of his stove. The heater is made of 3/16 stainless steel 2, 3/4 inch pipes, cold water inlet goes to 2 inches from the bottom of the tank outlet was flush to the top. the bottom has a securing tab the top had a bracket not pictured that bolted to the fire box and held the pipes. Measures 16x8x1.5. The reservoir was to be placed higher than the inlets. He used a vintage copper boiler as his reservoir, but it could have been something else. It is a bit dirty since I just pulled it off of the shelf in the shop. The stripes are from baffles i put on the inside of the tank and are heat discoloration not rust.  on the other side the baffles were not attached, and you do not see the heat discoloration. The baffles probably offer minimal flow direction but did add additional heat transference mass/material. Though I didn't intend for it to be used directly in the fire box I suppose it could be. My concern would be the bulkhead seals for the pipes. I'm not sure if you have the space for a reservoir for a heat sink but that might be an option. If the rear of your stove has a removable square of soapstone a heater similar to this could be made to replace a soapstone square.

The main problem is that most of the heat that you want to accumulate goes into the chimney 1. You need to put the stove in front of the niche 2. Lay the window and make the walls thermally insulated, then cover them with ceramic tiles or bricks 3.  increase the length of the chimney to the entrance to the roof (I made this assembly from old fire extinguishers) by 10 meters, while making several bends so that the pipe works as a heat exchanger, you can see an example in my profile.  4. Then you can install a turbine in the niche and send it to other rooms through the ventilation ducts, or leave the niche open and install a winter-rotating ceiling fan above it.  This will allow you to use the heat that is now going into the chimney and operate the stove at maximum power without overheating the chimney.

A simple and cheap way is to place a small fan in front of the heater. It will blow air under it and up the back then into the room. Adjust the fan speed to suit.

When I lived in an old school building made of concrete block (uninsulated) I dry-stacked more concrete blocks behind my drafty old woodstove for it to warm instead of the exterior wall.  In the bottom row of blocks I chiseled openings in the front at the bottom of each block to act as air inlets.  Air flowed up through the holes in the blocks as they warmed, and out the top row of blocks to disperse around the room.  I also had one of those heat-activated fans on top of the stove to help move the air into the center of the room, plus a wired ceiling fan to blow the warm air down off the 12' ceilings.  

The OP might not have enough room in that tiny alcove for a stack of 8" blocks behind the stove but it seemed to work pretty well for me, and used recycled blocks already onsite.  Perhaps using the smaller bricks that have the three holes in them would produce a similar result.  

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Every one says "Most of your heat is going straight up" so why not capture some that "lost heat"?
I have wood stove in a cement floor cinder block shop in CT. Once it gets cold, it's hard to heat up. I've looked at many DIY heat exchangers videos. Here is a good one https://youtu.be/iGZWx0gwRCo?si=HoxRmTWlJLq94LTP
In spite of many years of welding/fabricating I opted for a bolt it together fix. https://youtu.be/c2-XwqNiVL8?si=WOmTK_6LXs9SkA7q This works pretty well. But it is hard to prevent smoke leaks, red permatex failed, went to a higher rated sealant.  I bought 3 magnetic temperature gauges and can read the surface temp below and above this flue diversion so I can see what kind of heat is coming off the pipe. I have plans to build an RMH which would store heat in a work bench, giving me nice warm hands when the gloves are off doing fine work.