Building a massive hearth for a wood stove.

Maybe you should find a hearth shop in your area and speak
with them for the safety of your family, after all safety is first.

One of the first things to look at is will your floor system handle the load? I assume you have a wood main floor system, rather than on a concrete slab, since you mentioned the basement. Typically you design for a 50 lb/sf live load IIRC, it doesn't take long to exceed that the way you are talking about going. What can you tell us about the structure you are building on?

gnawrot said:

I did some research but I could not find anything pertaining to my case. If you can provide a link to interesting article I would appreciate it greatly. Otherwise, you can put your comments, suggestions etc, please

Based on some people suggestions I gave up putting wood stove in my basement (for now). Instead I would like to put wood stove (maybe insert) in my family room which has fire place. I would like to build massive hearth which would absorb lots of heat and radiate it later when wood stove will be off.

I was thinking of building a frame on the floor ( 2 x 4 or 2x6 if necessary ), puting 3/4" plywood or two 1/2" plywoods and pour concrete on top of it (1000 lbs or more). Than I could put tiles or stones on top of concrete. I would reinforce concrete using rebar or mesh. Can I pour concrete directly on plywood? Should I do it in sections? Can I put insulation underneath plywood (fiber glass)? Should I pour concrete blocks and glue it to plywood using construction glue. I would reinforce floor from underneath to make sure that floor would not be stressed too much (two steal pipes). How should I put massive concrete on the wall (maybe I should not). How do I insulate it in this case etc.

Any feedback, suggestions would be greatly appreciated. This is an awesome forum (very professional and friendly people).

EDIT: I also have lots of black locust shavings, sawdust (probably like 400 gallons of it now). I will still have probably another 300 gallons of it. What for can I use black locust shavings, saw dust. I dont' think it will be good for insulattion and it might not be approved by insurance (fire code etc). I know that R value might be marginal but it is still better than plain air.

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The largest amount of heat that comes from a wood stove or insert that would be capable of heating a massive hearth is out the front through the glass. It's all radiant heat, and I don't think in the course of some limited number of hours it would absorb enough heat to make your effort worth while. If your going to reinforce that much, pour that much concrete and go through that much effort, you should look into adding a masonry heater. Some of the small tulikivi's would work nicely for you, probably look neater, and provide the mass your looking for in a package that's designed to do what you want.

Getting the heat down to the hearth is an ineffective tactic. (Don't take my words as critical or blunt, I just know how mine reacts) I have an Osburn insert and my hearth is 2 layers of Wonderboard with ceramic tile over top, and the lower board barely (and I really mean barely) gets warm at the edge of the hearth.

If you want some sites to look at for Tulikivi's look here for an example of a small heater: Tulikivi 1000 model line

Well, the insert is out if you want heat storage. Inserts are specifically made not to lose heat to structures around them, instead it converts that energy into additional amounts of heated air that blows out the front.

Radiant heat, and I usually get blasted from people about this all the time, I hate. I think the reason my basement stove situation was so bad, was because of the radiant heat. Sunlight is a form. On a 75F degree day at noon, go out there and stand in the sun. It may be 75F but you'll probably feel like you're cooking. Maybe you've stood close to a fire and felt your pants were going to combust... that's radiant heat. It's a form of light, you can't move it down a hall, with fans, or move it through ducting and one of the reasons my basement stove situation was a big failure I think. I captured most of the convection heat of my stove, but the radiant heat being a form of light, my ducts & fans were useless and it made the area very uncomfortable particularly with the amount I had to burn to try to heat two floors above.

I think this isn't going to heat like you think it will. With Masonry heaters which sounds exactly what you're thinking about, you usually put them in the center and have all the rooms share a wall with it or the flue rising up through the other floors. It lets you keep the house in the low 60's and feel comfortable from the radiant heat. There isn't a lot of air movement with masonry heaters and their radiant heat, and generally it's healthier. But rooms that don't have a common wall with it, will be cold. It's like radiant floor heating, how effective will you heat your house if you have radiant floor heating in only that room you plan on putting this structure and don't have it turned on anywhere else. The rooms that don't have any connection, particularly the extremeties are going to be freezing. Big structures like that the surface doesn't normally get above 170F and on cold nights that's plenty to make you feel warm IN the room with it but, the other rooms that don't have a wall exposed to it, it won't make enough convection heat to move to those rooms and they'll be cold. It's something you build/design the house around, not something you retrofit normally.

Anyway, I don't think it's going to work like you're hoping and be a lot of work. I'd get a soapstone stove, it has the thermal mass as a buffer and makes decent amounts of convection heat you can move to other parts of your house, along with radiant heat that's more gentle.

I agree with Rhone... My point was that a 5k Tulikivi would be better than all the cash your planning on spending to put in a thermal mass. Your probably underestimating the cost of your thermal mass also, so a Tulikivi or as Rhone points out, a soapstone stove will probably be your best bet. Skip all the termal mass of a hearth and I think you'll be quite happy.

gnawrot said:

I thought that pouring concrete would be cheap unless I need some fire retardant stuff (wonderboard). If I get Encore with blower will I get double benefit of radiant heat and convection and will concrete mass absorb additionally heat which will be released later? I might get insert if needed (I am open minded). I think that insert does feel cooler because of air movement which makes us feel cooler and more comfortable compared to radiant heat.

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I'm not a thermodynamics engineer, but I think your trying to make a woodstove into a masonry heater. It really won't work. of course It's only my opinion, but let's see if anyone else thinks I'm wrong.

Concret ain't cheap and it is extremly heavy, that will make bucking up your floor joist with steel necessary and that ain't cheap either.
If your looking for cheap just get some electric baseboard . You can run them for a few years for the cost of all that constuction you will need to do with a concret slab in your living room.

Let me help you out, with your decision the massive hearth is the least effecient location for heat storage. Coupled with almost all Encores have ash pand and heat shields on the bottom. THE WALL BEHIND THE STOVE IS WHERE HEAT WILL GET STORED.
not the hearth. Mine is a center chimney location with masive granite stone facing It takes 24/7 to get heat into that facing But once there it stores and releases heat that no soap stone stove can I have 20" of soild stone and masonry floor to ceiling 12 wide to store heat. When the stove goes out when you walk by that amount of stone heat absorbtion 6 hours later you can feel the warmth still generating. I once used a heat shiled at the rear of the stove trying to notice the difference of not letting the heat transfere to the stones and more stove to the room It did not work as planned I remover the heat shield. My original design worked best.

It takes mass to store heat. granite stones the thickness I have, supplies that mass. Can not be compared to a coople small slabs of 1" thick soap stone on a stove

Maybe you can get your B.I.L. to estimate the support you need for that wall.

You might be better off building a masonary heater for you home, they really are cheap to burn and if you get stuff cheap ( stones, steel, and cement) then thats the one for you.

I wouldn't dismiss this idea quite as quicky. We have a Woodstock fireview soapstone stove. I have found that the floor directly in front of the stove can get very warm - nearly too warm to hold your hand to it. Note this stove sites relatively low to the floor. As Elk points out, an ash pan or bottom heat shield will keep the floor directly under the stove relatively cool - we notice this as well. The other place I find getting very warm is a half-wall we built just to the right of the stove. The distance is about 18 inches, and the wall is ~3 feet high and lined in 1/2" heavy dark slate tiles. These tiles get very hot - nearly too hot to touch. This has made me think that had I filled this dummy halfwall with something like sand or other cheap filler, it might have actually served as a nice heat battery for periods where the stove dies down. In fact, I think this may be the best option - even better than in front of or behind the stove. If your stove has something like a 16" side clearance, and you can frame it in like that, you could store a non-trivial amount of heat.

Also, this side wall happens to be opposite the cast iron door - unlike soapstone, it heats up and cools down fast. You might have better luck using a cast iron or steel stove if you surround it with a lot of thermal mass - you'd get similar benefit to what a more expensive soapstone gives you.

If I had to build a raised hearth, I would have started with the sheet metal sandwich floor protection on the bottom of the hearth to meet requirements, but on top of the that, I would have used non-combustible materials to build up a thermal stack before finishing top surface. Sand and concrete blocks topped with cement board and tile/brick might be a really cheap way to go if you're already looking at building a raised tile/brick hearth with a wood frame.

You can also do some simple heat capacity calculations to estimate how much heat would be stored in a given mass of material - do it in BTUs and that'll help put it in perspective. I concluded it would be quite easy to store 30-50K BTUs, which could stretch your heating cycle 2-4 hours in some homes.

Don't forget posts to support the floor if you try it.

-Colin

Colin,

He said hearth. Not walls. I do believe this would work if the stove had brick or some other thermal mass surrounding it. My fathers coal stove had brick on two sides and that brick got quite hot. With coal, it would take longer for the stove to die than the heat life of the brick to die down, but if the brick were 4-5 layers thick and the stove a wood stove, then your idea might work.

I'm still in the masonry heater camp for what your trying to do.

Warren said:

Colin,

He said hearth. Not walls. I do believe this would work if the stove had brick or some other thermal mass surrounding it. My fathers coal stove had brick on two sides and that brick got quite hot. With coal, it would take longer for the stove to die than the heat life of the brick to die down, but if the brick were 4-5 layers thick and the stove a wood stove, then your idea might work.

I'm still in the masonry heater camp for what your trying to do.

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I guess I consider hearth to be inclusive of the greater installation including whatever you build to accommodate a stove's clearances. For me, it included a wall, and it sounds like this guy is considering any options that accomplish his goal of a pseudo masonry heater. Doesn't change the point... as others note, it is relatively hard to get heat under a stove, and often hard to get heat behind a stove. But directly in front of some stoves, and to the sides, there may be good opportunity. I would pursue those options before pouring a ton of concrete - might be a lot easier. Especially if you go with something like a metal studded half wall on top of floor protection that you fill with something like sand, tile or brick it, and if you keep it free of combustibles, you could take it a bit closer to the stove, getting pretty nice heat uptake with a large heat capacity. Obviously don't want to go too close, but if 18" transfers a lot of heat on my setup, 12" might be really nice and still leave plenty of room for some air circulation.

BTW - I would find it hard to believe that the Tulikivi models would be $5K installed. A guy I work with was told they start at $6K list plus a lot of additional labor that can't be priced until they see your install situation, and that $10K was more typical entry price. With a $2K stove and a $1K budget for thermal mass, and a lot of hard labor, I think you could get close.

-Colin

Never said you could get a Tulikivi installed for 5k. I just mentioned a 5k Tulikivi. I agree...another 5 to install.

Might be interesting to get hold of the masonry heater association to see what one of these would cost: Small Heater

It almost looks like you could build it your self just from the pictures they include there. Face it with some nice brick and that thing might be s nice "little" heater.

NY Soapstone said:

Warren said:

Colin,

He said hearth. Not walls. I do believe this would work if the stove had brick or some other thermal mass surrounding it. My fathers coal stove had brick on two sides and that brick got quite hot. With coal, it would take longer for the stove to die than the heat life of the brick to die down, but if the brick were 4-5 layers thick and the stove a wood stove, then your idea might work.

I'm still in the masonry heater camp for what your trying to do.

Click to expand...

I guess I consider hearth to be inclusive of the greater installation including whatever you build to accommodate a stove's clearances. For me, it included a wall, and it sounds like this guy is considering any options that accomplish his goal of a pseudo masonry heater. Doesn't change the point... as others note, it is relatively hard to get heat under a stove, and often hard to get heat behind a stove. But directly in front of some stoves, and to the sides, there may be good opportunity. I would pursue those options before pouring a ton of concrete - might be a lot easier. Especially if you go with something like a metal studded half wall on top of floor protection that you fill with something like sand, tile or brick it, and if you keep it free of combustibles, you could take it a bit closer to the stove, getting pretty nice heat uptake with a large heat capacity. Obviously don't want to go too close, but if 18" transfers a lot of heat on my setup, 12" might be really nice and still leave plenty of room for some air circulation.

BTW - I would find it hard to believe that the Tulikivi models would be $5K installed. A guy I work with was told they start at $6K list plus a lot of additional labor that can't be priced until they see your install situation, and that $10K was more typical entry price. With a $2K stove and a $1K budget for thermal mass, and a lot of hard labor, I think you could get close.

-Colin

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That's consistent with my research. I was quoted a price of $6120 on special promotion on the TU2200/SA, plus installation, plus
concrete masonry support. For the model we wanted, with bake oven, we were told to expect 15k to 20k, all inclusive.

northwinds

gnawrot said:

Would addition of blower to the woodstove and redirecting of hot air more on hearth floor help heat absortion below the stove?I can do pretty much anything so I just want to make sure that I do it properly. I have lots of tools and am quite handy. I have plenty of sand on my property.

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I think you'll find diminishing returns here - I would try to locate things such that radiation does the job painlessly and free for you. It is generally accepted that it is fairly difficult to move a lot of woodstove heat with fans relative to what is already moving from radiation.

I think if you can manage to build a non-combustible half wall on either or both sides of the stove constructed out of cement board and metal studs, filled with sand, and faced with tile or brick could both look great and provide as much mass as your floor supports could take. And someone can correct me if wrong but I don't see any reason why you couldn't bring that fairly tight in with your stove to really soak up a lot of heat. Remember, I'm finding a lot of heat gain in my half wall located 18-19" away from one side of the stove! Would be very interested to see how something like that would work out for you. You could estimate the heat holding capacity in BTUs if you multiply volume of sand X heat capacity of sand X anticipated temperature difference. (probably about 75 degrees above room temp when fully charged). All that data is on the web - then assume in a well built home, on average you'll only need to release 10-20K BTUs/hr to stay warm and see just how well it works out for you.

Warren's point is valid though in that if you're really handy, you might just build a masonry heater. But I suspect complying with code will get trickier for a homebuilt model, and if you are willing to put in a lot of sweat hauling your own sand or other bulk material, you might have a cheap solution.

-Colin

gnawrot said:

I would prefer not to build masonry heater because I don't have too much time and I have still some outside projects to finish. Building hearth with/without walls will be enough for me.

Can I put Madison VC wood stove or Dutch West wood stove (medium or large)? Is Madison VC a good stove? Encore is a bit pricey for my taste. Does anybody knows about Lennox wood stoves? Are they good?

Putting walls on both sides would make wood stove easier to protect from my kids and would achieve my goal. However, some kids might be tempted to climb those walls (I would if I were still a kid).

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Building the walls will block the heath you get from the unit.
You will need to burn longer and hotter to achive the desired
comfort level you are seeking.
I don't understand your thinking about time to build a masonary heater
seems if you have the tools ,skills, and knowledge it would be a piece of cake for you.

The best hearths I have seen in homes from Washington to New York are supported from the ground up with
solid masonary work.
I owned a home once that had a masonary heater that had 3 flues and was 20 ' tall and 6x 8 at the base down to 3x 3 on the top floor where there was a smaller hearth for hot coals to provide quick heat for the sleeping areas.
That heater was built in the early 1800's
Wish I had one like it today.

Good luck to you gnawrot