I'm blogging this in case anyone else is in the same position as I was and is wondering about their options . I bought a lovely little woodstove , an Aarrow , and wanted to put it in a room with a newly laid carpet . I wasn't sure if the woodstove would be leaving with me when I left the property , or perhaps moved to another room , so I didn't want to cut a piece out of the carpet where the hearth would be.
Looking at stuff on Google , it seemed the accepted way to do things was by putting the hearth tiles on a massive concrete slab , 5" or so thick , and/or putting the stove on a stand . Now I had a stand with the stove but was trying to keep everything in the room on a low level, beanbags etc , so I really wanted to avoid lifting it up any higher than necessary . Without the stand there's only a couple of inches of gap between the floor of the stove and whatever was going to be underneath it , and reading some of the horror-stories on the web , that kind of proximity worried me. There were accounts of wooden floors under hearths catching fire and so on , and I began to wonder if I should really press ahead with this plan .
But.. I've lived with quite a few woodstoves here and there , and the one thing I'd always remarked on was how little heat they had radiated downwards below the firebox. I guessed that a small stove would not need a massive hearth to stay within safe limits as far as the temperature of the subhearth material was concerned - the trick was going to be designing something that would stay cool enough to avoid the problem of materials being made combustible by the cycles of heat-cool-heat-cooling , and be able to withstand the conditions when you wanted a roaring fire as well.
The government publication on woodstove installation recommended a massive hearth unless the woodstove was rated as radiating less than100 degrees centigrade maximum below the stove - if less than 100 degrees then the requirements for the hearth were only a 22mm depth of non-combustible material. Which was great , I could lay my quarry tiles on top of a sheet of cementboard and that would be it . But I really had no idea what my stove was rated at (it was an early Aarrow and they said they couldn't be sure of the rating) and 22mm seemed awful close to me.
Then I remembered a programme I had seen on the TV about termites , where an engineer looking at ways to design buildings with good inbuilt heat control had poured plaster into an old termite mound and washed away the earth to reveal a structure of vertical channels that took air in from around the mound and released it at the top , thus keeping it at a steady temperature.
See Scott Turner Termite Research website.
This seemed to offer a possible way to keep the hearth under the stove cool. Warm or hot air was always going to be rising off the sides of the stove in use , pulling air in from around the bottom of the stove . If there were holes rising through the concrete pad underneath the stove then air would be drawn through them , thus cooling the pad . ( And it might even make the stove work slightly better).
The next step was to cast a block with holes in -
I used a piece of chickenwire to locate the ends of the rods and hold them in position . It would also reinforce the concrete should it crack due to repeated heating and cooling and stop it falling apart .
I started out using dowel for my channels , then ran out and found a much better solution in the form of plastic hosepipe , it is released very easily by the concrete once set and has a nice curve and a smooth finish . The model looked great , like a maquette for a really groovy building ,,I always knew I should have been an architect....
I used random spacing and direction for the channels , but it's possible a better flow might come from a different and more organised arrangement
The concrete poured in
Note the exquisite finish on the formwork box , a craftsman to my fingertips ..a finishing layer was added to cover the chickenwire and give a smoothish top to the block. I guess you could use a high-temperature cement if you wanted.
So now it was just a case of designing the rest of the hearth. There were all sorts of websites offering information on the web , some giving huge distances from the stove to any other surface , some very low tolerances of temperature change (and some strangely high) , and confusion did reign for a bit .
I also found a few references to radiant barriers and their effectiveness in insulating material that was at risk of combustion , and the use of a space-blanket style silvery film as a radiant barrier . Most of the suggestions for hearths made use of cement board as a noncombustible base , but cementboard is a pretty terrible insulator and comments were made that the back of a cementboard barrier will actually get hotter than the front due to trapped heat. But putting cementboard together with spaceblanket and my termite-inspired block , I ended up with this design .
The air is drawn in from the side of the hearth ,travels between the sheets of cementboard and up through the concrete block. The block has space blanket on the top , and there is a layer of spaceblanket on top of the bottom sheet of cementboard and another on the bottom of the top sheet of cementboard . The heat below the stove therefore has to travel down through a radiant barrier , then a concrete block , then an air gap , another radiant barrier and then cementboard before it reaches my nice wool carpet. At the sides of the stove the heat has to travel through quarry tiles , cementboard , a radiant barrier , and air gap , another radiant barrier and more cementboard. There are of course thermal bridges where these are supported by bricks or bits of tiles but they are quite good insulators and ,, well, we'll see when it's finished ! I reiterate that I'm not an engineer and a lot of this is just trying to think things through from experience and basic physics.
The hearth laid out
You can see the four red bits of quarrytile that will support the concrete block with the holes through it. So this picture shows a round piece of cement board with a flat on the wall side
which is covered with space-blanket , then the bricks arranged around where the concrete block will sit on its supports. The bricks have to support the second piece of cement board with space blanket on its lower surface (it's leaning against the wall in the first picture , showing the hole cut in it that sits over the block.) and also be positioned to take the weight of the stove itself where the legs sit outside the perimeter of the block. They must do this whilst also allowing as free a flow of air as possible all around the area between the two sheets of cementboard.
With the the block in place.I then put a piece of the spaceblanket over the block , with holes cut in it where the holes emerged from the block to allow the air to flow unimpeded,,, I just thought the more radiant barriers the better. I glued it on with high temperature mastic I had around. No pic of this , sorry.
The next piece of cementboard went on , just held by its' own weight , then the quarrytiles were cemented on top . No pics of this either , did it in a hurry....but I had to hire a wetsaw to cut the tiles , them's hard buggers.
Carefully waggled the stove into place and fitted the flue.
And painted the stove
And lit it!
The wire coming out of the side is attached to a heat sensor that gives me a reading of the temperature directly under the stove above the lower cementboard sheet. I also put a probe-type thermometer inside one of the holes going down into the concrete block -I wasn't at all sure this was going to work with only a small airgap between the bottom of the stove and the hearth and I wanted to know if Farenheit 451 was approaching!
The flue is a recycled CO2 bottle ,very thick steel and a good thermal mass . Cut with an anglegrinder . Oversized but that meant the curved top could be cut in such a way as to make a corner for where the flue meets the fire, simpler and a lot cheaper . It will possibly collect condensation in the well of the curve is the only problem , but it's very thick stuff and won't rust through in a hurry.
The collar and flue plate are made from a stainless steel frying pan that had seen better days. The bottom of the pan made the adjustable flue plate which is out of sight inside the flue , mounted on a long bolt (should be stainless steel but I couldn't find one that size locally) , the handle is the handle , and the sides of the pan made a collar where the flue passes through the cementboard chimney-plate.
The sides of the hearth needed to let the air in so I used a cheap cane blind cut into strips .
and there we go.
Does it work? The temperature readings I get when I've got down to my T shirt and had to open the windows to cool the place down a bit are ,
for the probe under the concrete block ,10 degrees C above room temp
for the probe inside the concrete block ,15 degrees C above room temp.
Which is pretty much a resounding success , given that the subhearth temp is never going to be less than 65 degrees C under the maximum safe level of 100 degrees that the regulations give for a 22mm hearth.
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