IS and the hearth pad

this is a follow up on a thread a number of months back. the old stove was a jotul oslo 500 and to this day the floor requirements are fine with the 1/4" slate. not so with the IS and to get ahead of any insurance inspector and so forth a 2" reinforced mortar pad got poured with paving brick over that meeting the R requirement. it's over 650# of masonry.
the former thread, this was the summer and it was an intellectual rather than practical wonder, posed the question, how much heat storage would the hearth pad hold and what would be the ramification on overall performance. there was the normal division of opinion on the matter but i had my hopes and as a building tradesman know that thermal mass is thermal mass.
with this open question the warm months went by and i figured a good guess could come from common observation. i had lived with the jotul and a vermont castings defiant before that. when the normal 400# stove goes out, the hot stove gives a couple hours heat at the end and keeps the house up. i have a 600# stove which could give another half again to 3 hours, and about equal weight in the hearth pad so that could double to 6 hours based on mass alone.
it's not the winter yet but cold enough to give an initial report. the combination of stove and hearth makes a profound difference and holds heat a long time when the stove goes out, in this transition weather easily over 6 hours. the stove itself is a wonder with the steel and soapstone and holds forever, and the hearth pad is warm to bare feet and helps in its way. the house, well insulated and all, was previously always colder than the outside feeling and didn't really hold. now my wife says it's like having an oil burner in the house and a very even feel to the whole place. the question now is how far into the winter can the cycle of leaving the stove die for many hours continue. look at the math here. if the stove give an extra 3 hours of heat, and the hearth pad gives 3 hours, that's 10% less wood consumed just from installing the hearth pad which was a bit over $100. in material plus my labor to get it done. i feel 10% is a reasonable rough number. the actual pile of wood not consumed is something you wouldn't want falling on your foot.
i did not go for the ash pan as to leave things simple and am very glad i didn't get it. it would have been an additional sort of baffle chamber between the stove and the hearth pad. the brick absorbs most heat and stays warmest directly under the stove. there's that. then i have to pick up the soapstone panels that insert into the fenders as thermal mass is being my very good friend and let the party continue.
this scheme is geared for having a burn time and a rest period. maybe this fits to a 24 hour schedule maybe not. for those interested how much masonry weight is needed to hold heat from a 12 hour burn. on the extreme end, there clearly isn't time to heat up tons and tons of material. i feel the weight can be held to something modest, at the minimum 350# or about the weight of some stoves, where i have it at about 1300# between the stove and the masonry is also a good point, and at the top end 1000# to 1500# just for the pad. you can go heavier and it won't hurt, this is just a guess on how much is actually necessary.
there's more to report on the stove's performance for later.

 

Are you an engineer by any chance?

 

I know my hearth and the brick wall behind it must hold some heat, my stove top is sitting at 300 degrees right now the hearth under the stove and the wall are 95 degrees, but my stove only requires amber protection and does not radiate much heat out of the bottom or the back , sides a little more but most of the heat from this Stove comes right out the top and front.

 

no, career tradesman as i mentioned, jack of all trades carpenter cabinetmaker everything else nearly competent brick mason as the hearth pad attests. i'd rather be the guy who does it rather than the guy who thinks it. surprising too to find doers on a firewood forum. actual work, what an insult.
where an engineer would be handy is making a system to store wood heat in water columns or a large masonry mass, as in the idea behind the russian fireplace. you might be able to fire off a stove for a day and have heat stored for 3 more days. that's where actual calculations would be handy. on a practical basis there's usually only so much weight that can be put into an existing wood frame building and you're doing well to support 400# to 800# of a masonry hearth pad in addition to the weight of the stove. really good thermal storage systems are going to be many tons and can be found in designed from the ground up structures.

 

I suspect that if the pad gives you 3hr of heat and the stove gives you 3hr of heat, you will get 3hr of heat (not 6hr).

{edit: re-reading and see that you aren't saying it is additive.}

 

My PH sits on a pad of slate and brick that is 6" x 6' x 4'. That, in turn, sits on an older hearth made of a single stone that is about 1' x 8' x 2.5'. Behind the stove is the fireplace/chimney, and bread ovens. It is "mass"ive. But the temperature differential doesn't get very high because these stoves are designed to stay relatively cool on the back and bottom. When I come down in the morning, I don't find much heat radiating off the pad and brickwork.

The IS beta, before they changed some bottom shielding, would get our hearth up to 90+ degrees. But a foot away it was considerably cooler. So even 'though we have the mass, it isn't being used to its full potential.

 

Maybe you should have made a hollow hearth and filled it with water for more heat storage? I kid I kid.

I asked if you were an engineer because you are over thinking the whole thing. Plus I think you gotta wait until it gets colder to see the difference. I think your house might need more btus than a warm hearth will provide unless you have a very small or very insulated or "tight" house. I have a feeling your calculations are off because there are way too many variables to take into consideration. Your calculations are very linear which might make sense on the surface but not be correct. The IS gives a lot of its heat off the combuster. The bottom gives off some heat but I can tell you my previous stove made my hearth very warm while my hearth stays much cooler with the IS. I've got a corner install and I'm still amazed how cool the brick is.

In short, I don't think there's enough data to determine anything yet. Wait for the colder weather is my suggestion and then reassess.

 

my initial thread on the hearth pad during the summer produced the same sharp divisions of opinion and skepticism. i am not over thinking things at all. thermal mass makes a huge difference in how buildings perform. i lived with a jotul 500 for about 15 years and a vermont castings before that and am quite familiar with how my house holds, or needs heat. a good bit of the good performance i am now getting is from the IS unit itself which takes forever to completely cool down. this has to greatly aided by the hearth pad. the house seems to hold heat forever and has a more even feel. i don't put a stop watch on it but it is way beyond 6 hours of heat after the fire has burned all the way down. in this transition weather it's more like 8 to 10 hours and i won't know until i actually do time it and get through the worst of winter. in this transition weather the house holds for a very long time. we can be out of the house on appointments and quests for very long periods and just forget about needing to get home. i feel the 10% wood savings guesstimate is conservative and it is probably a bit higher. it is valid in transition weather which is going to be the greater percentage of days the stove is run during the season. that type of savings would of course not occur when the stove is run continuously.
there is a church in south america, a famous one, where the walls are 4' to 5' thick adobe (?) at the base and i believe it's not heated and the temperature has a very small variance the entire year. i didn't invent thermal mass and its qualities. i am getting fantastic performance and am very happy with how things turned out.

 

dennis

if you reread my last post i mentioned that the paradigm would not apply when the stove was running continuously which it would in january. on a theoretical only basis, if you could get your house insulated and tight enough, the stove could actually be run loading only once per day through the winter. i don't expect to have those circumstances. it's nice to burn continuously through the cold weather anyway. most of use are in it for the great physical feel of the radiant wood heat. why deprive yourself. it's healthy to put heat directly back into your body.
the weather is colder in MI. it's cold enough, but not all that cold here. the transition period where you will get 40* or better daytime temperature is about twice as long as when the stove will need to be on all the time. it's handy to have improved performance for those times. even in winter there are going to be a fair number of days that are unusually warm. there's going to be some wood savings and more importantly less fuss for a good bit of the time.
i couldn't give a figure on wood usage save it is obviously less than with the jotul. your 50% savings is impressive.

 

I have a ton of masonry around my stove... well perhaps even more than a ton? You don't get any extra heat of course, btu's are btu's and thermal storage just means it takes longer to heat up, then longer to cool off. Sure, if its mild and you burn for just a portion of the day it may help when otherwise the swing between too hot and too cold is severe. But in my case once I start burning its usually 24x7, and there is hardly ever any too hot! It's always just too cold lol.

I am putting in an IS myself, hope it heats better than my Jotul but I know asking for it to heat the house fully is a stretch but maybe...

 

Interesting thread, chucker. I am neither an engineer nor know the math/physics wizardry to grok how much heat absorption-give back the surrounding masonry provides via thermal mass. After reading this thread I realize the loss caused by my pad being concrete poured right atop the cement basement floor. It's been said that basement installs lose bunches of heat to the earth through downwards conduction through concrete floors. Yet there must be some (small) temperature gradient from one inch to the next from the warmed surface to the adjacent floor or to the sand below. So even though the warmth given back to the living space after the fire burns out would surely not be enough to support the home's heat requirement, it does at least add a little bit of warmth. Looking back, pouring the brick-colored pad on top of some fireproof insulated material (between the pad and cement floor) would have been better. Oh well, it is what it is....

 

There was one fella I think on here (or maybe hearth) that set out to build a super efficient house and purposely didn't insulate the concrete pad (just 1 story on concrete) so that the concrete and earth below acted as thermal storage mass. But I'm not sure how cold of a climate that would actually work.