brick surround helps slower, even heating?

[dvellone]

I installed my castine last fall and it heats the house very well, but it's too easy to over heat the house as well. Aside from small and frequent fires which would be a little bit of a pain, I'm wondering if a brick surround would help to absorb heat and then release it into the room thus reducing the amount of smaller fires I might have to light and absorbing heat to help a little with overheating.
Even though it'll look good whether it works or not, I'd only do it if it made much of a difference.

The stove is in a corner installation.

 

[Skier76]

We have the same stove. I've only burned it this fall...but it does put out a lot of heat. One thing you can do is let the fire die down a bit before you had more splits. I'm still getting used to that myself. I'm not saying leting the fire go out, but let those logs "coal" for awhile. I've noticed this will cause the temp of the stove to drop. But I don't beleive it's smoldering by any means.

 

[bluefrier]

That is a good idea and I think would work well in the shoulder season. My stove is installed in a fireplace and one fire will keep the bricks warm long after the fire goes out. Only problem is that when it gets cold that slow even heat won't keep the place warm, you will need a stove blower to speed up the heat transfer and also feed the stove more often.

 

[Corey]

Technically yes - but in any practical sense not really. I think that stove is rated around ~50k btu/ hr. Brick holds about 0.2 btu/lb/ºF. So if you could drag in 500 pounds of brick and get the whole mass up to an average of 200ºF, then cool it off to 70ºF room temp you would be storing about:

0.2btu x 500 pounds x (200ºF - 70ºF) = 0.2 x 500 x 130 = 13,000 btu

13,000btu / 50,000 btu/hr output = ~ 1/4 hour or 15 minutes worth of heat

So you would notice a few minutes lag in heating up the room and a few minutes lag in cooling, but it isn't going to allow you to cut back to say one fire per day - or instantly give you a masonry heater which would be more like 1000's of pounds of masonry heated to several hundred degrees and emit heat over several hours.

 

[savageactor7]

I surprised small fires aren't working out for you dvellone...split everything in half again and try it that way till it gets colder.

 

[Slow1]

Corey - is that fibure of 0.2 btu/lb/ºF only for brick or does it apply to most stone/masonry? Just curious in general.

Related topic - I was noticing as I played with the IR thermometer just how much the walls (and other things) around my stove warmed up and retained the heat for a while after the stove was cooling. With stove down to 250 top temp I realize it was still radiating some, but the room air temp was reading around 75. Walls were reading high 80's so I imagine that they were helping to keep the room warmer. My thought (and point) being that everything in the room is absorbing that radiant energy and will release it over time as the stove cools down. I think someone in another thread suggested heavy furniture such as a stone table etc around a stove for a cabin to help preserve heat - perhaps that isn't such a bad idea, but I guess unless you make your stove room look like the Flintstone's cave you still won't really achieve long-term heat storage...

Maybe a very large tropical fishtank in the wall behind the stove with very tolerant (or fake) fish?

 

[woodgeek]

Corey nailed it. The heat storage is (mass X temp swing). So, you need a large mass AND a large temp swing to get thermal storage (e.g. a masonry heater). In your furniture example, the mass might be high, but the temp swing is much smaller than in your brick surround example. Water, with its huge heat capacity helps, but those guys in "The Boiler Room" are still building very massive tanks.

To the extent soapstone works as a thermal battery, its b/c the ~500lb mass has a decent specific heat and is getting a few hundred degree swing.

Anyone ever compute the BTUs on their Fireview swinging 300F?

 

[spirilis]

hey along this same line, would it be possible (or highly INADVISED?) to throttle back a large stove by surrounding it with panels of sheet metal, or aluminum foil draped off some kind of hanger, to concentrate more heat in the stove (so you can turn back the air control and reduce its output, but without making the fire burn cool enough to produce creosote...)

 

[Slow1]

Anyone ever compute the BTUs on their Fireview swinging 300F?

Well - doing a quick calc in my head using the .2 number above for brick (I don't know what soapstone would be) a 300 degree swing would be somewhere near 30Kbtu (unless my head put the decimal in the wrong place... happens) so with a 55Kbtu/hr rating that would be 5+hrs of heat stored there.... I need to do this on paper and check my numbers as I think there must be a rounding error there somewhere.

 

[firefighterjake]

This may be stupid, but I'm assuming you don't load the stove to the gills when you're just trying to take the chill out of the air during this time of year. With the Oslo I fire the stove up on these chilly evenings, let it get to coals and then typically only do one or two more loads (depending on just how cold it is in the house) . . . and even then it's only a partial load with some of the "junk wood" -- softwood, slabs, odd-ball shaped wood that wouldn't allow a nicely packed firebox, etc. This seems to work pretty well for me as it gets the house warm, but not overly warm.

 

[woodgeek]

Slow1--I think you did drop a decimal 30/55 ~ 0.5 hrs output.

I just worked out that the air in my house weighs in at an impressive 1380 lbs, and found that it has the same heat capacity as brick 0.2 BTU/lb.degF. So, I guess If I heated the air in my house by +200F, I could store a few hours of heat. Probably wouldn't be 'code', though.

Jesting aside, I guess that we are using our whole houses as thermal batteries--house movers say houses weigh 60 lbs/sq ft of living space, I'll figure 40 pounds in the insulated space. A 2000 sq ft house has an effective thermal mass of ~80,000 lbs. Using the same 0.2 BTU/lb.degF figure, that is 16,000 BTU/deg. So, a 5degF swing can store 80 kBTU, or the total output from burning a ~20 lb load of wood. Going back to the OP, if you don't think you have enough storage, you might consider improving your distribution!

 

[branchburner]

As Corey said, I think the best you'll get is a bit of lag time: slower to heat up, slower to cool down. That isn't always a good thing - it requires some guesswork as to how warm you think the room is going to get. You also get less "instant" heat from a quick morning fire.

Having a lot of thermal mass around the stove is a plus when it's really cold, but I think it's a minus in shoulder season. My stove sits in a massive brick hearth and if I want a small fire to take the edge off, a lot of heat gets sucked up by the cold brick. Trouble is, the brick does not give back much real effective radiant heat unless the fire has been burning hot and long enough. By then, it is often more heat than optimal if outdoor temps are not real low.

Rather than a quick morning fire, I prefer an overnight fire in the fall. The fire is out in the morning, but the brick (and other thermal mass in the room) is still warm. Air temps come down a bit towards morning but are still comfy. The kids aren't shivering, asking for a fire, which would only start to warm the place just as they're heading to school!

 

[dvellone]

To address some of the feedback...

I do use small splits and build up a smaller fire, let it run it's course, then while there are just enough coals start over again. It works pretty well, but my house is 820 sq' so it's pretty easy to over do it even while being vigilant. The only time I really have to load the stove heavily is the sub-zero nights, and it'll top out near 80* downstairs and we can sleep comfortably with the windows cracked upstairs.

My thinking was that given the size of the house a bit of thermal mass might make enough difference to make it worth my while. Corey's calculations are a great insight. I just have to try to apply it to the size of my house.

All in all, I have no regrets to moving up to the castine from the f100. The castine provides me with much greater versatility and heating range. During the shoulder season, or warmer winter temps the f100 was right on, but when it got COLD it was a lot of work to keep the house warm. The castine allows me to keep it warm when it's mild, and sleep through the night when it's sub-zero. Plus ease of ash removal.

 

[Corey]

Yes, most stone / concrete is around 0.2 as a general number. It's called 'specific heat capacity' so you can stick "of XXXX" what every material you want to look in google and find a reasonable approximation. Soapstone is not much better at ~0.23 - but it does provide a balance of heat capacity and thermal conductivity (another important factor)

A fairly inclusive lists:

http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html
http://www.watlow.com/reference/files/nonmetallic.pdf

Surprisingly, a lot of people think "if stone won't work, I'll drag in a block of iron!" but that is even worse at 0.11 btu/lb/F.

The best would be a tank of water at 1.0 btu/lb/F (by definition), but then you are limited to 212F (again by definition) I would also urge caution on your 300F figure. To be accurate you need to consider the average temperature of the entire mass. It's doubtful you could pile 500 pounds of brick around a stove with all it's air holes, seams, gaps, etc and get the entire mass up to 300F. Slabs of concrete might be slightly easier to bring to an even temperature, but again this is all a very large amount of work for an additional 15-30 minutes of heat.

Some hidden surprises: cork = 0.5 (5x the heat capacity of iron), though 500 pounds might be a rather large volume, polyethylene plastic (recycle your milk jugs?) = 0.54, paraffin wax (ring the stove with a giant candle?) = 0.70 Though all these materials have troubles in other areas - despite the high heat capacity.

 

[Dune]

I don't know...500lbs is not a lot of brick, plus I imagine the heat loss in an 820 sq.ft. house is pretty small. How much of your corner can you cover with brick? Can you build a big thick(heavy) hearth underneath? Maybe get the thermal mass up to a couple thousand pounds and it should have some good benefit, especialy in such a small house.

 

[Slow1]

Slow1--I think you did drop a decimal 30/55 ~ 0.5 hrs output.

I just worked out that the air in my house weighs in at an impressive 1380 lbs, and found that it has the same heat capacity as brick 0.2 BTU/lb.degF. So, I guess If I heated the air in my house by +200F, I could store a few hours of heat. Probably wouldn't be 'code', though.

Jesting aside, I guess that we are using our whole houses as thermal batteries--house movers say houses weigh 60 lbs/sq ft of living space, I'll figure 40 pounds in the insulated space. A 2000 sq ft house has an effective thermal mass of ~80,000 lbs. Using the same 0.2 BTU/lb.degF figure, that is 16,000 BTU/deg. So, a 5degF swing can store 80 kBTU, or the total output from burning a ~20 lb load of wood. Going back to the OP, if you don't think you have enough storage, you might consider improving your distribution!

Thanks for the correction - I figured there was an error there somewhere as 5 hours seemed just too good to be true.

You know the key thing to actually pick up about the thermal mass of the house air, contents, and walls is that it does take a decent amount of energy to get the space up to temperature. Thus the reality that if you let the house cool down (i.e. come back from vacation to a cold house - say 55*) and try to warm it up to whatever you like (perhaps 70*) that 15* differential is going to take a while to stabilize - a decent amount of fuel will be spent just heating up the home. Once things are up to temp it is mostly a question of making up for whatever heat is being lost which is largely a function of the delta between inside and outside temperature.

So - back to OP, part of the issue with your overheating is that not only are you putting out a decent amount of heat, you really are not losing heat anywhere near as fast due to the warmer outside temps and your home is already quite warm (relatively speaking). I think the suggestion of simply building a smaller fire or using softer wood is likely to be the simplest and most cost effective solution. I have to learn to do the same thing here - I accidentally overheated my house this weekend and had the wife complaining since I took the house up 10* in about 2 hours and it stayed up there all day... she isn't going to be all that willing for me to light up on marginal days if I don't learn to build smaller fires.

 

[dvellone]

So - back to OP, part of the issue with your overheating is that not only are you putting out a decent amount of heat, you really are not losing heat anywhere near as fast due to the warmer outside temps and your home is already quite warm (relatively speaking). I think the suggestion of simply building a smaller fire or using softer wood is likely to be the simplest and most cost effective solution. I have to learn to do the same thing here - I accidentally overheated my house this weekend and had the wife complaining since I took the house up 10* in about 2 hours and it stayed up there all day... she isn't going to be all that willing for me to light up on marginal days if I don't learn to build smaller fires.

We built the cabin just to get on the property and to be able to walk out the door to work on the main house. But now that I've been here for a few years (and will be here a couple more) the low cost and rustic cabin keeps getting more of my attention towards improvement because, well, I live here.

So if it won't make a big difference the smaller fire detail will have to do. Or the longer burn and bermuda shorts in January. :coolsmile:

 

[gibson]

I have an insert and I need to get that hearth heated up real good before radiant heat starts pumpin'.

 

[FisherZip]

digging through old posts here... thanks for these calculations Corey. a few new thoughts, since I am just now studying how to extend my heat release phase:

- 500# bricks might 'only' store the heat equivalent of 15 minutes burn time, but that is 15 minutes at max heat output (~50k btu/hr), right? if we assume a lower and tapering burn rate as one might expect in the wee hours, that might be 45-60 minutes of low heat release, which is considerable, especially when you consider how few bricks 500# really is. if a brick is ~5 lbs, that's only 100 bricks, or a pile 11"x20"x32". Doing a 3-sided "surround" around my hearth just 1 brick thick, 100 bricks would only give me a wall 26" high, only about 3/4 the height of my stove. I was thinking of something a little taller, thicker, more expansive, maybe like this: (broken link removed to http://www.lowimpactliving.com/blog/wp-content/uploads/2009/04/cob-house-stove.jpg) which should store significantly more heat and release it over a longer time.

- but now I am thinking about water! water has 5X the heat capacity of brick, so 500# of water could potentially extend a slower burn 5 hours! and 500# of water really isn't much at all, just 60 gallons! Hell, my fish tank is bigger than that! so I am thinking that a couple 55-gal metal drums (one on each side of the stove, maybe with some copper pipes running across the back of the stove (outside) from the bottom of one barrel to the top of the other) could soak up some considerable heat. it's no masonry heater for sure, but would seem significantly better than nothing! just need to convince the wife of how those drums can be made purty...

 

[savageactor7]

^Well that has some charm appeal to me.


dvellone...I may be projecting but it could be your splits are too big. It's a nuisance to split smaller but with good hardwood smaller splits make it super way easier to make the fire do what you want it to. When we're burning our prime hardwood all it takes is 3 splits in different burn stages to reach our magic stove top number. Actually the stove looks almost empty blazing away the way it does. btw every time we put another split on we rake the coals forward. With small splits and an overnight extended fire is required just sandwich them together with the least air space as possible.

The thing is ...wood stoves require tending & fiddling. Now if you load it up and off to work your go the main requirement is a safe burn and have some coals for your return. Now if you're in the house that's a different story, you have personal demands. And that's where secret fiddling knowledge comes in handy...but you have to enjoy fiddling.

We live in a 'turn key' society and many can't make the accommodations necessary to heat with a pre-industrial revolution piece of equipment. Don't allow yourself to be frustrated just keep experimenting and build up a supply of 'seasoned' wood.

...as far as using bricks goes. Meh...I'd sleep on it if I were you.

 

[cycloxer]

Building a smaller fire and reducing the air will make a bigger difference than adding a brick surround. The brick will look nice though.

 

[dvellone]

^Well that has some charm appeal to me.


dvellone...I may be projecting but it could be your splits are too big. It's a nuisance to split smaller but with good hardwood smaller splits make it super way easier to make the fire do what you want it to. When we're burning our prime hardwood all it takes is 3 splits in different burn stages to reach our magic stove top number.

I've slowly realized this. Went to the larger stove for more heat output and longer burns, but while the f100 needed to be loaded to the brim and burned hot to keep us warm, the castine with a full load will easily overheat unless it's in the double digits below zero. What I've realized is that to achieve comfortable temperatures indoors while it's in the teens or above outside I'm loading the castine with the same volume of fuel as I did the f100. Where the castine benefits me is when it's bitter cold outdoors - we can keep the house easily comfortable. And on overnights I don't need to get up to reload. It may get a tad too warm but the fresh air and a solid night's sleep can't be beat.

I guess I was trying to make my wood stove into a masonry heater which would have worked great in this small cabin, but it's too late for that and the surround which might help a bit would be a lot of construction in a small space for too little benefit. That heating design will go into the main house!

 

[cycloxer]

Yeah you are on the right track. No cast iron stove is going to give you a perfectly consistent temperature. A soapstone stove will be smoother, but even with that you will get fluctuations.

 

[EatenByLimestone]

I think you are making the mistake of assuming the stove always burns at it's peak output. A 50K btu stove may burn at peak for an hour, but the rest of the time it's probably running 30K btu or so.

Matt

 

[dvellone]

I think you are making the mistake of assuming the stove always burns at it's peak output. A 50K btu stove may burn at peak for an hour, but the rest of the time it's probably running 30K btu or so.

Matt

yeah, I'm aware of that, but heating a 800 sq' cabin with the castine doesn't take peak output to get indoor temps above 80*. Just a constant burn will do it.