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Author Topic: Masonry heaters  (Read 495 times)

MaineShark

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Masonry heaters
« on: April 14, 2015, 10:48 PM NHFT »

Given all the combustion design I've done over the years, folks have asked me to look into doing masonry heaters.

There are a few different types of fireboxes that are used, and a few different ways of storing the heat, but the general principle remains the same: to use a hot, quick, clean fire to heat up a large mass, which then slowly releases the heat into the space over the course of the day.

Typical masonry heaters are fired twice a day (morning and night) and burn for one to two hours at each firing.

Some use fairly-conventional combustion, usually with a secondary burn to make sure that there is no creosote released.  These tend to burn typical split firewood, and are usually loaded and fired, then left to run on their own.

Others use "rocket" combustion chambers, which run best on smaller pieces of wood (usually 1-2" diameter), and require constant feeding during the burn, since they only hold a small amount at a time.  On the other hand, their fuel is usually stuff that others consider trash, so it may very well be free.

Heat storage may be in vertical columns, horizontal benches, or a combination of both, regardless of which sort of combustion technology provides the heat source.

There are a lot of sites showing some very inexpensive builds, but they usually end up noting that the builder found some of the most expensive components for free, or are using inappropriate materials (using conventional brick where firebrick really needs to be used is common - and will tend to fail in a matter of months, if not sooner).  If built using proper materials, prices tend to be between $5,000 and $25,000 for no-frills heater.  Some folks go crazy with the aesthetics and may use hand-cut soapstone and other exotic materials, so plenty of heaters have been installed with pricetags as high as $40,000 (or even more), but for purposes of discussing the value of the actual heater, those high-priced facades don't add any performance that can't be achieved with brick or block.

One big factor is weight: for a masonry heater to store heat, it must have mass in which the heat will be stored.  That can be multiple tons, in some cases, and is almost always more than one ton.  Installing something that heavy usually requires a foundation that specifically supports the heater (an "average" masonry heater will probably require 16" of reinforced concrete).  Installing a few more cubic feet of concrete during a build is not a big deal, but it's something that needs to be considered, and can be difficult (but not impossible, by any means) in cases of a retrofit.  It's also one of the most common places that a DIY install may end up having problems, because most folks don't realize the load requirements, and assume that a concrete basement slab will support anything they choose to place on it... which then results in a cracked slab, at minimum, and may result in the masonry heater collapsing (potentially during a burn, with catastrophic results).

Unlike a woodstove, which heats and cools rapidly, a masonry heater absorbs and radiates heat slowly.  As the beginning of the heating season, it can take several days to get the masonry heater back up to temperature.  Not something you can just light to take the chill off, on occasion; if you want to use a masonry heater to heat your home, you have to be prepared to consider it your primary heating source, and keep it tended.  On that note, the earlier prices are not out of line with what one would pay for a conventional heating system.  Unlike many heating systems, though, the masonry heater is primarily heating one space (fans can help, of course), so it is best suited to open-concept floorplans, where it can be placed centrally.  On the flip side, a centrally-placed masonry heater can actually be used for space division, splitting a single space into two, three, or four rooms (which all share a portion of the warm heater's surface).

It would be best to have a backup source of heat, if you wanted to leave for a few days, both because the heater will not continue to heat, and because it will take a day or more to get it back to operating temperature, even if it did not fully cool down during your absence.  Better heaters may include a second firing chamber that can release heat quickly (the equivalent of a woodstove), for mild days, or quickly re-heating the room.  But backup is important, to prevent frozen pipes and the like.  However, if the masonry heater will truly be the primary heat source, then the operating cost of the backup is not a big concern (since it will rarely be used), and even electric heaters may be acceptable, given their low cost to install.

Masonry heaters may also include an oven, which can help release heat a bit more quickly during a burn, and for actual cooking purposes, of course.  Ovens may be "white" or "black," with the former being sealed off from the flue, allowing cooking during the burn, and the latter being part of the flue path, storing heat for cooking after the burn is completed.  The names are a bit misleading because, while a black oven does have flue gas in it during the burn, masonry heaters are clean enough that there should not be black soot forming (wood-fired pizza restaurants use black ovens, for example).

I figure the information above may be useful for some folks.  And, of course, I'm in the business of selling such things, so I would not complain if anyone contacted me themselves, or with a referral.  If someone were to purchase a masonry heater and wanted to host a seminar, I would not be opposed to doing some teaching during its construction.
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Russell Kanning

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Re: Masonry heaters
« Reply #1 on: April 14, 2015, 11:08 PM NHFT »

good info
seems to me that most people would start firing up a heater like this as it gets colder, so it will get warmed up before it is suddenly cold :)
it also seems that a clean burn with nice mass is so much better for when it is not that cold outside. With many wood stoves or fireplaces it is hard to get a good fire going .... say when it is cold and wet outside in the fall, but not really cold.
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MaineShark

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Re: Masonry heaters
« Reply #2 on: April 25, 2015, 07:41 AM NHFT »

Yeah, you definitely want to plan on starting it up before it gets seriously cold.  Folks who run them consider it a lifestyle, rather than a "hobby" like occasionally lighting a woodstove because you like the ambiance.  Since many folks here heat primarily with wood (and may be loading their stoves more than twice daily), I don't think it would be a hard thing to handle.

I've also seen that some folks apparently install electric elements in the mass for backup, so they don't have to worry if an emergency comes up and they can't get home for a day or two.  Seems like a smart idea, and they could connect it to a separate meter and take advantage of off-peak electric rates (ie, run the heaters at night, when electricity is cheaper if you have a time-of-use meter, and then let the hot mass radiate all day when electricity is more expensive).
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