Newbie questions… large DSR2 with bench

[iau461]

Hello all,

My son and his wife have a large house (approximately 572m^3) in eastern Pennsylvania. There is a steel wood stove in the basement that we would like to replace with a BBR and a large (approx. 4m^3) thermal mass bench. Leaning toward the DSR2, to avoid DSR3 horizontal tube procurement issues. In this picture, the outline of the future bench is marked on the floor with blue tape.

photos.app.goo.gl/bRgccRUfYQ1vBSvm9

Questions:

The core size tables go up to a 10” riser. Is there a reason not to go bigger than that, say around 12?

Does the existing 8” flue pose a constraint on the stove size?

Is there a reason (other than structural soundness of the floor) not to add thermal mass to a bench by filling with packed soil and vent pipes, vs. a hollow bench?

Thanks in advance for any replies.

[fuegos]

Hello and welcome to the forum.Im a novice by the standards of this excellent forum having only built one stove so ill answer within my knowledge.Rocket stoves are scalable the only limit being the flue pipe size.The narrowest part of the system should be the venturi so an 8 inch flue will mean a 8 inch system.Bench or bell(hollow bench) ? good question but im not sure of the answer except to say a bell will start to radiate heat quicker than a solid filled bench.Im not sure what you mean by vent pipes ?To my knowledge a 10 inch system is the biggest anyone has built so far.You have to bear in mind that the difference in cross sectional area (CSA) between a 10 and 12 inch system is substantial and with that the heat output and wood consumption.There is a spreadsheet out ther that can be used to calculate your system size factoring in variables such as the insulation value of the house and the delta T (difference ) between the lowest outside temprature and desired inside temp. Im assuming the proposed stove is to heat the whole house and not just the basement ? This in itself presents a challenge that some builders have solved by building a staircase bell. Hopefull some other members will be along soon to add their 2c worth.

[iau461]

Hello fuegos, and thanks so much for the reply! This board is indeed a great resource, one just has to work to mine all the gold nuggets. :-)

You answered my biggest question, on the flue being the limiting factor. Which I kinda expected. It's disappointing, because this house needs a LOT of heat. It was built around 2000, so not too bad on the insulation, but my son and wife are going to use around 5 cords of wood this winter in the wood stove in the picture, and they only keep the house at 60-66F. They have 16 wooded acres, so the fuel supply is no problem, we just want to reduce the amount of time spent cutting, splitting, moving, and burning it. Still, the spreadsheet says we need about 18kW of heat capacity per day, which is ~8 fires a day in an 8" stove! Of course that is on the coldest days; we don't even burn that much now. But like the spreadsheet says, it is better to oversize and fire it less often. So had been hoping for a 10" or even 12".

The heat is actually pretty even in the house now. It is 2 stories plus the basement. The stove is in the workshop/utility room, which takes up half the basement and gets to typically 95-105F when the stove is running. We keep the doors to the basement stairs open, plus we have also rigged up a 6" fan that pushes warm air through the HVAC system ducts (there is an oil-fired boiler available for backup heat, which they haven't used). You can see the fan (black vertical thing) in this pic, with its controller on the far wall. We have it set to kick on automatically at 80 degrees.

photos.app.goo.gl/KUN9RdmxVDZrYvz76

In 2011 I built a masonry stove from a kit (Temp-Cast). So I have some experience with thermal mass. That one has about 2.5 tons of cob with ducts running through the bench, plus the weight of the core. It has no problems with draft, works beautifully. photos.app.goo.gl/DgsSRM1iuBawWz9m6

We want more mass for my son's house, to keep the temps more stable overnight. Planning a brick bell along with the bench.

I'm also still chewing on the DSR3 afterburner thing. Might post an idea on that subject soon.

Thanks again, and any other comments welcome.

[fuegos]

Wow thats a lot of wood.One of the problems i see, if the basement is subterranean, is that there's already a lot of mass with probably little or no insulation between it and the ground.Heating this space & relying on what rises up is not very efficient, to say the least.66 F is bloody freezing ! This is our bedroom temp at night, daytime the house is between 22 & 24 c.Have you thought about building a rocket powered cooking range in the kitchen ? Even with wooden floors it's possible, a lot easier with a basement,to put in enough steelwork to carry the weight. That way, you're heating less of the house, enjoying the radiant heat, and you can cook with it.Being in Pennsylvania, I'm assuming mostly hardwood, ideal for coppicing. Once it reaches 10 to 15 cm in diameter its perfect for a batch box, just saw to length, no splitting needed.

Your temp cast build looks great btw.

[iau461]

You're probably right about the subterranean space. 3 of the walls in the stove/utility room are exterior/CMUs, uninsulated indoors and almost certainly on the outside too. And those walls are ~75% underground. Was planning to insulate the new bench from the floor and walls with foam board, but we should probably do something with those 3 walls in their entirety.

I think we'll focus on the basement stove first before destroying the kitchen. :-) But yeah, something to consider when it is time for a remodel.

Yep, mostly hardwood in our forest, a few pines. Don't think we want to fell living trees, there are so many standing snags and so much "pre-dried" fallen deadwood that we have our pick.

Here's a wider shot of my Temp-Cast in all its glory. It has a secondary burner on top, but it takes 20-30 minutes to really get up to steam with a top-down burn. During that time, the upper door glass gets sooty and I do have to clean it every 3-4 fires if I want to watch the violence in there. That's my only complaint with it.

photos.app.goo.gl/ekiwDWVZUsDY3EbW9

Thanks again!

[josephcrawley]

If the basement isn't a living space it doesn't make sense to put the stove there. Masonry heaters are meant to be lived with especially considering the slow release of heat. You will always be trying to heat that uninsulated basement first.

In your situation an air jacketed stove that can integrate into an existing HVAC setup would be a much better choice.

If you want the masonry stove in the living space it isn't too much work to lay up a cmu block box and pour a slab on top. It is a two to three day job and won't set you back more than $250 if you do it yourself.

[iau461]

Feb 9, 2023 15:20:43 GMT -8 josephcrawley said:

If the basement isn't a living space it doesn't make sense to put the stove there. Masonry heaters are meant to be lived with especially considering the slow release of heat. You will always be trying to heat that uninsulated basement first.

I would say it is about 75% living space and can easily become more so if we insulate and finish the stove room walls. Half the basement is the (insulated, finished) rec room with our only TV, 4 recliners, weight bench, ping pong table, and sewing table. Even the current stove/utility room, humble as it is, gets a lot of use, mostly in the morning after the stove is first lit and the son/dau-in-law sit on cushions and read with cat nearby. With a heated bench and finished, well-insulated walls (not difficult) it can definitely be a winter destination.

In your situation an air jacketed stove that can integrate into an existing HVAC setup would be a much better choice.

That is worth thinking about. Would "air-jacketed" just be an enclosure over the core and bell, with direct service from HVAC supply and return ducts?

Thank you for all the input, it is good to be challenged and you are both affecting my thinking.

[fuegos]

"Yep, mostly hardwood in our forest, a few pines. Don't think we want to fell living trees, there are so many standing snags and so much "pre-dried" fallen deadwood that we have our pick"

There's nothing wrong with cutting live trees if it's done within some sort of plan with regards to sustainability.Im an arborist, along with other trades & would advise extreme caution with standing dead wood.I think that you're on the right track here, exploring options and aware of the issues, like the importance of insulation.

[iau461]

We've owned the property since Sept 2021. There are relatively few young trees on our property due to deer pressure, and 2-3 acres have been overrun by invasives (privet, honeysuckle, bittersweet, multiflora rose). We want to improve its health, so cutting live young trees is not in our plans. We do inspect our wood for termites before splitting and stacking. Bark beetles are common but we've found very few termites.

[fiedia]

Sorry, I will be deceptive. You will have a very hard time to heat a two stories house with a mass heater in the basement. The principle of mass heater is to radiate. You surely know it since you have already built one (a very nice one). You will not get as much hot air as from your existing stove and the fan will not heat as well the rest of the house.

If you plan just to heat one room in the basement, you do not need  a 12". 6 to 8" should work perfectly. You can use the present flue. But the house temperature will not improve.

If you really want to build a 12", you will have troubles to find insulated pipes for a new flue. Do not forget that you will have to fill up the firebox (probably more than 60 pounds of wood each time). If the box is not properly filled, you will have a hard time to reach the post combustion temperature.

[iau461]

Feb 10, 2023 11:45:05 GMT -8 fiedia said:

You will not get as much hot air as from your existing stove.

Fiedia, thanks, I understand and agree with most of what you said except this one sentence. It seems to me that, all else being equal, for a given amount of wood:

a) a rocket stove of any size will extract more of the heat capacity of the wood than the current stove does, and
b) a big thermal mass (hot bell and warm bench) will reduce the amount of heat that currently just goes up the chimney. 

Therefore, we should be able to heat the house at least as well as we do with the current stove, using less wood. What am I missing?

[josephcrawley]

Feb 10, 2023 14:12:54 GMT -8 iau461 said:

Therefore, we should be able to heat the house at least as well as we do with the current stove, using less wood. What am I missing?

The cooler exterior of the masonry heater will transfer heat to the air at a slower rate than the much hotter metal stove. So moving a large volume of whole house air through this room will be a poor strategy.

If you build an 8 inch it may be practical to put the top half of the stove through the floor and get better direct heat.

[iau461]

Feb 11, 2023 9:08:04 GMT -8 josephcrawley said:

Feb 10, 2023 14:12:54 GMT -8 iau461 said:

Therefore, we should be able to heat the house at least as well as we do with the current stove, using less wood. What am I missing?

The cooler exterior of the masonry heater will transfer heat to the air at a slower rate than the much hotter metal stove.

Compared to the current metal stove, the bell and bench of the masonry heater will have a much larger surface area from which to heat the space, through radiation, conduction and convection.

Since it is collecting the heat more effectively and less is going up the chimney, then if the thermal mass is at too low a temperature to heat the house as much as the current stove, it will simply heat up more, until the rate at which it releases heat reaches equilibrium with the heat being put into it by the core.

It is possible (though I think doubtful) that to reach that point of equilibrium, the bench might have to get so hot as to be impossible to sit on. Perhaps a good strategy would be to start with a metal bell, which will give the same quick response as the current stove. If it turns out that the response is too quick and the bench is not warm enough to satisfy, we can simply insulate the bell until its response slows down and more heat goes to the bench.

[josephcrawley]

Feb 11, 2023 10:23:59 GMT -8 iau461 said:

Compared to the current metal stove, the bell and bench of the masonry heater will have a much larger surface area from which to heat the space, through radiation, conduction and convection.

Since it is collecting the heat more effectively and less is going up the chimney, then if the thermal mass is at too low a temperature to heat the house as much as the current stove, it will simply heat up more, until the rate at which it releases heat reaches equilibrium with the heat being put into it by the core.

It is possible (though I think doubtful) that to reach that point of equilibrium, the bench might have to get so hot as to be impossible to sit on. Perhaps a good strategy would be to start with a metal bell, which will give the same quick response as the current stove. If it turns out that the response is too quick and the bench is not warm enough to satisfy, we can simply insulate the bell until its response slows down and more heat goes to the bench.

I have built quite a few of the stoves and have experience in living with them in three different spaces. Also I have the feedback of my customers as additional data.

Based on your own data of current wood consumption and estimated KWs of output needed the best thing you could do is reduce the heated area to something more manageable. I and I think I can speak for fuegos just want you to have the best possible outcome. After all building a masonry heater is a significant amount of work and resources.

In reference to your first post. The DSR3 "tube" can be constructed from soft firebrick if you settle for an octagon shape and change the volume of the shoebox. So don't let that hold you back.

Don't let the momentum of an idea carry you on a bad direction.

[iau461]

Feb 11, 2023 18:43:38 GMT -8 josephcrawley said:

I have built quite a few of the stoves and have experience in living with them in three different spaces. Also I have the feedback of my customers as additional data.

Based on your own data of current wood consumption and estimated KWs of output needed the best thing you could do is reduce the heated area to something more manageable. I and I think I can speak for fuegos just want you to have the best possible outcome. After all building a masonry heater is a significant amount of work and resources.

In reference to your first post. The DSR3 "tube" can be constructed from soft firebrick if you settle for an octagon shape and change the volume of the shoebox. So don't let that hold you back.

Don't let the momentum of an idea carry you on a bad direction.

Thanks. Interesting idea about a firebrick DSR3 afterburner. Haven't seen it discussed, much less done. Do you have a sketch or drawing?

My son and his wife live in the 1st floor space. When my wife and I are not there, they close the doors and vents in the 3 upstairs bedrooms and bath. Can't really reduce the heated area any more than that without a major tear-down and rebuilding. We bought the place thinking there would be other residents on the property (living in tiny houses) who would also share the main house. For legal/zoning reasons that dream died. We would not buy a house that big today, but it is the one we have and we love both the house and the 17 acres. So we aren't moving either.

I have no basis to argue with your experience, or your customer's experience. I can only guess. I presume they were disappointed in the amount of wood needed. or the inability to heat the house to desired levels compared to the central HVAC system they were using before. All I know is, our current system heats adequately enough for us, it just burns too much wood. And we can improve on that, simply based on the higher efficiency of a rocket stove. Do you disagree? In other words, do you think we'll burn MORE wood, or the house will actually get COLDER, after replacing a dated steel box stove with a rocket stove?