Kim Woodson wrote:I have a 12 inch pipe thro my ceiling. I plan on building a 6" MRH. Has anyone ever tried piping in make up air thro the annular space between between the 12 inch hole and the 6 inch pipe?
Second: the average 6 inch Batch MRH would equal to how many BTU of heat?
I'd like to see an answer to the idea of using outside air for combustion. My biggest concern about a RMH is using room air for combustion, and creating a draft of cold air through the walls, around windows/doors, etc...
My first wife's parents had a masonry heater which ducted in outside air for the combustion. It worked like a dream.
For what it's worth, the Wood Heat Organization, Inc, says outside combustion air is unnecessary.
I tend to believe them, because their only bias is toward people using wood heat.
The average air consumption of a modern wood heater is in the range of 10 - 25 cfm, which is very small compared to the natural leakage rate of houses. Building scientists say that the air in a house must be exchanged at least every three hours, or one-third of an airchange per hour, to control moisture from cooking and washing and to manage odors. One third of an air change in a 1500 square foot house is 4000 cubic feet, or 66 cfm. Note that this is the absolute minimum air change for healthy living and that most houses older than 20 years have natural leakage rates far higher than this in winter. So the air consumption of a wood stove is a tiny part of a much larger exchange of air between the house and outdoors.
The main disadvantage of taking air from inside the house is that the pressure environment can be adversely affected by powered exhausts. However, depressurization caused by powered exhaust flows is predictable and manageable, unlike the more random and unpredictable effects of wind on outdoor air supplies. The worst-case indoor air pressure environment can be measured using the house pressure test procedure, and can be controlled either by limiting exhaust flows or by installing a powered make-up air system.
In general, therefore, wood stoves and fireplaces that are vented by natural chimney draft should draw the air for combustion from the room in which they are located. Where necessary the indoor air pressure should be controlled to minimize depressurization.
My in-laws had a powered air-exchange system with a heat-exchanger. They were in central Wisconsin, and it was a passive-solar designed house. I think they had a pretty high-tech solution to low-energy living. The masonry heater did a good job of keeping the house warm, even in the depths of winter. 2 burns in the evening would keep the house warm all night. Burns were a paper grocery bag full of kindling.
Air exchanges are important, as you say, but can create a problem of uneven heating in the house. If the RMH is drawing room air for combustion, and warming the bodies in an inverse cube law around it (radiant heating), yet the cold-air drafts are at the periphery of the house, we create a nice warm-zone around the RMH, and cold zones at the periphery of the house. If you have bedrooms off the main chamber where the RMH is likely located, those bedrooms are likely to suffer the effects of the drafts.
What if an air intake pipe were to run through the mass of the RMH from the outside, pre-warming the outside air?
My room is a undergroundConcrete garage that is very tight. I do have some leakage around the doors i'm sure but was worried also about pulling todays air of 25 degrees in and defeating the efficiency of the RMH.
Hmm... If your space is that tight, then depending on occupant and usage loads you may be getting barely enough air exchange for a healthy environment. In that case, do you want the fresh air to all go to the fire while the room air stays around and gets staler? I appreciate the desire to minimize cold air leaks; you might want to somehow draw in outside air and have it ducted past the fire and released a ways away so old room air is what goes to the firebox.
The room is not so tight that air does not get changed out. I have some leaks at the sides of the overhead door. This past week I had a old wood 80K BTU stove running for about 10 hrs with a 18 inch fan blowing on the 6 inch flu and stove to get all the warm air I could in the room. The walls are 8 inch solid concrete along with the concrete ceiling. 8 inches of styrofoam insulation on outside top, then a sheet of 30 mil visqueen and 2 foot of clay on top of that. out side air was 25 deg F and the inside of the garage pretty well maintains between 52 - 55 deg F with no heat. After 10 hr of hot burn it raised the temp inside to 61 deg. It takes a while to heat up the concrete walls which have earth back filled agaiist them. I really don't need the drafty air being pulled through the room. My question is can I pull air down from the annular space between the 12 inch hole in ceiling and the 6 inch exhaust pipe ?
I would say if you can, you can
What is the upper/outer opening like? How is the flue sealed against water coming in around it? Is there space for an air intake above the roof that will not leak water?
If you can work the logistics of air intake, the cold air coming in around the flue will tend to preheat it and moderate its temperature while lowering the flue exit temperature. As it warms, and whenever there is no fire, the room air is going to want to exit that opening rather than coming in. You will increase the negative pressure at the lowest points and be sucking harder on the garage door edges.
Kim Woodson wrote: It takes a while to heat up the concrete walls which have earth back filled agaiist them. I really don't need the drafty air being pulled through the room. My question is can I pull air down from the annular space between the 12 inch hole in ceiling and the 6 inch exhaust pipe ?
I am in the process of building a hobbit home, which is basicly a buried basement covered with a foot or two of dirt. I am being told I have too have a heat recovery ventilator.
One of the main, and most important things in building one of these buildings is too insulate between the concrete and the dirt outside. The back fill acts as a heat sink, constantly and forever pullling the heat out of the concrete. So you insulate to hold the heat in the concrete mass.
In your situation, I would put in a rocket stove with an air intake to the outside - for one, everytime I opened a door in my workshop, my rocket stove would backdraft - https://permies.com/t/40107/hot-barrel - you would not have this problem, or it would be reduced.
You do not want too cool the exhaust when it goes thru the roof, since with my rocket stove, the exhaust gas starts out at the top of the barrel at roughly 1,000 degrees. By the time it reaches the bottom of the barrel, it is roughly 400 to 500 F. Running thru a 10 foot 3 x 3 foot mass - 2 directions - by the time it reaches the hole in the wall, my exhaust gas was barely 180 F. Some problems with steam condensing in the pipes, as well as loss of stove draw (backdrafting - smoke in the house - not nice ). In a properly designed rocket mass heater, there is no more heat to extract from the gasses going outside, as you have heated up the mass with it.
My vote - seperate air intake vent from outside directly into top of J tube.
Daves Hobbit Home Build progress
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