Draft vs heat extraction?

Hey,

I have been watching videos and learning about rocket mass heaters and it seems like there is a conflict between drafting well and extracting heat from the exhaust. Obviously the colder the exhaust is out of the chimney the better in terms of efficiency, but colder flue gasses mean more issues with smoke coming back the other way and potentially with pulling enough air to burn hot.

So what is the best way to do well in both categories? I've heard insulated flue pipe is good, but then your just not getting as much heat into the house. I suppose insulated pipe outside the house is good, though.

Obviously a bypass when starting a fire is good.

How about chimney cap dampers so that the flue starts warmer?

Are there other things to consider in order to get the best of both?

Are there other philosophies on the topic?

Hi Brandon;
It all seems confusing before building your first RMH.
A new build almost always has some issues getting started. A cold, wet mass can literally take weeks to fully dry and start holding heat.
The style of RMH you build affects how long it takes to warm up.
A J-Tube piped through a solid mass will take much longer to dry out and warm up.
A batchbox into a bell will dry much quicker, and warm up faster.

All RMHs should have a good bypass installed.
Exhaust pipes can be single-walled inside, and any exposed outdoor pipe needs to be insulated.
The ideal exhaust temperature is 140°F- 220°F, which is measured from the gas stream inside the pipe.
Digital point-and-shoot temperature guns read pipe surface temperatures, not exhaust gas, and do not give accurate readings.

If all the build specifications are closely followed and a bypass is installed, your new build may have a few startup smokes, but they will quickly go away
If you cut corners, get it close enough, or use inferior products to save time or money, the old saying "you'll get what you pay for" comes true.


The best advice I can give you is that reading alone is not enough.
Go get some bricks and start building a test core in your yard.
Get hands-on and dirty, and before you know it, you will have built your very first RMH

Brandon Hands wrote:Hey,

I have been watching videos and learning about rocket mass heaters and it seems like there is a conflict between drafting well and extracting heat from the exhaust. Obviously the colder the exhaust is out of the chimney the better in terms of efficiency, but colder flue gasses mean more issues with smoke coming back the other way and potentially with pulling enough air to burn hot.

So what is the best way to do well in both categories? I've heard insulated flue pipe is good, but then your just not getting as much heat into the house. I suppose insulated pipe outside the house is good, though.

Obviously a bypass when starting a fire is good.

How about chimney cap dampers so that the flue starts warmer?

Are there other things to consider in order to get the best of both?

Are there other philosophies on the topic?

Brandon, Tom hit a lot of the things on the nail head spot on. but I would like to add a couple of things.

1) I don't think there is a conflict with a "either drafting or extracting heat" as you put it.   Simply, if there is not enough heat going up the flue, things slow  (flue gas) Thus picture in your mind, a plane flying strait up, and simply running out of power (in this case HEAT) to keep going up. The saying "what goes up will come down" is very true in this case.   Flue gas that can no longer go up, will come down, if it is NOT OUT of your chimney, it will tumble back down. Thus as Tom stated, a bypass can be a very little used but very useful tool at times   And very hard to put in later. I use my three times per year.   Once, when I start in the fall,  2nd time, tends to be when we have the first warm up in the spring and I don't fire for a week or two.  3rd time, is when we have a second cold snap in the spring after another time off.  I wouldn't say, it is used for more than 10 minutes each time, but the help is instant.

2) So with the above in mind, Extraction of TOO much heat either from to large of bell, bench,flue tunnel even flue pipe to your chimney can have detrimental effects. but fear not,  Our testing guru, Peter V. has already figured out what works best for sizing, and these figures are out there. if you have not run across, just ask.

3) On your second paragraph, you mentioned using a insulated flue pipe to your chimney may have a lowering effect on efficiency for  your stove. (at least that is how I took it)  This can be simply said, if your connecting flue pipe is hot enough you feel it is warming a room, it is to hot... at the internal flue gas temps of 150-220F-- you will never know it is casting off "additional heat"  The temp is simply to low to be radiant into your room.

4) "chimney cap dampers"?   The stove is 100% planned to burn as wide open as possible,  As stated in #1, if you try to keep something from going up, it will come down.   That is never good.

5) I don't mean to repeat, but all the figures are out there, you will have the best of both, if you build to proven numbers. This would be like trying to build a house by looking only at the outsides of a nice house.   if the insides are not correct, it may look like a house but that is about it.

The chimney cap damper would be to completely close the top of the chimney to keep it from getting very cold when not running.

My father and I did that when I was a teenager to close off a fireplace. We made a hinged metal flap on top of the chimney flue, with a piece of iron pipe running down to the fireplace throat. We would reach up inside the fireplace with a poker and raise the pipe end and set it on the throat shelf before starting a fire. With a ranch style house, it was easy.

thomas rubino wrote:Hi Brandon;
It all seems confusing before building your first RMH.
A new build almost always has some issues getting started. A cold, wet mass can literally take weeks to fully dry and start holding heat.
The style of RMH you build affects how long it takes to warm up.
A J-Tube piped through a solid mass will take much longer to dry out and warm up.
A batchbox into a bell will dry much quicker, and warm up faster.

All RMHs should have a good bypass installed.
Exhaust pipes can be single-walled inside, and any exposed outdoor pipe needs to be insulated.
The ideal exhaust temperature is 140°F- 220°F, which is measured from the gas stream inside the pipe.
Digital point-and-shoot temperature guns read pipe surface temperatures, not exhaust gas, and do not give accurate readings.

If all the build specifications are closely followed and a bypass is installed, your new build may have a few startup smokes, but they will quickly go away
If you cut corners, get it close enough, or use inferior products to save time or money, the old saying "you'll get what you pay for" comes true.


The best advice I can give you is that reading alone is not enough.
Go get some bricks and start building a test core in your yard.
Get hands-on and dirty, and before you know it, you will have built your very first RMH

thanks for the advice!

Scott Weinberg wrote:

1) I don't think there is a conflict with a "either drafting or extracting heat" as you put it.   Simply, if there is not enough heat going up the flue, things slow  (flue gas) Thus picture in your mind, a plane flying strait up, and simply running out of power (in this case HEAT) to keep going up. The saying "what goes up will come down" is very true in this case.   Flue gas that can no longer go up, will come down, if it is NOT OUT of your chimney, it will tumble back down. Thus as Tom stated, a bypass can be a very little used but very useful tool at times   And very hard to put in later. I use my three times per year.   Once, when I start in the fall,  2nd time, tends to be when we have the first warm up in the spring and I don't fire for a week or two.  3rd time, is when we have a second cold snap in the spring after another time off.  I wouldn't say, it is used for more than 10 minutes each time, but the help is instant.

2) So with the above in mind, Extraction of TOO much heat either from to large of bell, bench,flue tunnel even flue pipe to your chimney can have detrimental effects. but fear not,  Our testing guru, Peter V. has already figured out what works best for sizing, and these figures are out there. if you have not run across, just ask.

3) On your second paragraph, you mentioned using a insulated flue pipe to your chimney may have a lowering effect on efficiency for  your stove. (at least that is how I took it)  This can be simply said, if your connecting flue pipe is hot enough you feel it is warming a room, it is to hot... at the internal flue gas temps of 150-220F-- you will never know it is casting off "additional heat"  The temp is simply to low to be radiant into your room.

4) "chimney cap dampers"?   The stove is 100% planned to burn as wide open as possible,  As stated in #1, if you try to keep something from going up, it will come down.   That is never good.

5) I don't mean to repeat, but all the figures are out there, you will have the best of both, if you build to proven numbers. This would be like trying to build a house by looking only at the outsides of a nice house.   if the insides are not correct, it may look like a house but that is about it.

thanks for the advice.

I'm kicking around a couple of ideas. one being to start with a Liberator Rocket Stove and add some material around the bell/flue in order to extract heat. the Liberator is nice because it's UL listed to help avoid insurance fights. so I've been thinking of ways of pulling heat off of it, but I wonder if I would be overkilling it.

the other thing I'm thinking about is a more tall masonry heater type of design instead of the long bench, so that makes some of the numbers from Peter less useful. if I were to go with this kind of design, I would also want more of a batch box, which adds another element that makes the standard ratios hard to apply

does Peter have masonry heater designs?

also, for #4 (chimney cap dampers), I meant for the purpose of closing off the chimney between burns so that the stack stays warm instead of having the cold air pour back down.

Brandon Hands wrote:


thanks for the advice.

I'm kicking around a couple of ideas. one being to start with a Liberator Rocket Stove and add some material around the bell/flue in order to extract heat. the Liberator is nice because it's UL listed to help avoid insurance fights. so I've been thinking of ways of pulling heat off of it, but I wonder if I would be overkilling it.

Depending on your abilities,   BUYING a a Liberator Rocket Stove and BUYING pellets for the rest of it's heating days? This defies much of what heating with  a Rocket mass heater and split wood is all about.   And adding the words- "add some material around the bell/flue to extract more heat from the flue" Goes against the advice you were given in the past few replies to you.

  I think most "insurance fights" are self brought on?   Kinda like asking the question and giving a negative answer at the same time, Such as  telling your insurance man, " You probably won't insure me, because you've heard that you won't insure wood stoves?"  If you give most people a easy way out, they will take it, Seems like I often hear a home owner trying to convince the insurer NOT to insure them and often get their wish.  Probably a better way to word that, But it rings true in many cases.

                                     

Brandon Hands wrote:
the other thing I'm thinking about is a more tall masonry heater type of design instead of the long bench, so that makes some of the numbers from Peter less useful. if I were to go with this kind of design, I would also want more of a batch box, which adds another element that makes the standard ratios hard to apply

A "more tall masonry heater type"? your thinking about?  with a a batch box unit,  which essentially is a batch box with a bell ( no bench ) that is super well documented by myself, Glen in CO, Tom in MT and many others And backed up by ALL of Peter V's numbers, also well documented. or are you thinking of something else? Such as a so called Russian Masonry Stove that is much harder and way more expensive to build?

Brandon Hands wrote: does Peter have masonry heater designs?  

See above

Brandon Hands wrote:
also, for #4 (chimney cap dampers), I meant for the purpose of closing off the chimney between burns so that the stack stays warm instead of having the cold air pour back down.

For air to pour back down, would you agree that it has to have a place to go in order for this to happen? if you have your air inlet door to your stove closed as recommended by every rocket design when not burning,  there would be " NO pouring back down" to take place.

I hope this does not come across as trying to rain on your parade, But in all honestly I see few things that should cause concern, every bit of information is there for proven designs.  Best of all, before you build, you will have a plan on paper, with all of the dimensions, brick count, and final look, and you can toss this to the forum. There may be some tweeks suggested, but they will be all helpful.  Once in hand, there should be nothing stopping you. Best of success to you and all.

I had another thought on bypasses: If you have a combined mass-cook stove like Matt Walker's designs and you want to run it year round, it makes a whole lot of sense to eliminate the mass from the burn path. That would avoid trapping heat in the house when you just want to cook your dinner.

I have also read someone say that they did not think a rocket mass heater would suit them because their temperature went up and down a few times in spring and autumn. I think using a bypass appropriately might mitigate in those conditions too.

(disclaimer - still rocket dreaming here)

Scott Weinberg wrote:
Depending on your abilities,   BUYING a a Liberator Rocket Stove and BUYING pellets for the rest of it's heating days? This defies much of what heating with  a Rocket mass heater and split wood is all about.   And adding the words- "add some material around the bell/flue to extract more heat from the flue" Goes against the advice you were given in the past few replies to you.

.

The liberator is just a j-tube, so it can burn sticks like any other j tube. It has an optional pellet hopper.

How does adding mass around the flue or bell go against the above advice? I'm not sure I understand what you're getting at.

Scott Weinberg wrote:

A "more tall masonry heater type"? your thinking about?  with a a batch box unit,  which essentially is a batch box with a bell ( no bench ) that is super well documented by myself, Glen in CO, Tom in MT and many others And backed up by ALL of Peter V's numbers, also well documented. or are you thinking of something else? Such as a so called Russian Masonry Stove that is much harder and way more expensive to build?

.

Well I'm not quite sure the exact type yet. Do you know where to find good examples of the batch box design you're talking about? I can search but if you have the link handy, I would appreciate it.

Scott Weinberg wrote:

I hope this does not come across as trying to rain on your parade, But in all honestly I see few things that should cause concern

.

Well I'm just trying to learn a bit before I start drawing things up or tinkering in the back yard, so I appreciate your help with knowledge and links. Cheers 🍺

Nancy Reading wrote:I had another thought on bypasses: If you have a combined mass-cook stove like Matt Walker's designs and you want to run it year round, it makes a whole lot of sense to eliminate the mass from the burn path. That would avoid trapping heat in the house when you just want to cook your dinner.

I have also read someone say that they did not think a rocket mass heater would suit them because their temperature went up and down a few times in spring and autumn. I think using a bypass appropriately might mitigate in those conditions too.

(disclaimer - still rocket dreaming here)

That's a good idea. You could have the top of the j-tube exhaust across the cooktop and then have the bypass after that so you could choose between sending it through a mass or sending it straight outside.

I think a chimney cap damper would depend on the situation. For a larger flue like 8 x 8 or more that a fireplace would have, it could be very useful even if the bottom was closed off. For a 6" diameter flue, air might not be able to circulate within the height of it so a top damper might not make a difference. An exterior chimney would get cold full height whether or not it had a damper on top.