Rocket mass heater output feeding into a chimney that is also used with a fireplace?

I have a traditional brick fireplace, and in the room behind that fireplace there is a stove pipe opening where they had a coal stove hooked up a long time ago. I am attaching an  diagram of the situation, the fireplace on the left, the rocket mass heater on the right. My question is simple, but I have had a hard time finding a similar situation. If I put a rocket mass heater on the back side of the chimney (where the old coal stove used to be) would I be able to use the fire place at the same time? Or would I only be able to use one at a time? I know certainly the rocket mass heater should be more efficient, but I would like to have both running for nights where its really cold. My concern is that the massive heat of the smoke going up through the chimney would mess up the convection of the rocketmass heater output and perhaps it would not flow as well...After thinking about it longer I have no idea if the convection would just take all the heat up or not. Any thoughts or experience would be appreciated.

Hi Joshua;
To start, fireplaces are horribly inefficient and really only good for romantic evenings not staying warm.
My answer is that you can not run both stoves simultaneously.
Does your fireplace have a working flue damper?
You would need the fireplace closed off before attempting to run the RMH.
Were you thinking of building a batch box style RMH?
Is It possible to build your RMH  in the room with the fireplace?
You can utilize that brick mass as a bell.

Hi Joshua,

No self-experience yet, but I am currently doing my own preparations for adding an RMH or masonry heater.

1) Do a web search on "shared chimney".  You will see many links.

A quick summary:
-possible flow of exhaust gases into other parts of your house
-some regions have bylaws against shared chimney/flues

2) RMH (or masonry heaters) predominantly rely on line-of-sight radiative distribution of heat, so it is common to contemplate more central location of the heater vs against the periphery.  Depending on the era or house designer your current chimney maybe along house edge.

3) It might be possible to put separate flues inside the current chimney to keep the gases separate and not need to create a new perforation in your roof.  It will depend on if there is enough capacity in the existing chimney for both, does it still allow chimney sweeping both flues or if even allowed.  Check with your local experts and bylaws.

4) You might be able to route the new chimney flue horizontally out a side wall then up the outside of your house.  I think there have been some discussions at permies.com around this topic in relation to this because exhaust gases are much cooler than typical chimney.  Again check with your local experts and more ideally find some local RMH users to visit.

Have fun with the new project.

Cheers.

I have the exact same set-up as you picture. I've been using it for 35 years. It's doable. It doesn't burn down the house. But. Fireplaces are lousy for heating a house, unless you keep a really big fire going all night. If you allow the fire to burn down at all, the fireplace no longer really heats, it just sucks the warm air in your house, up the chimney. That causes two problems. Your house gets colder, and the now colder air going up the fireplace side cools off the smoke going up the wood stove side, and you end up with increased creosote buildup in the chimney. Then you have to clean the chimney much more often, or eventually you have a fire. So the answer is, you can do it, it works. But it doesn't work well, and you waste wood and lose heat. Ending up in deficit, not gain. My solution was to install an airtight stove into the fireplace side, stick the stove chimney pipe up past the flue, and seal off the rest of the flue opening so it doesn't/can't draw air (I used pink insulation). Every Spring I would haul the 500 lb. air-tight out to the barn so the fireplace looked better in the Summer. That was fine when I was young and stronger. Now, not so much. A better solution, at least for me, was to simply build another chimney on the other side of the house, and run a separate, as needed on colder days, airtight. It's safer. It uses less wood. I can better control the house tempt, according to what the weather is doing. ~~P.S. It doesn't actually have to cost much. Big box lumber stores often sell broken bags of concrete and mortar mix cheaper. Just go consistently to accumulate enough to do your job. And there are often people on craigslist giving away, or selling cheaply, bricks and cement blocks. You may get pick and match, but so what. Be creative in laying them up. Brick yards and contractors also often have piles of extra bricks left over from jobs. They sometimes get rid of them cheaper because they don't have enough of any one style or color, to do the next job. --Good luck.

Jim Fry wrote:I have the exact same set-up as you picture. I've been using it for 35 years. It's doable. It doesn't burn down the house. But. Fireplaces are lousy for heating a house, unless you keep a really big fire going all night. If you allow the fire to burn down at all, the fireplace no longer really heats, it just sucks the warm air in your house, up the chimney. That causes two problems. Your house gets colder, and the now colder air going up the fireplace side cools off the smoke going up the wood stove side, and you end up with increased creosote buildup in the chimney. Then you have to clean the chimney much more often, or eventually you have a fire. So the answer is, you can do it, it works. But it doesn't work well, and you waste wood and lose heat. Ending up in deficit, not gain. My solution was to install an airtight stove into the fireplace side, stick the stove chimney pipe up past the flue, and seal off the rest of the flue opening so it doesn't/can't draw air (I used pink insulation). Every Spring I would haul the 500 lb. air-tight out to the barn so the fireplace looked better in the Summer. That was fine when I was young and stronger. Now, not so much. A better solution, at least for me, was to simply build another chimney on the other side of the house, and run a separate, as needed on colder days, airtight. It's safer. It uses less wood. I can better control the house tempt, according to what the weather is doing. ~~P.S. It doesn't actually have to cost much. Big box lumber stores often sell broken bags of concrete and mortar mix cheaper. Just go consistently to accumulate enough to do your job. And there are often people on craigslist giving away, or selling cheaply, bricks and cement blocks. You may get pick and match, but so what. Be creative in laying them up. Brick yards and contractors also often have piles of extra bricks left over from jobs. They sometimes get rid of them cheaper because they don't have enough of any one style or color, to do the next job. --Good luck.

Thanks Jim. I'm glad you said something. All the warning and regulations, make it seem like anything will end up burning down everything. I certainly understand how you feel, In my young energy I have moved several woodstoves, even a 500lb wood cookstove by myself. But I am certain in the future that will not be possible. The fact is, its getting pretty cold out here in Ohio. I can use the fireplace now, though it may be inefficient, it will work to keep me warm. If I get the time throughout January, I will be trying to build a rocket mass heater on the back side. Depending on how that works, I might just retire the fireplace on most days and close off the airflow and use the RMH primarily, but perhaps I will use both if its a cold and romantic evening. A fireplace has some good uses, an open fire feels different. I am planning on building a basic rocket stove out of cob and bricks rejected by a large building company near me, I have used their broken and rejected blocks for many things now and they work just fine.

thomas rubino wrote:Hi Joshua;
To start, fireplaces are horribly inefficient and really only good for romantic evenings not staying warm.
My answer is that you can not run both stoves simultaneously.
Does your fireplace have a working flue damper?
You would need the fireplace closed off before attempting to run the RMH.
Were you thinking of building a batch box style RMH?
Is It possible to build your RMH  in the room with the fireplace?
You can utilize that brick mass as a bell.

Thanks Thomas. You are correct, the fireplace is inefficient, but compared to nothing, they are amazingly warm. The fireplace does not have a damper, but I could install one. I was not thinking of building a batchbox RMH, I was thinking of building one out of cob and stovepipe, the common J pattern. It might be possible to build the RMH in the living room and take it up through the fireplace. However it seems more logical to me to leave both options open. Why do you say that burning them both simultaneously would not work? My thought is that if I get the RMH running first and spitting out exhaust, then starting the regular fireplace would be possible, and the fireplace smoke (creosote buildup) would not be able to get into the RMH exhaust.

Gordon Chinnick wrote:Hi Joshua,

No self-experience yet, but I am currently doing my own preparations for adding an RMH or masonry heater.

1) Do a web search on "shared chimney".  You will see many links.

A quick summary:
-possible flow of exhaust gases into other parts of your house
-some regions have bylaws against shared chimney/flues

2) RMH (or masonry heaters) predominantly rely on line-of-sight radiative distribution of heat, so it is common to contemplate more central location of the heater vs against the periphery.  Depending on the era or house designer your current chimney maybe along house edge.

3) It might be possible to put separate flues inside the current chimney to keep the gases separate and not need to create a new perforation in your roof.  It will depend on if there is enough capacity in the existing chimney for both, does it still allow chimney sweeping both flues or if even allowed.  Check with your local experts and bylaws.

4) You might be able to route the new chimney flue horizontally out a side wall then up the outside of your house.  I think there have been some discussions at permies.com around this topic in relation to this because exhaust gases are much cooler than typical chimney.  Again check with your local experts and more ideally find some local RMH users to visit.

Have fun with the new project.

Cheers.

Thanks Gordon. I did some searches on shared chimneys.
https://inspectapedia.com/chimneys/Shared_Chimney_Flues.php#:~:text=A%20%22shared%20chimney%20flue%22%20means,fireplace%20and%20a%20heating%20boiler.
It seems like the main concern is exhaust gasses getting back into the flue that is higher up. In this case the RMH exhaust. The simple solution I can think of to solve this would be to start the RMH first, because the smoke would not want to flow into a hotter pipe when it can just go up the chimney. Perhaps since the fireplace smoke is so much hotter it would heat up the higher parts of the chimney, and then want to flow into the RMH exhaust.

It seems like the only problem would be needing to scrub the chimney more often depending on how often I use the fireplace, and perhaps check the RMH exhaust and scrub it out if need be. However it does not sound like anything would get choked up and be unable to burn.

Joshua, what size is the existing chimney?

One more thing, ... you may be getting more complicated than you need to. My set up is just like you have pictured, except I used an airtight stove instead of a rocket. I just cut a hole in the back side of the chimney and inserted a stove pipe. Then I just hooked up the stove. Didn't hardly take an hour to do, and it's been working great for 30 years now. You will get A LOT more heat out of the stove than the fireplace. And it's really simple. -- The stove I got on craigslist for hauling away.

Joshua - If you were starting out, then the RMH would be a no-brainer.  The issue seems to be that you have a fireplace.  Now perhaps it would be better to tame that beast a bit.  Place sealable doors over the entrance of the fireplace and run a separate intake line from outside.  A damper would be highly useful, but unlikely to work well in most simple fireplaces.  I had a fireplace in Denver that I modified a bit and it did not creosote up much.  I ran a stovepipe up the flue from the fireplace and then installed a small cast iron stove on a metal pan in front of the old fireplace.  I cut off all outer flow and used a double damper on the horizontal pipe from the chimney.  Worked well.  No reason I can think of why that mod might not work with a RMH set up in front of the fireplace.  Using separate stovepipe inside the chimney provides some degree of insulation that lowers production of creosote and also lowers potential for damaged liners allowing heat into combustible chimney surrounding materials.  I would like to hear if others more experienced in RMHs think this is do-able.  It worked for me with a far less efficient wood stove.

Hi everybody
i hope it is ok to ask related question to this topic. I am also building a RMH and want to link the exhaust to an existing chminey.

My question:
1) because of the smaller diameter of the chimney pipe of 120mm do I need to adapt the area of the burn tunnel to this? Or should I calculate  for 150mm diameter of the heat exchange pipes? I am aware that in Evan excellent book he writes that the burning tunnel needs to be the tightest part of the RMH. But in this case i have a quite high chimney of almost 10m. So that is why I an wondering if it might be different to Evans recommendation.

2) does it make any sense to have 1m vertical heat exchange pipe?

3) does it improve the burning if I try to round the corners by the edges from burn tunnel to the heat riser?

4) will it work if i fill the bench with sand (850kg) and topping it with a concrete floor layer and ceramic plates as finish?

5) do i need to seal this kind of pipes to abvoid any co/co2 issues? Or will it be enoug witht the sand around?

Thanks a lot in advance for any comments on this

Tobias - Quick question.  Is the wall concrete?  Even so, with the heat riser and stove pipe against it, I would expect some discoloration.  Most code requires a minimum separation between heat sources and building materials.  It's a safety concern.

Richard Henry wrote:Tobias - Quick question.  Is the wall concrete?  Even so, with the heat riser and stove pipe against it, I would expect some discoloration.  Most code requires a minimum separation between heat sources and building materials.  It's a safety concern.

Hi Richard
Thanks for you reply and question.
The wall is a 56cm deep stone wall (natural & mortar) and the ground is concrete.
So I guess it is quite save and I intend to use this mass for heating storage too.
Any thoughts on my guessings are welcome!

So, since most stone walls are exterior, is this on the outside of the home?  If so, is there anything to insulate the exterior to maximize the heat being provided into the room?  If not, this may turn into a great exterior radiator.  As you are probably aware, water is head and shoulders over stone for heat storage.  Placing a reservoir over the pipe would allow significant heat storage, and during the summer, it could be used to cool the room if needed. A cob or concrete thick skin at least a couple of inches (5-6 better) would provide moderation of the temperatures.

The holes in the brick might allow circulation if fully aligned and with intake at the floor and exhaust near the upper level of whatever you build on them.  Running the brick heater air through an accessible exhaust would also allow placing herbs to brighten the air in the room and perhaps reduce incidence of mold/mildew if the correct herbs are used.

thomas rubino wrote:
My answer is that you can not run both stoves simultaneously.
Is It possible to build your RMH  in the room with the fireplace?

Would it be possible to have rocket mass heaters in both rooms? Would the exhaust cause a problem if there was two? The internal brick chimney is 14'' by 25''

Hi Josh;
Well, you have a nice large chimney... Maybe...
It might be possible to use a stove in both rooms but I would be super cautious if you try it.
After burning your 6" extended-length batch box with brick bell that you build, using the original fireplace as part of the bell.
And your chimney is nice and warm you might discover that it does such a good job that you do not need a second stove!    
If you are still not warm enough, you could try lighting a small box stove in the other room.
If that works flawlessly then build a second RMH.

The Rocket Mass Heater Builder's Guide pg 16 wrote:
Some efficiency-minded colleagues glory in seeking out the absolute minimum ex- haust temperature. A few of Evan’s original heaters ran fog-like exhaust well below its dew point at 60 or 70°F (15 to 20°C). This is very exciting for efficiency fans, as almost no heat is being lost outside the building. **Unfortunately, this exhaust emerges as a cool, dense fog that will not rise in a conventional chimney, and it lacks the force to overcome even the slightest wind pressure.** These ultra-low-temperature exhausts must exit nearly horizontal or downward, and are difficult to protect from normal wind and household pressure conditions. Too-low exhaust temperatures create draft problems in almost all conventional buildings, and are especially unreliable in winter storm conditions (just the time when an electrical exhaust fan is also vulnerable to power failure).

Something I had not considered.... My chimney is the length of an old two story house. 28 ft. Do you think I would have problems with the exhaust not even making its way out?

Maybe not the cheapest option but with a chimney of that size, you could instal two separate internal flues.