exhaust pipes on rocket mass stove

We have recently installed a Rocket mass heater and are having a backdraft after it burns for a few minutes that comes back and blows out the fire. My husband has checked everything and all seems to be correct.
He asked someone that works with installation on fireplaces and pellet stoves etc. and they told him that he did not have to have the exhaust go out as the tallest room in the house as long as it was 10 feet away from that point. Also the exhaust has 2 elbows in it making it exit on the second story at a 45 degree angle. Could this be causing the problem? It is a beautiful stove the way he has done it and I do not want to see him tear it out, but he is about to that point. Please help! I love the idea of this stove and would love to see it work. Thank you for any help you can give.

Teresa; We need more info on your setup, dimensions on feed tube, tunnel and riser. Is your core firebrick , or insulated cast? describe your transition area, what is your vertical pipe like ? where does it go out of the house? Most important is ... is your mass dry yet ? is it heated up at the farthest end? Until its dry and warm it will fight you on drafting. Likewise a firebrick core will take longer to come up to true operating temps than an insulated cast core.

ok it is all done in firebrick, and 6" exhaust. It is 6 1/2"x6 1/2"x16" is the feed tube. Tunnel is 6 1/2x4" high and 25" long, riser is 6 1/2x6 1/2x50" high, the barrel has a " hole in the bottom of the drum with 6" exhaust pipe. Used white shat and retaining wall block for the mass. out of the drum goes 7' elbows back goes another 9' goes up 16' 45's for 7' and then 45's out of roof and 3' out of the roof. I hope I did not miss anything:)

as far as the heating up, it never burns long enough to heat up.

OK; It sounds like everything is in the ball park , the burn tunnel at 6.5 wide and only 4" tall seems small, but should work. Have you tried using a small hand held fan over the feed tube when it wants to back up ? You must dry out and heat up your mass before it will start drawing like a dragon. Using a fan is an easy trick to try and get your draw going.

That pretty much all sounds good. One thing, it has been found that if the burn tunnel is not square, it works better to be taller rather than wider. I don't know if that is a problem here. The combustion zone cross section is a bit bigger than the duct, even accounting for square sections being less aerodynamic than the same diameter round section. What kind of insulation do you have around the burn tunnel and heat riser> Do you have a 55 gallon drum for the barrel? Does the duct go straight out the lower side of the barrel, or out of the bottom and turn to horizontal? You missed typing the size of the hole in the barrel.

It sounds like you may be getting a condensation-induced cold plug. How cold is your weather now? How wet is the thermal mass? That will absorb vast quantities of heat until it dries, leaving you with nothing for draft. Does the mass surround the length of 45 degree duct? Can you feel warmth at any metal parts or cleanouts along the route? Can you see water anywhere, especially after the fire backdrafts? How strongly does the fire burn at startup? Is there a cleanout where you could do some preheating? Newspaper burned in the base of the chimney is one common method, or you could put a heat lamp on the duct near the chimney.
The chimney top is 3' above the roofline at the eave? How steep is the roof? Is there anything significantly taller within 10'? Do I understand correctly that all the ducting and chimney is 6" diameter?

I would estimate that you have about 22' of horizontal run, plus two and two halves elbows at 5' equivalent, making around 37' equivalent, which is close to the suggested maximum of 35-40' for a 6" system.

The retaining wall block is presumably dry, but I don't know what "white shat" is...

Teresa Wouk : A little more information from you and some careful reading by us should locate your problem areas ! One very important question that we failed to ask

Is the location of the Rocket Mass Heater RMH inside your home - especially if Your husband has placed it in a basement that is below ground level ,* if so you are definitely

going to have problems ! Often the fix is in locating those upper areas of your home that are not air tight and leak heat out of your Home. Hopefully this short video

from the good people at Cold Climate Housing (These people KNOW Cold ) will help you understand the Twin issues of "Stack Effect' and ''Whole House Stack Effect'', It is

often hard to understand why we go to the ceilings and Electrical and plumbing traces in the upper parts of the house to solve problems of back draft ! Link below:





So potentially any leak in the upper stories of Your home can cause the problems you are experiencing now !

Unfortunately several things we do automatically can help create or worsen your problem ! Any other heating/ household appliance with an exhaust fan running at the same

time can easily defeat the natural /unassisted draft of Your RMH. something as simple as opening an up-stairs window for fresh air can cause your draft problems

We need to eliminate/ shut off All of the potential problem areas and try again ! these include

1) any other Heating unit located inside the house and drawing in outside air to run !

2) Any Clothes Drier vented outside ( consider venting this inside to raise household humidity )

3)The range hood above your kitchen stove

4) even the bathroom exhaust fan can cause your problems

5) any vented (non-electric) water heater

Actually if the house is new and built very air tight any one of these can limit the amount of draft you can create or even reverse it !

So with all these heaters /forced air appliances switched off and the door or window closest to Your RMH open -this would be the time to retest your RMH . *

A crutch often used by New Rocketeers until they learn the care and feeding of their Rocket is to use a Lit Handheld Propane torch to warm up the Burn Tunnel /

Heat Riser to create a good draft the 1st several times they use their Rocket !

I don't feel that I totally understand the nature of the Final vertical chimney pipe as you have it . Most of it runs inside the house, and thats to the good !

A well built Final Vertical Chimney Should be located on the Lee or Downwind side of the House and end at least one length of stovepipe above the peak of your roof,

or any other close object bigger than a TV Antenna !

Remember that Your RMH is supposed to act as a primary heating source, a shorter chimney not located on the downwind side of the house may work perfectly 80%

of the time - for a heating unit that is a Failing Grade !

I hope this is timely, these steps should greatly help with your problem ! For the good of the Craft. Big AL


* If as I fear your RMH is located in below grade basement These tips may not help !

Hi Allen sorry it took so long to get back to you. It has a 6" hole in the bottom of the barrel and the other is White Chat, sorry about that. He has not watched the video yet, just looked at the forum. That may be the problem, the part of the house that we put the heater in is not finished, we use the room all the time but I know there is a hole in the ceiling and part of a wall that is not sealed off. Ok will look at the video and seal that wall off and see what happens. Thank you for all of your help I will let you know the outcome.

Teresa Wouk : Please calm my probably unjustified fears There is a world of difference in what "Chat'' means to a landscaper or to a Mining Engineer !

To a hard rock miner "chat'' means 'Mine waste' or 'Tailings' specifically those from Lead and Zinc mines. Some of the areas where huge mounds of

this 'Spoil' were made -are now listed by the E.P.A. as " Superfund Sites " Chat is usually considered to be fine sand or even Rock flour, but to a Landscaper

It could be course gravel !

The person that sold you the "chat' should have a M.S.D.S. sheet supplied by the Manufacturer, in this case You are interested in the inhalation hazard -

this is where it would say that it contained lead !

This is just an old-soon-to-be-Great-Grandfather, who probably is worrying about nothing !

NOW, Lets get ready to light off your Rocket your need to get

1) fine very dry kindling and tinder, about a 5gal buckets worth nothing bigger than you thumb!

2) A Hair Drier with Multiple Speed settings and hopefully a fan-only-no-heat setting !

3 )We want to find a would object like a 1gal or a 1.5 gal bleach bottle that we can tape to the Hair Driers discharges that we have a funnel the size of the

Feed Tube*

4)A couple of bricks to cap off the top of your Feed Tube

So You can use a incandescent lightbulb in a "Trouble light'' or the hand-held propane torch to warm up the burn tunnel and the base of the Heat Riser,

or the Hair drier on high The lightbulb maybe 20 minutes, 10-15 for the torch, and somewhere close to that for the Hair drier !


Then while everything is Hot- Test your Hair Drier and build a small fire, it may help to encourage the fire with the hair drier from a a couple of feet away

or blow on it !

For This fire you only want to feed it small stuff and fill the feed tube 3/4ish full. Then use the hair drier to get the fire roaring and see how it works for you !

Because we have to be prepared for less than perfect results having a couple of bricks to place on top of the Feed tube to block that hole of is just a good

precaution ! Any time you put a cap on top of the Feed Tube leave it there until the whole stove is dead cold !


Try this trick and then get back to us with your results, If there is a Rocket in your future we will help you find it ! For the Good of the Craft ! Big AL


*As an alternate tool you can place a Angel food bowl/mold turned upside down on top of the FeedTube and blast your Hair driers exhaust through the Center
of the mold

Late note : My wife tells me I should be saying Bundt Pan - a lot of the Angel Food pans are spring form- and probably not work as weLL ! A.L.

It is 1/4" limestone. We have been working on putting windows in so have not gotten back to the stove yet. He has been doing all of that to no avail! We are going to seal off the one wall today and see if that makes a difference. Again thank you for all of your help.

Found a large crack above the stove that lead to outside, plugged it up and the stove works great, Praise YAH! It has been burning for almost an hour now and works great. Thanks for all of your help

Teresa W : Great ! Glad to get your positive report, If you have carefully reviewed the video from the C.C.H.R.C. group at University of Alaska Fairbanks ,

You will note they very specifically mention going on to seal all the Leaks high up in your house as 1st priority !

One of the most interesting things is in a well build RMH to exhaust gases should never be higher than 150ºƒ or 1/2 to 1/4 of the heat a regular wood stove

needs to run at to not produce creosote ! Most of the time your RMH will NOT be running even though it is still warming your house !

This is just one of the ways your Rocket Mass Heater is so much more efficient -

Now is a good time for you and your husband to look at some of the other forums here at Permies and to make some new friends - Didn't I mention before

that we all started where you started ? For the Good of the Crafts! Big AL

Hi Allen, sorry to bug you again, I have another question for you. I am burning the stove today and have been for awhile now to make sure everything is working well and I notice that it is smoking a lot through the chimney outside, my understanding is that it should be putting out steam not smoke. Your thoughts on this please.

Teresa Wouk : You may want to go back though the Threads previously posted, there are still a few unanswered questions, As this is your 1st build

I expect that your husband has a small list of things he would do differently if he had the chance to do it again, #1 would be a Taller Burn Tunnel-


We want to quickly create a super hot fire that pulls in lots of air around the small very dry wood we are using. This keeps the top part of the Feed Tube

from getting too warm (which also could have caused our original smoke back problems) So hot fire, pre warmed air, and cool Feed Tube- You are off

to a great start- Ideally you should have several chunks of small very dry wood in your feed tube all the time at start-up


I often see when looking at other U-Tube Videos a couple of bad habits that are spread by these videos that is cramming in so much wood that the

Burn Tunnel gets filled up with Charcoal this can slow down the intake of air, reduce our Rockets roar, and cause smoke.


The first 15 minutes are critical in getting Your rocket up to temperature -if everything has been close to perfect, at about 15 minutes you should be

able to see (in a darkened room) that the visible part of the Burn Tunnel has reached a red glow, if the fire is close to out due to poor tending it may

be dull red otherwise a bright red ! A piece of wood added now should immediately burst into flame !


These are signs that you can now use Larger pieces and fewer in number ! Some times after the barrel itself is up to temperature and radiating off

gobs of heat we want to cut back on the amount of wood we are feeding the combustion core. If you have wood that is just shorter that the full length

of your Feed Tube you can reduce the size of its top opening with one or two well placed bricks to better channel your combustion air past the reduced

amount of wood you are burning. If we don't do this we are likely to be feeding it to much air and actually cool down the Internal Temperature of Your

Combustion Zone, this will allow smoke to leave your barrel without being burned up.


You can read though this a couple of times - if any part is not clear I will try hard to make it clearer. There Still maybe a gremlin hiding in your RMH

But I am comfortable saying that Following these steps should make a big improvement. For the Good of the Crafts ! Big AL