Questions about heat exchanging. And a few others.

Hello everyone,

New person here and I have been doing a little internet reading about RMH's and watching a few vids on youtube as well.

I have been thinking of how to get more heat from the barrel that sits over the rocket stove and wondering about why not having the thermal mass go a bit higher around the outer barrel? Possibly store more heat in the thermal mass?

Also why not use some fins on a band around the top 8-10 inches of the barrel to bring more heat from the barrel to the room it is sitting in? (Think about a band around the barrel that is 8 inches wide and fins that are 8 inches high by 5 inches wide.) Or would this rob to much heat when starting and force a longer warm up time?

For hot water could you run a copper spiral on top of the RMH and use thermal siphoning to cycle the hot water without robbing to much heat from the room? (Or use something like many of the Amish houses use to heat the water needs they may have, a coil around the chimney 3-4 ft from the firebox or secondary burn chamber.)

How long are the RMH lasting, are they having to be taken down every 5-10 years or are there any that have been burning for over 20+ years and still going strong?

I am sure these have been discussed but I have been reading threads and I have only managed to get through the first to pages around here and may have missed these points. Thanks for any answers, and this is an exciting time to see what is going on for the rest of the world.

These are all good questions and worthy of evaluating with some engineering experiments. But first let me ask you, why do you think that the RMH is losing too much heat up the stack? Is the exhaust gas temperature too high? The hotter the gas being evacuated to the outside, the more heat you could be trying to capture with some sort of fin or tube arrangement inside.

Then again, as the temperature of the exhaust gases goes down, you reach a point of diminishing returns in trying to capture those BTUs. The whole thinking behind hypocausts and long runs of pipe after the burn chamber is to allow plenty of time for the gases passing through to lose their heat to cooler surfaces. There are too many engineering factors to take into consideration beforehand in the design, so it's nice to have an after-build modification if you find that there is more heat to be extracted. At which point it's worth trying the things that you suggest.

John Elliott wrote:These are all good questions and worthy of evaluating with some engineering experiments. But first let me ask you, why do you think that the RMH is losing too much heat up the stack? Is the exhaust gas temperature too high? The hotter the gas being evacuated to the outside, the more heat you could be trying to capture with some sort of fin or tube arrangement inside.

Then again, as the temperature of the exhaust gases goes down, you reach a point of diminishing returns in trying to capture those BTUs. The whole thinking behind hypocausts and long runs of pipe after the burn chamber is to allow plenty of time for the gases passing through to lose their heat to cooler surfaces. There are too many engineering factors to take into consideration beforehand in the design, so it's nice to have an after-build modification if you find that there is more heat to be extracted. At which point it's worth trying the things that you suggest.

I am thinking of trying to bring the overall burning temp down to extend the life of the barrel (second gas burner) not sure of the burn temps but Tearing the thing down and rebuilding it every 10-15 years could be a lot of work.

I figure that If I need to build the barrel much heavier instead or a new stainless steel barrel to extend the life of the build that is a good thing, and if bringing the temps down 200-400 degrees and still getting a clean burn with a bunch of scavenged BTU's for heating a bit more of the house, that too would be a good thing.

I understand the diminishing returns but I figure if no one asks we could miss a small jump that could be the stepping stone to something that makes our stoves even a bit more than what they are today. That and I really like the idea of using this to heat the home hot water.

That's the unfortunate thing about metals, they oxidize and it's just a question of time how long they will last when they have flames swirling around inside them. The standard trick is to bond some sort of ceramic to them, but ceramics are poor thermal conductors, and if the coating is thick, well then you lose more heat up the stack.

One solution that you might be interested in is painting some zirconium oxide refractory on the inside of your metal burn chamber. That would really extend the life of your barrel. This was developed courtesy of your tax dollars at Oak Ridge National Laboratory and there is a spin-off business there that now sells this kind of paint. Before you click on the link and look at the price, better sit down.

Strayz Arund : Todays 55 gal drums are made of a heavier gage (thickness ) than the Heat Exchanger on a conventional Fossil Fuel Fired Furnace (Air to Air )
these exchangers are usually rated for 20 years ! Here I should stop and recommend the 55 gallon drum with a top that is completely removable ! It makes
installation easier and though noticeably lighter it is still a thicker gauge than most F. F. F. F. Heat Exchangers !

With ` ~ 100,000 ~ R.M.H.s built there are many now approaching 20 years old w/ zero reported failures, this could be related to seasonal use,older heavier
55 gal drums, or other factors, but where are the reports of failures if they have happened !

Fins would be Great ! Before attempting any system that combines the Rocket stove and hot water heating, I would recommend buying and watching The
'Boom' 'Squish' DVD video offered here at Permies.com Big AL !

allen lumley wrote:Strayz Arund : Todays 55 gal drums are made of a heavier gage (thickness ) than the Heat Exchanger on a conventional Fossil Fuel Fired Furnace (Air to Air )
these exchangers are usually rated for 20 years ! Here I should stop and recommend the 55 gallon drum with a top that is completely removable ! It makes
installation easier and though noticeably lighter it is still a thicker gauge than most F. F. F. F. Heat Exchangers !

With ` ~ 100,000 ~ R.M.H.s built there are many now approaching 20 years old w/ zero reported failures, this could be related to seasonal use,older heavier
55 gal drums, or other factors, but where are the reports of failures if they have happened !

Fins would be Great ! Before attempting any system that combines the Rocket stove and hot water heating, I would recommend buying and watching The
'Boom' 'Squish' DVD video offered here at Permies.com Big AL !

Ok, I was not sure if the 55 gallon drums were or are still up to the task of 20 years of service.

Good to hear that there are only designs that drift form the original design that are having issues. (Saw one on youtube that was using wood pellets and in some sort of Geometric dome that had some issues. He may be a member here and if I am wrong please feel free to correct me.) I am more than likely to try something that has a good track record VS something that requires a lot of maintenance after 10-15 years.

Good to hear you like the fins idea, as a older motor cycle fan every time I hear about heating and running a fire I cant help but think temperature transfer and how to maximize that. I know it is always not wanted but with temps as high as what are being described, I can not help but think get more from the higher temps when they are available and then get everything else as you can(longer chimneys ect) along the way.

I will have to look in to the DVD.

Thanks for the good information Big Al and John.

If you are really worried, you can get an end welded onto a piece of gasline pipe and have as thick of walls as you want--but you will spend a whole lot of money to get something that won't work as well (too slow to heat up) but will last WAY longer than you care about.

the barrel needs to discharge heat rapidly to create the differential needed to drive the exhaust draft. if the temp inside the barrel/bell is close to the temp coming up the stack, the heater will stall.