Just curious. I suppose it depends on how cold the outside air is.
If I could put on infra-red sensing glasses and look at a recently stopped RMH chimney exit, would I not see a plume of heat dribbling out, sort of like gun smoke out of a revolver barrel in an old western movie?
It seems to me when the stove goes out in the evening and airflow stops in the piping in the mass it would be good to close a damper between the end of the mass and the chimney outlet. Even just a thin galvanized cap or plug on the end of the chimney pipe.
With no cap, or a wide open damper, should not the warm air inside the pipe keep rising until it exits the chimney? And would not that warm rising air be replaced by cold outside air flowing into the piping in the mass? And then the heat in the mass is warming both me on my bed - and the cold air in the pipe? And once the mass has warmed the air in the pipe, will not that air rise out of the chimney to be replaced by more cold air from outdoors?
I suppose how noticeable this is depends on how cold the outside air is. Also, probably a good idea to cap the air intake and then wait, what maybe thirty minutes or so before closing the damper?
I saw three ways to do this when I was looking at Russian stoves online. The convoluted Rube Goldberg way involved running a steel cable through some bent metal tubing encased in the masonry. The two methods that appear simple and dependable are in this pdf on pages six and seven.... http://www.neo.ne.gov/reports/russian_fireplace.pdf
I guess the gas flow through the flues will stop once the temp inside drops below what's needed to keep things moving and I suspect (though don't know) that a drop below this critical temp will tend to lessen or mitigate an on-going all-out leakage of heat from the stack without the need for a damper. However, I have seen some builds on Youtube where folks do close off the entry to the feed-tube when not in use. I'd imagine this doesn't need to be a perfect seal and would stop the leakage of a lot of heat. Probably a whole lot cheaper than adding a damper too.
Edited to add: Just saw your location. Ok, with you being in Alaska perhaps the external temp. is so low that you'll never hit the point where the stack stops pulling till the heat has been robbed from the bench. In which case I'd think a well sealed cover over the feed tube would probably be enough.
Michael Scott : First, many of these new fangled smart phones have cameras that 'see' in infrared ! Many people take static pictures of their Rocket Mass Heaters RMHs, and
find that they have proof of High Temperature Combustion !
The J bend in your Rocket, is supposed to work like the P trap under your sink, and prevent thermal syphoning, If you are worried about this just stop feeding your fire before
you go to bed and when you are down to nothing but coals, Cap off the Top of your Feed Tube. Without a door to seal off your combustion chamber, a damper is an attractive
nuisance that will get closed at the wrong time and fill your house with smoke ! For the Good of the Craft! Big AL
Success has a Thousand Fathers , Failure is an Orphan
I dunno fellas. Maybe its just because my outdoor ambients are so low.
When I open the door to the clothes dryer if it has been off a few hours - horizontal flue- the temperature inside the dryer drum can be quite low. This is especially noticeable when it is colder than -20dF outdoors, but now that I notice it I can feel it at +15dF.
Similar with my box style wood stove. Mine is a EPA cert non cat - vertical stack-, all the air control is between the intake and the box. It's wide open from the firebox to the chimney cap above my roof. Possibly analagous to a mass heater the firebrick in my stove holds heat after the fire goes out, but if I wait long enough in winter time it gets pretty cold in the fire box too, colder than inside room temps.
Allen, your point about the damper getting closed at the wrong time is well taken.
I linked you straight to a picture of the masonry heater they have in their lobby.
It has two dampers on it. The one below and to the right of the firebox has a black handle for the intake air, a tube under the floor leads to outside air.
The other damper handle is brass, up above and still to the right of the firebox, very near where the vertical chimney leaves the masonry mass.
I was talking to one of the engineers, and I asked him, more or less, "So the damper at the top prevents a two way convection current from setting up in the chimney, right? The mass would cool off more quickly if you left the chimney open, right?" And he said, "Exactly."
I am not suggesting all y'all in milder climates need this, let me throw you three things about my local winter.
1. I know the point where the Farenheit temperature and the Celcius temperature is the same number is at -42. I see it on the bank thermometers at stoplights every year, over and over.
2. In January (and December and Febuary and sometimes March) we stop saying "negative" when talking about the weather. When it warms up to "20" we mean the temperature has finally climbed up to -20dF for the first time in a while.
3. My limit to plan to do something outdoors is -30dF. Below that, I basically commute between home, work and the grocery store only. I took the first week of January off in 2013 for an outdoor project, and I took the first week of January off in 2012 for an outdoor project. On the Thursday in 2012 the forecast high was -28dF, but we never saw it; both of those outdoor wintertime projects remain unfinished, because the daytime high temp never exceeded -30dF either year.
Do you need a damper between your thermal mass and the cold outside air? Maybe not. However, if you have a cap of some kind in your woodshed that you can use to cap your chimney on those three or four really cold nights you experience in your local climate (-60dF anyone?), you might notice the difference in how warm your mass is the next morning. -- But you'll have to take your cap off the next morning before you fire your stove.