Holding the heat overnight, the importance of shutting off the air flow.

Hi All;
For years now, I have been running two RMHs in outdoor uninsulated buildings.
I called them extreme burning stoves, as they were lit in the morning and continuously roaring until evening.
As they were freshy stoked when we walked away, the air intakes were always left wide open.

Last year, when building Shorty I installed temperature sending units, in the riser (quickly failed) the top of the arch, and in the exit chimney pipe.
I monitor these to  see how Shorty is performing.  (Shorty performs above and beyond ALL expectations!)

One of the little details of running an RMH was to close off the air intake after the fire coaled out to stop natural convection from sucking your stored heat up and out the chimney.
With a J-Tube, this is easy, two bricks were placed over the feed tube.
My batchbox doors have a closable primary intake and the secondary tube can be plugged with superwool.
Also easy if your fire  is coaled out...
With my extreme burning stoves this was not even attempted, and with no monitor it was a non issue. (If you do not know... it is easy to not be bothered!)

With Shorty's temperature probes  I am able to see the instant reaction of shutting off the air supply.
The exhaust stack temperature will drop by no less than twenty degrees in the ten seconds after shutting the intake and taking two steps to check the monitor.
That is a pretty impressive result!

Yesterday afternoon after Studio Dragon was coaling out, I checked the temperatures inside the chimney pipe.
With some coals still glowing my gas temperature was 211 F , shutting the primary intake instantly dropped that down to 170F, and after stuffing superwool into the secondary it dropped to 150F... all with 2 minutes or so!  Quite impressive!

This morning before opening any intakes the gas temperature inside the exit chimney was 101F.
First I removed the secondary plug, and gas temp went up to 110F, I then opened the primary intake and watched the gas temp rise to 130F...
When I opened the main door to load my wood, the temp went up to 148F.
Had I left my intakes open all night that 130F would have spent 12 hours venting and my studio Dragon would have been much colder this morning when I went to light it.

Upon lighting her off this morning, the chimney temps started climbing quickly.
The moral of the story is...
Heat your mass and then shut off all incoming air, to receive the most benefit to using an RMH!

thomas rubino wrote:

Also easy if your fire  is coaled out...

How "out" is "out"? For us people who would like to upgrade from a metal wood stove to some sort of rockety stove when renos happen, I need to know at what point air can be closed without risking indoor air quality issues.

and:

My batchbox doors have a closable primary intake

I will assume this is as obvious as the typical air control on a quality metal wood stove? So remembering to adjust it will be easy, however...

the secondary tube can be plugged with superwool.

This is less obvious. There's a high possibility that ours will have more than one person operating it, and I wouldn't want to find out the hard way that the secondary tube stayed plugged.
1. Is there an obvious way to know this?
2. How messy is the procedure of taking the wool in and out? (Like should I have a dedicated bucket for it to live in?)

Hi Jay;
Good questions, I'll try to give you clear answers.

A fire is considered "coaling out" when there is no more wood , just glowing coals.
I personally wait until the coal pile is still there but much smaller and lower than initially.
Closing the air intakes with coals inside has no effect on the indoor air quality.
The exit chimney remains completely open, any gasses inside are still drafting up and out, just not rapidly.

Not all RMHs are built the same
On my shop / studio dragons, and also on  Matt Walkers stove designs,  there is an exposed secondary air tube.
On Peters first generation designs, he keeps his secondary tube inside the firebox, so not needed to plug it off.
On Shorty core there is only the one air intake, very easy to close off.

With an exposed secondary tube, anything can be used to close it off.
Tin foil would work, a small piece of ceramic board with a handle, I used superwool as it was handy. There is no mess at all.
A nice painted metal piece, with ceramic board plug bolted to it, as well as a small bead chain to attach it to the bricks or door frame so it leaves no doubt about where it is used, or of it becoming lost.

Do you find charcoal in the firebox the next morning? Or have the coals mostly burned away?

Charcoal if you close the intakes, ash if you leave them open.

I gain a small handful of charcoal for my bio tub each morning, when cleaning out the Liberator stove.  I have not yet built any kind of mass along the sides to store any heat but I do close the two air 4 inch intakes once it reaches the coal stage.

Peace

Here is an example of a "coaling" out fire in Shorty.
Her chimney temp was running 178F, after closing the air intake it quickly dropped to 140F

Note, I choose not to use angled bricks on the floor of my stoves.
Using them would consolidate the coals in the center allowing for a more complete burn.
Leaving them out, give me more room for odd shaped wood.

Am I correct in my understanding that it is much more important to block the flow in a ducted mass than a stratification chamber? Seems the ducts would be much more efficient in drawing heat straight out of the system.

I'm not sure how familiar you would be with this concept Thomas, but I was at Wheaton Labs when Caleb designed a custom plug for the feed of the RMH in the Fisher Price House. I've since been wondering if the difference between something like that and the two firebricks I use would be measurable...?

The tighter the seal the better it will work.
You could fold up some foil to cover the feed and place your firebricks over that.
I suspect both piped mass and a strat chamber are very similar in heat loss, if allowed to draft.

A piped mass might flow better, but by volume, it is tiny,
A strat chamber is huge in comparison.
Ultimately, both rise through a vertical chimney pipe.

thomas rubino wrote:The tighter the seal the better it will work.
You could fold up some foil to cover the feed and place your firebricks over that.
I suspect both piped mass and a strat chamber are very similar in heat loss, if allowed to draft.

A piped mass might flow better, but by volume, it is tiny,
A strat chamber is huge in comparison.
Ultimately, both rise through a vertical chimney pipe.

I kinda suspect the foil would be too much fuss on a daily basis for most users. The plug for the FPH is a well crafted piece. I'm surprised Paul hasn't done more bragging about it, maybe he's just too shy and timid to say anything. But seriously, you should check it out, this might be another design that could be offered by Dragon Tech, if there is enough of a J-tube market out there any more. I'm just curious how much draft actually will still get around two firebricks.

For piped vs. stratification, both systems have say an 8" exhaust going up. The ducted system is designed as one big chute, usually ascending through the entire mass (particularly if following the Wisners' advice from the Builders Guide). I think the strat chamber would function as a bit of a plug compared to the inverted sink of heat that would flow through the ducts. The bottom of the strat exhaust is pulling the coolest air, the heat of the mass will work with the warmer air to stay above that level and resist flow out the exit...

I agree that a ducted mass would continuously lose heat with any amount of residual draft. I think a stratification chamber would be more dependent on details of the configuration. If the combustion core exit (riser top or secondary chamber exit port) is near the top of the stratification chamber, then it will be introducing cooler air and diluting all the stratified hot air remaining. If the core exit is low in the stratification chamber, it can move more directly to the chimney exit and leave the hot upper part of the chamber undisturbed. This is assuming that draft is restricted from full burn-time flow and does not "jet" at all.

Ever since I started playing with thermal mass with my sand batteries, I became much more aware  of how small changes = big results over time.    

Being able to log the outside temp and plot that on a graph and being able to overlay the outside temp to that graph was most valuable to get the big picture.

It seems to me that the standard damper could be designed better to stop airflow...   but on the other hand don't want CO2   build up.     I guess your move to stopping the air entering the rocket probably best option.

Today, I lit the stove in the morning, but it wasn't coaled out yet when we left for a doctor visit.
When we returned home a few hrs later, I closed the primary intake but left out the secondary plug.
I went into the shop and used some scrap copper pieces and a piece of ceramic board.
I now have an easily installed and removed plug for the secondary air tube.
I have decided to wait until next summer before creating a new, larger backing plate for the front.
I will attach a bead chain at that time, so I can never lose it.

With just the secondary tube open, the chimney gasses were exiting the bell at 163F, thirty seconds or so after inserting the plug they dropped to 147F.

Glenn Herbert wrote:I agree that a ducted mass would continuously lose heat with any amount of residual draft. I think a stratification chamber would be more dependent on details of the configuration. If the combustion core exit (riser top or secondary chamber exit port) is near the top of the stratification chamber, then it will be introducing cooler air and diluting all the stratified hot air remaining. If the core exit is low in the stratification chamber, it can move more directly to the chimney exit and leave the hot upper part of the chamber undisturbed. This is assuming that draft is restricted from full burn-time flow and does not "jet" at all.

As many may know, I have tremendous draft, even when the stove is not lit.  and this is nearly all the time.  Yes, even in summer.  Thus it can be concluded, that if something ( air ) goes in it has to come out, and of course if it comes out and makes it through the bell or any other part of the mass, it is just the law of physics, that X degree cooler air, will gain temp if allowed to travel through, X degree of warmer environment.  The speed of such can be expressed as CFM (cubic feet per minute)   Thus the tiniest level of inlet, left unchecked but having a clear shot of air travel, can loose just as much  heat as loosely sealed but large area of air inlet. such as a door.

Perhaps one of the easiest ways of knowing, is simply when you shut down your stove, before the wood is completely coaled out, and it is nothing but ash in the morning.  Something allowed this to burn. i.e  air entering your stove burn chamber.  Basically if you have a great drafting stove, you will also have a stove that wants to just keep the cycle alive, ( heat going up )  And of course, this can be simply tested, if your exit pipe is very warm 3 foot from your stove, it is getting that way from flowing heated air.   I have come to believe that some will simply migrate up the flue, but stiving for the lowest CFM will save a great deal of heat.

When burning let it roar, when not, shoot for the lowest snore.  A true sleeping dragon so to speak.

Best of success.  
Note- if any of the areas that need to be sealed up, never gets over 500 degrees F, some silicon sheets come in handy for air stoppage. And can last for years. Just like the stoves themselves, a little ingenuity can go a long ways.