Air Intake effect on chimney temps

Hi All;
When building my new Batchbox Shorty Core I installed three high-temperature heat sensors, one in the riser, one at the top of the arch, and one in the outgoing chimney just above where it exits the bell.
As of this morning, during full burn I showed a peak riser temp of 1375F
The top of the arch was showing 402F
And the outgoing chimney was showing 168F

Looking inside the core while it was coaling out, my firebricks were perfectly clean, indicating a clean hot fire.
At that point, my riser temps were just over 900F and my chimney temp was 168F and the air intake door was wide open.

When learning about RMH operation we are all told to close the air intake when there is nothing but coals left, to stop our mass heat from being drafted out of the chimney.

This morning I watched how dramatic an effect this little thing has!
After admiring the view of Shorty coaling out.
I snapped a photo of it and a photo of my temperature readings.
Then I closed the air intake and snapped another picture of the core.
I stepped around to snap a shot of the temperature gauge expecting to see it start to drop.
Imagine my surprise to see that in the 20 seconds, while I repositioned myself, the outgoing temp had dropped from 168F down to 132F !!!
That fast it dropped 36 degrees!

Proving to my satisfaction, just how important it can be to close your incoming air off whenever your RMH has nothing but coals left and is being allowed to go out.  

It is now over ten hours since I shut the air intake.
At that time I had a Riser temp of 900F and an exhaust temp of 132F

Currently, ten hrs.' later  the Riser temp is down to 365F but the Exhaust temp has only fallen to 125F
Shorty will not get relit until morning, I'll be curious what the exhaust temps are before lighting.

The moral of this story is, "Children ALWAYS shut the air intake when your burn is going out... Always!"

24 hours later, the riser is still 179F, and the exhaust stack was 93F. Overall, external brick temperatures are 87F!  
That's almost  450 clay bricks at 87F sitting in my Living room.
5 minutes after lighting the riser is at 1186F and rising.

Great information and great advice Tom. I often fire up the batch rocket and let it do it's thing, heating up the mass for long lasting radiation of heat. I then move on to other things and often forget to close off the air intake once it reaches the coaling stage. I guess like so many other things, I need to set a timer reminder to check the heater and shut the air intake.

You are absolutely correct though, shutting off the air is of great benefit to retaining the heat inside. Then when you're ready to fire up again, you can instantly see how the system is drafting when you light it off. A warm batch rocket doesn't require the bypass to be used, it just starts breathing properly as soon as the door or air intake is opened up, which proves that air continues to flow through the system and cooling it off unnecessarily when not burning. On the other hand, I always need the assistance of the bypass when lighting it cold. Marvelous!

thomas rubino wrote:24 hours later, the riser is still 179F, and the exhaust stack was 93F. Overall, external brick temperatures are 87F!  
That's almost  450 clay bricks at 87F sitting in my Living room.
5 minutes after lighting the riser is at 1186F and rising.

There's a remark to be made about the stack temperature when the air inlet is closed. The stove pipe, from which inside you take the reading from, is made of steel and it sticks all the way through the bell's wall down to the inner surface of it. The steel is conducting heat out from the bell, even when there's no air streaming through it. You would be able to check this phenomenon with an infrared thermometer. The closer to the bell, the higher the reading. The higher up the stove pipe, the lower.

A (very) long time ago, I didn't trust the air inlet of my heater of the time. The readings of the chimney thermometer suggested that there was heat transported through the pipe. I let the coals die down completely, to the point the ashes where just hand warm and not a single spark was to be seen. Then, I disconnected the pipe, stuffed it with rags and connected it again. The next morning, the pipe was still warm *above* the stuffed spot, despite the fact that there couldn't be an air stream through it.

In short: the steel of the pipe was conducting some heat out but lost it through its perimeter to the air in the room, so much higher up there almost wasn't a temperature difference between the pipe and the room air. This same phenomenon is happening with our current heater, the red bell.

Interesting, isn't it?
Very respectable temperatures you mentioned, by the way.

As I understand the heater is now declared to be dry. Are you still using the bypass when starting up?
Your temperature readings are good, but your bell ISA is a bit lower as compared to a 'standard' batchrocket. Would you recommend to build the bell some percentages, 3 or 4, smaller than the recommended figures which are on the website?

Thank you, Peter;
No, I do not use/ need the bypass now that things are warm and dry.

I am very much in love with Shorty and her performance as is.
Having the highest brick readings at chair level is much preferable to heating my cathedral ceiling.
As I'm only a few feet shy of the traditional ISA  I think that Shorty could heat 57 sq' but she is doing such a fine job with 55 that I see no reason to push her.

@Peter, thanks as always for your insights and experience. Always educational!

Question for you and Tom as it relates to the "original" tall riser combustion core with secondary air intake. As you may recall this is the core design of my masonry heater. It occurred to me the other day when closing off the primary air when I reached the coaling stage that there is still air entering the core via the secondary. Per Tom's comments to remember to close off the air intake to minimize system heat loss up the chimney, what are your thoughts for fabricating a removable plug that can be inserted into the secondary tube at this stage?

I already have one Glenn.
A 2.5 x 1.5 piece of CFB mounted on a piece of 1/8" metal.
I built it for the studio stove but never use it or shut the primary out there.
Indoors, Yes, you should use one.

Peter keeps his secondary air tube inside the firebox, unlike Matt and I, who bring it out past the door frame.

As a side note, I upgraded my temperature meter to a higher-quality one.
I can now monitor all three probes at the same time, much nicer!

thomas rubino wrote:I already have one Glenn.


Peter keeps his secondary air tube inside the firebox, unlike Matt and I, who bring it out past the door frame.

If your batch rockets were made with the secondary air tube (tunnel) at the base of the batch box it could easily be cut to be flush with the frame inlet for the fresh air.   Thus one mini door closing, closes both. This secondary tube often was replaceable, so you can easily figure out the exact length needed.

With a smoking piece of paper/punk wood or what have you, You can also check that the secondary air tube is in fact drawing air into it.  (Checking to see if any ash fell into the short tube by your exit port for the fire)  This is always good to know.

After a year of burning, always closing the inlet with a seperate piece, this year, I installed the mini door.  While it don't seem like much, it sure is nice to close this in seconds vs fiddling with the extra piece.  

Some don't have the ability to remove the front area of the stove, but two quick holes and self tapping screws, works well.

Best of success.

thomas rubino wrote:I already have one Glenn.
A 2.5 x 1.5 piece of CFB mounted on a piece of 1/8" metal.
I built it for the studio stove but never use it or shut the primary out there.
Indoors, Yes, you should use one.

As a side note, I upgraded my temperature meter to a higher-quality one.
I can now monitor all three probes at the same time, much nicer!

Thanks Tom. I'll cobble together something that will handle the temperature. I just checked the temperature of the inlet of the secondary tube with the dragon roaring and it's around 425 deg.

And, like you I purchased twice... my first temperature meter was a single input. Being a bit of a geek my second purchase is a 4 probe input unit that is also a data logger. As we are now moving toward more seasonal cold mountain temperatures I expect to be firing the dragon daily and will play with the data logger and charting the 3 thermocouple probe readings over time.

Scott Weinberg wrote:If your batch rockets were made with the secondary air tube (tunnel) at the base of the batch box it could easily be cut to be flush with the frame inlet for the fresh air.   Thus one mini door closing, closes both. This secondary tube often was replaceable, so you can easily figure out the exact length needed.

Scott, my primary air inlet is up high on the door and has a shutoff (Tom Rubino's design available here https://dragontechrmh.com/how-to-build-a-batchbox-door/). It will be pretty easy to fab up a plug for the secondary tube.