Batch Rocket - Build Question

I love it when a story has a happy ending.

Peter van den Berg wrote:The second suspected flaw is the position of the blast gate. It looks like it's above the level of the riser, this is a tricky position. The temperatures might be too high for the materials of the gate, possibly deforming it.

Is your concern the integrity of the blast gate alone, or all of the metal components of the bypass in this position?

I'm wondering what you would suggest as an alternative here? Would lowering the bypass below the top of the riser be sufficient? If so, at what point does lowering the bypass impact its overall effectiveness? I imagine there may be a delicate dance between keeping it high enough to vent effectively, but low enough to maintain its overall integrity in the presence of such heat...

Hi Miguel;
Peter's concern would be the Aluminum blast gate,  it is exposed on one side to bell top temperature.
The steel portion of the bypass is in no danger of high heat.

With luck, you may not need to use your bypass other than during a stone-cold startup.
I would monitor the gate, checking its sliding function for warpage.
If it appears to be affected by the high temperature, you will want to proceed to plan B...

You may be asking, what is plan "B"?
Plan B would be, to use a traditional cast iron stove pipe damper with a circle of ceramic blanket wired on to seal it from the hot side.

Yes, to Tom's reply.

I'll be monitoring the integrity of the blast gate. I only had a minimal number of burns before spring arrived so time will tell how well the current piece survives my mistake of mounting it too high. I'll adapt and overcome if the need arises in the future.

This build is superb. Thanks for sharing your path.
Glenn:  Do you have any news to share on the performance and endurance of the blast door?

I just want to say thanks to Glenn for posting this in such detail.  I've read the entire thing at least 4 times, and every time, I've picked up on something new.

I can't wait to start my build.  It will be in the greenhouse I'm building.  I've already started clearing the site for the greenhouse.  I'll be pouring a slab for the RMH to sit on.  It's pretty cold here right now, -2F this morning and windy, so I'm really looking forward to the temperatures going up a bit.  I'm still watching for bricks on all the typical places.  I put an ad on one asking to buy chimney brick.  If I don't get responses this week, I'll go to the guy nearby that has the bricks with the holes.  I was hoping to avoid having to use those and fill the holes, but if that is what it takes, so be it :)

Pablo Kulbaba wrote:This build is superb. Thanks for sharing your path.
Glenn:  Do you have any news to share on the performance and endurance of the blast door?

Hello Pablo, so sorry that I missed seeing your post. I hope you see my reply.

The door is functioning perfectly. I do typically leave it cracked open for the first half of the burn on start-up in the morning or evening. Typically about 1" (~25mm) for the first 15 minutes and then about 1/2" (~12mm) for the next 15 minutes. Keep in mind that I live at 8,000' above sea level (2,450 meters) so my air density is much less than most people and I simply made an educated guess on making my primary air intake larger. Once I have a good amount of coals it is burning fine and I close the door fully.

Note that this is Thomas Rubino's door design for which he offers a build book at Dragon Technology: https://dragontechrmh.com/how-to-build-a-batchbox-door/.

Trace Oswald wrote:I just want to say thanks to Glenn for posting this in such detail.  I've read the entire thing at least 4 times, and every time, I've picked up on something new.

I can't wait to start my build.  It will be in the greenhouse I'm building.  I've already started clearing the site for the greenhouse.  I'll be pouring a slab for the RMH to sit on.

I'm super glad to hear that my build thread has been helpful. It serves as a double blessing. First, the participation by the community was invaluable to me as a first time builder to get it right the first time and have a superb functioning heater. Second, it helps folks like yourself to get a jump on build planning.

For your greenhouse build I'll throw out a thought that I failed to do with my greenhouse which I keep growing year-round, but did learn when I built my home which is slab on-grade with hydronic floor heating in the slab. Floor mass, whether concrete or dirt can serve as a thermal mass but up front planning will help with efficiency. If you insulate the perimeter you will minimize heat loss out the sides. For my slab I laid down crushed stone with a vapor barrier on top then 2" thick construction foam board. The perimeter was dug a little deeper and foam board was cut and laid vertically to insulate from heat loss out the sides. You can definitely count on the slab that the RMH will be built on as a part of your mass. The slab that my heater is built on maintains about 80+ degrees 12" out from the footprint of the mass. at all times. If budget allows you may also want to consider trenching around the perimeter of the greenhouse and placing foam insulation vertically to help retain the warmth of the dirt mass of the greenhouse footprint. I'm not sure how deep to go but I would think at least 18" or more.

I would like to add to Glenn; if you are using the buildup with insulation under the slab, if you have the option, it's best to use XPS or EPS. Especially XPS does not suffer from moisture, PUR (nowadays the standard insulation, at least here in the EU) does. I will be using XPS for my underfloor heating (backup/alternative to my RMH).

Julian Adam wrote:I would like to add to Glenn; if you are using the buildup with insulation under the slab, if you have the option, it's best to use XPS or EPS. Especially XPS does not suffer from moisture, PUR (nowadays the standard insulation, at least here in the EU) does. I will be using XPS for my underfloor heating (backup/alternative to my RMH).

Thanks for the additional detail Julian. You are correct, I couldn't recall the exact material I used as it was 9 years ago, I checked and it was XPS board.

Glenn Littman wrote:...
For your greenhouse build I'll throw out a thought that I failed to do with my greenhouse which I keep growing year-round, but did learn when I built my home which is slab on-grade with hydronic floor heating in the slab. Floor mass, whether concrete or dirt can serve as a thermal mass but up front planning will help with efficiency. If you insulate the perimeter you will minimize heat loss out the sides. For my slab I laid down crushed stone with a vapor barrier on top then 2" thick construction foam board. The perimeter was dug a little deeper and foam board was cut and laid vertically to insulate from heat loss out the sides. You can definitely count on the slab that the RMH will be built on as a part of your mass. The slab that my heater is built on maintains about 80+ degrees 12" out from the footprint of the mass. at all times. If budget allows you may also want to consider trenching around the perimeter of the greenhouse and placing foam insulation vertically to help retain the warmth of the dirt mass of the greenhouse footprint. I'm not sure how deep to go but I would think at least 18" or more.

My climate is definitely challenging in the winter.  A couple, maybe three, years ago we had -40F two nights in a row.  That was an anomaly, but we get -20F most years, sometimes for a week or two, so everything I can do to minimize that will be necessary here.  I'll definitely use those suggestions when building the greenhouse.  I don't plan on it being overly large so I can maximize the heat I can keep in, but I'm already thinking I may need a RMH at each end.  The current plan is 24'x12' with underground air tubes, but the tubes probably won't be put in initially.  It would be easier, but the time for trenching would set me back further than I like, so I think I will see how well I can do without them initially.  Anyway, I don't want to sidetrack your excellent thread overly much, but again, thank you for putting it out here in such detail.

Hey Trace;
Our greenhouse/ studio is 12'x24' with 15' peaked roof.
One 8" J-Tube did the job, but required tending once an hour.
Now a 6" Batchbox heats it easily, feeding every 2.5 hrs.
We go below zero each winter but not nearly as long as you do in the upper mid west.
One 6"-8" batchbox with a large bell, would do the job in your greenhouse.

thomas rubino wrote:Hey Trace;
Our greenhouse/ studio is 12'x24' with 15' peaked roof.
One 8" J-Tube did the job, but required tending once an hour.
Now a 6" Batchbox heats it easily, feeding every 2.5 hrs.
We go below zero each winter but not nearly as long as you do in the upper mid west.
One 6"-8" batchbox with a large bell, would do the job in your greenhouse.

Hey Thomas, how many times a day do you think I would need to do a burn in mid-winter?  This next winter(2025)  it would be a struggle for me to burn often enough I think, but I'm retiring early in 2026, after that I'll be home most all the time and it will be no problem.  It may be that this coming year, the greenhouse can only be used to extend my seasons, but in coming years, I would like to be able to keep cold-hardy greens alive year round.

Hi Trace;
This year might be tough if you are working and get a long, deep freeze.
I light the studio dragon off from 7 to 10 a.m., depending on the outside temperature. It usually is in the low 50s or upper 40s when I light it.
It is burned every 2-3 hours throughout the day. Liz loads it between 8 and 11 for the last load of the evening.
We do not coal it out and shut the intake like we do with Shorty.
We burn more than the average bear...
As this is really just an artist's studio, Liz likes to be warm. It is not uncommon for it to be 80 degrees inside and 20 degrees outside.

Understood.  The greenhouse will be as heavily insulated as I can swing, but if the winter just makes it impossible to keep warm enough the coming winter, I'll let it go for the year.  I can live without year round produce for one more year :)