Adding a grate to a block arch - need your input.

[Cjadamec]

I don't think you need a bigger fire box if you are going to insulate the block. All a bigger fire will do is use more wood. As you said you are also draft limited by your stack.

On the insulation note, you should put the insulation between the fire brick and the cinder blocks. It will protect the insulation and perform better.

Make the area under the pans and leading to the stack as air tight as possible. It will force air through the fire and give you a better burn. Air being drawn in around the pans will cool the pans down.

A solid improvement would be to transition to a round metal stack. The cheap round metal ducting either 6 or 8 inch is cheap and effective and it comes in 5 foot lengths. You can make a square opening at the back of your arch with brick and use insulation to seal up the corners around the stack. It will make a huge draft improvement. I used a stacked cinder block flue for a season and I know it's a really poor draft even when stacked up 6 feet.

[jimmol]

2013 Stack.jpg

I don't think you need a bigger fire box if you are going to insulate the block. All a bigger fire will do is use more wood. As you said you are also draft limited by your stack.

So you feel the 11x 14 would be fine.

On the insulation note, you should put the insulation between the fire brick and the cinder blocks. It will protect the insulation and perform better.

I am assuming that if it gets compressed by the firebrick - the weight of the brick against the insulation - it will be fine?

Make the area under the pans and leading to the stack as air tight as possible. It will force air through the fire and give you a better burn. Air being drawn in around the pans will cool the pans down.

I was toying with putting insulation inside the 3 sided of the arch, then putting the cinder/solid cement blocks that build up the vertical arch back in - 15 5/8 + 1/2 +1/2= 16 5/8 and the pan is 18 wide. I may be able to do that in summer. For now I will insulate exposed sides. The firebrick will stack just about to the top, though somewhere in a firebrick installation, it said something about leaving and inch or two, which I am thinking would allow the heat to get to the pan at the edge.

A solid improvement would be to transition to a round metal stack. The cheap round metal ducting either 6 or 8 inch is cheap and effective and it comes in 5 foot lengths. You can make a square opening at the back of your arch with brick and use insulation to seal up the corners around the stack. It will make a huge draft improvement. I used a stacked cinder block flue for a season and I know it's a really poor draft even when stacked up 6 feet.

In 2012, the arch had just the cinder/cement block stack. Several blocks cracked - and now that I think of it, spending 3 dollars to replace 3 of 5 blocks is way cheaper than running a stack - So in 2013 I added 4" stacks inside - pictured below. In either 2014 or 15 I added an additional 4 foot. I put a piece of galvanized mental across the bottom, cut out a 4" hole wth a hole saw for metal, put the stacks in them and out and folded the stack below the metal to be flush with the metal. Across the arch side I put a piece of 3/4 L frame to hold up that side of the metal.
I will have to give thought to the 6/8 inch with brick base. I will see how this year runs.

2013 Stack.jpg

[jimmol]

I don't think you need a bigger fire box if you are going to insulate the block....

After reading around some more, I checked my wood and all mine are cut to 16 to 20 inches so I choose to make it 4 inches bigger so I do not have to recut the wood I want to use for my arch.

Picture shows insulation cut to size.

I am putting together the firebricks in my basement and will set them in when the mortar is dry.

Attachment 19396

[Cjadamec]

Not having to re-cut wood is plenty of reason to make the box a little bit bigger.

I'm finishing up a 200 gallon run from this past week today. The insulation is worth the effort.

[jimmol]

Good to know. Still below freezing here so will not tap till March.

[Tater]

The purpose of firebrick is to protect the insulation from being damaged by the wood. Thus, it's fine to leave your brick well below the top of your arch if the ceramic blanket will hold itself in place and you won't be stacking wood all the way to the top against the side. I have AOF in my arch, and the 2" square tubing (which is wrapped in 1/4" ceramic blanket) sits on the top row of firebrick with about 4" of exposed blanket extending up to the top rail the pan sits on, meaning my firebrick stops like 6.5" below the pan.

The material you use for your grate is less important than the design of your grate. As you can see from reading this thread, some have problems with angle and cast, while others believe one or the other is better. If you think about this from a physics perspective, heat rises. The fire may move the heat around a bit (especially if you are injecting high pressure air), but if you have a proper amount of draft, the chimney will literally be sucking the heat up and away from your grates. I have been around a campfire on 4" of ice on a lake at a skating party, and the ice under the fire barely gets wet, much less melting away. Steel takes a bit more than ice to melt/warp, so obviously something else is going on. If your grate has 1" wide slots for large chunks to fall down through, you will get a lot of heat under your grates. If you add AUF to that, and the airflow hits those large coals that drop though the grate, you have just made a forge, and no grate will handle that well. Thus, you need to minimize the size of chunks that fall through the grate. Angle layed side by side (vvvvvv) with a 1/8"-1/4" gap will keep the heat up away from your grate (the v fills up with ashes plus "rounds over," which keeps the coals well above the grate. Air under fire, whether natural draft or forced, will help to cool the bottom side of the grate, and the narrow gap is plenty to allow ashes to go down and air to come up.

As Cjadamec said, increasing the pipe for the chimney will help. As we all know, heat rises, and the warmer the air is the better it rises. A larger pipe has a larger volume to surface area ratio, which means the exhaust gases (aka smoke) gets cooled less than it will in a smaller pipe. Thus, it will rise faster. The hottest and fastest moving part of the draft is in the very center of the pipe, and it slows as you get closer to the cooler outside of the pipe.

AUF (air under fire) is a great tool, but without enough oxygen above the fire, you are literally blowing your wood out the chimney. I finished my steel arch 2 years ago a bit after the season had already started (yeah, planning ahead is good, and doing ahead is even better, [and hindsight is always easier than foresight]), and only had time for AUF. I used 2 high CFM computer fans and blew a lot of coal black smoke out the chimney. That black stuff is unburned fuel (ie wasted). I had an airtight arch (planning for AOF), and I had to run with the door open because, as you have read in that thread linked in post 1, my AUF was burning the wood, but it wasn't burning the gases generated as there was a severe lack of oxygen in the actual heat-producing part of the fire. I added AOF (air over fire) plumbing the following year, and switched from the computer fans to a bounce house blower I bought from fleabay for around $45. It was a fair amount of work, but the fire burns hotter, my boil is better, my stack temps went from 1200++ to 750-900, the jet black column of smoke that used to pour out of the stack is gone (it is actually unusual to see anything but heat waves coming out the stack), and of course the best benefit is that I use less wood. The only downside is listening to that raging inferno inside my homemade arch and wondering whether it might suddenly break out, but so far so good!

If you have an unlimited supply of wood (ie split and stacked by someone else at no charge to you), AUF will get you a hot fire (hotter than natural draft). AOF is generally more work (maybe more work than is worth it for a block arch), but the results are absolutely fantastic. I started on a block arch with the previously mentioned computer fans blowing from the front of the arch on to the base of the fire. It worked, and I made syrup. Gotta start somewhere!

PS

This forum has too much information... This was originally a post asking about adding a grate to a block arch.

We've all been down that rabbit hole. And it's doubtful we'll ever get out! Be careful about spending too much time on here, or you and your money will soon be parted.

[ecolbeck]

If you think about this from a physics perspective, heat rises.:

Just to be picky:
heat doesn’t rise. Warm air rises because it is more buoyant than the surrounding air. Heat actually flows from areas of greater heat to areas of lesser heat (cold).

[Tater]

Just to be picky:
heat doesn’t rise. Warm air rises because it is more buoyant than the surrounding air. Heat actually flows from areas of greater heat to areas of lesser heat (cold).

I knew that line was gonna get me in trouble! Almost reworded it, but very little of the heat "flows" from the firebox to the ashbox. Certainly not enough to ruin my post with advanced thermodynamic theory. But yes, I did oversimplify things, maybe too much.

[jimmol]

Thanks for the information and the stories. I need to get to work as this weekend could be in the 40s.