8" Batch Rocket Heater with Masonry Bell

[etownandrew]

Nov 18, 2018 9:47:44 GMT -8 coastalrocketeer said:

Another “active” way to use that old furnace ducting would be to use it in “fan only” mode (if it has that. Would depend on where the return air for your furnace is, as it would need to be sucking the air from a “warm” part of the house.

It so happens that I have typically used fan mode for the conventional heating system to circulate air to the side rooms. However, you have a good idea that would make that more powerful. If I can add a door to a supply vent in the basement, and then open it up in the winter, I would be doing a better job of pressurizing the basement and forcing warm air from the basement to the upstairs. The previous owner had cut a sliding door in a return vent near the wood stove but that just depressurized the area and sucks smoke out of the wood stove. Running the fan all month seemed to only cost me $10 in electricity.

I put the ceramic boards back on today and also finished the door that I had planned. Door is probably not the right word for what is shown in the picture below. It is more of a cover. It does seem to make a tight seal. The weight of the cover is being used to hold it against the brick face. I'm sure that there is something better that I can do for a door. 

I fired the stove, as shown, late this afternoon and it is doing OK but is having to dry out again.

[etownandrew]

I noticed a difference in how I need to load this batch rocket heater as compared to my old 55-gallon steel drum stove. I have been used to putting big logs into my wood stove as they prolonged the burn and worked fine. For this masonry stove, I tried that and noticed that the burn was not as hot and took longer to burn. So I can see that I need to split my wood down into smaller pieces when possible to help get a hot fast burn. You can see in the picture above some split wood that is the typical size I had used before but I need to split each of those at least once or twice more.

My exhaust temp is often 100F and rises as high as 175F so I'm capturing plenty of heat in the bell but may also be in danger of having incomplete burn and potentially collecting creosote in the stove pipe.

The bell seems to stay warm for maybe 6 hours but is certainly cold if the last fire burned was 12 hours previous.

[etownandrew]

I thought it would be interesting to post my costs and comments on the materials I used. In round numbers, it cost me $1,100 US dollars in materials. So the payback on this should be 3.6 months at $300 a month if I had been heating with propane gas.



Comments on the materials and design used.

• I had considered using Geo-polymer materials but finally decided they were too risky and so I went with more commercially available materials.
  
• The red bricks are small and took a lot of time. I wish I had looked for a bigger brick that might have reduced that work by half or more.
• The red bricks I used had holes in them which reduces their cost but ends up consuming a lot of sand/clay mortar. I could have used a solid brick but it cost twice as much. So using more sand ultimately cost less than buying a more expensive brick.
• I needed to have paid attention to what type of insulating board I got to avoid ones with organic binders that would burn off when first used. 
• The merging of the 5 minute riser (took more like an hour) with the firebrick port could use improvement. So I have about a 3 1/2" deep port area that might be slowing down my burn rate.
• I used more expensive "play sand" because whenever I looked at my local "masonry sand" it had round river stone in it. Also in one test, the masonry sand expanded and a test brick crumbled after a couple days.
• I agree with comments that the door can be one of the harder things to figure out. I'm still puzzling over how to attach a door to a clay mortar masonry stove. What I probably should have done is bury mounting brackets way back in the void between my firebrick box and the surrounding red brick that I filled with stabilized pearlite. I would have needed to have that planned out from the start because that would have been one of the early items placed so I could put weight on it and fill around it.

 

[wiscojames]

Thanks for posting all that useful information. Regarding the door, I agree that providing for a structural connection is best. That said, I used another method that worked well in my situation. Last winter I built a 6 inch batch box heater built mainly from 4 by 8 by 16 concrete blocks (much faster than regular bricks). For a door, I had a guy fabricate 2 simple frames that could be held together with a few s.s. bolts. Sandwich some glass between with a ceramic fiber gasket, and it's a decent door with a pretty good seal. Next time I'll have air adjustment in the door frame (and maybe fix it to the stove) For now, I set it on bricks and can regulate primary air pretty well.

drive.google.com/file/d/1AwBeYyQFUqsDgXkCaZyipCEd3Jde2MvJ/view?usp=drivesdk

[etownandrew]

Your stove ended up looking nice. Did you make all of it from concrete blocks?

[drooster]

Andrew how about a steel sleeve around the firebox with a hinge for your door?
It would be solid and good-looking and not require you to re-drill into your finished masonry,
and your firebox is narrow enough.

[wiscojames]

Andrew - yes, all concrete block except directly above the riser I used half of a small steel drum, so the block was all below the top of the riser. The thing worked well, maybe too well. That, or I built it too close to the wall, because I became concerned about the temperatures I was exposing the wall to. I can't give long term durability endorsements, but I didn't see any degradation of the blocks when I disassembled it after about 10 or 15 burns.

[wiscojames]

Forgot the visual aid!

drive.google.com/file/d/1tWZFlpdHYGa0hNU42tI6gOzhjy6Ksy3u/view?usp=drivesdk

[etownandrew]

Yesterday morning I added sheet metal walls around the masonry stove to help force the warm air to go up through the floor vent openings overhead. The picture below shows the finished product. Not pretty but functional. The sheets hang several inches above the floor so air can easily enter but mostly has to go through the overhead floor openings to go out. It looks like that was the final item needed to get it working without further changes. With the walls, I was getting a noticeable flow of warm air into the main house area. This is the same solution that I had used for the last 15 years with a 55-gallon metal barrel wood stove to get the heat up into the rest of the house. I experimented for a few hours with opening up an HVAC duct supply port in the basement but that did not seem to have the desired effect. I also looked at creating more return openings from the main level to the basement but did not see any easy way to do that. So I moved forward with adding the walls. These were the same panels I had used with my old stove and so one panel has a cut-out that is not needed for this arrangement. Last night the house was 78F and this morning when I got up it was 75F with the outside temp being 36F. So the masonry bell seems to be doing good at retaining the heat and releasing it over about the next six hours. I did run the HVAC system in fan mode some to move the heat into the bedrooms and bathrooms. This had the side room temp about 73F which was 5F less than the core of the house.


I've been telling people that I think this stove will cut my wood consumption in half. Based on a few days use, I think that may end up being correct. I won't really know until I go through a complete heating season and compare my overall wood consumption. I didn't have a baseline measurement for pounds of wood burned to outside temp that I could use to make a more accurate comparison.

[drooster]

Wow Andrew that is extreme!
I was expecting a small panel near the ceiling from what you had previously described ...

[coastalrocketeer]

Beautiful! And I doubt you were waking up to a still warm house with that old smoke belcher!

Love the passive convective heat distribution... Depending on how well sealed that basement is, some portion of the air convecting upward will be bringing in fresh air from outside.

In my very moderate maritime climate, where it rarely gets below freezing, I would like to build a system that is outside and convects warmed outside air into my house in a similar fashion. A set of good quality furnace filters on the intake and I will have warm, positive pressure of clean fresh air anytime the mass is warmer than the home interior.

[peterberg]

Now I see what you've done, Andrew.
So what you did is a novel way to transport warm air from the heater in the basement to the house above!

Incidentally, just today I installed a corrugated steel shield behind my heater. There's about 4" of space behind the heater and the wall but the plasterboard wall behind it did get worrying warm last winter. Now with the shield in place, the heater is as warm as before but the shield is barely handwarm. The air streaming up behind it is keeping the wall cool.

[coastalrocketeer]

The better those sheet metal panels are sealed to the ceiling (or floor & joists above, the better the "stack effect" will work for you... If a ceiling, painter's tape could work, and if at joists, pliable foam, or batt insulation in bags could stuff any gaps.

Perhaps you've already addressed this somehow, can't see that part from the pics

[etownandrew]

A report on how it is doing. This past week we had temperatures in the mid 20s Fahrenheit. This 8" batch system could not keep my house fully warm as built and installed. So we ended up running our conventional Propane heating system in tandem with the wood heater. The wood heater can handle outside temps in the 30s.

There are a few compromises in the back of my mind that I am thinking about.
1) I made the batch box 32" long which is longer than recommended. At the time I understood that would not matter but have since seen that there is a recommended limit. The result seems to be that wood at the back part of the box does not get hit by the primary air flow and so tends to remain as red-hot coals. The length is convenient to avoid having to recut some of my longer wood supply.

2) My 5-minute (1 hour) riser sheet metal form is blocking about 1/2" of the port top and so may be limiting air flow. I expect that metal will rust away quickly and hopefully performance will improve a little.

3) I did not include a 55-gallon barrel top for quick heat. I did not have the proper safety clearances for a metal bell. However, my family misses the quick hot heat. I could lengthen the stove pipe run to the chimney but I am already extracting what seems to be as much heat as I can and I don't want to cool the smoke too much.

4) Of course, the big compromise is that it is in the basement.

[coastalrocketeer]

Is the basement getting excessively warm?