5" j tube water heater not heating

hi all, here is my 5" rocket stove prototype made from old bricks and blocks.  I'm using sand and cement to seal any gaps between the blocks.

In this image, the feed tunnel is 5", the burn tunnel is 9" and the riser is 20" (I'm measuring these as described in the Rocket Mass Heaters book).

The stove gets a good draft and there's very little smoke. The problem is even after warming up properly, it doesn't get hot enough to boil water (the steel can contains 1 litre of water at 20 degrees c, and after 15 minutes it still hadn't boiled).

I've checked there are no leaks by dropping some smoking charcoal down the riser. I've tried increasing the burn tunnel height, and decreasing the gap between the can and the top of the riser. I've also tried a longer burn tunnel (14") and a taller riser (30"). I've also tried varying thickness of fuel. No luck with any of them

The one obvious thing I've not tried yet is insulation, but the blocks/bricks are only slightly warm on the outside so I don't know if that would help.

I'm about ready to give up on this, so any help or suggestions appreciated!

It will be hard to get good temps when there isn't any insulation. I'm not sure what those bricks you used are made from but they will grab a lot of the heat from the burn without insulation. They also look to be quite thick and will take up even more heat. 5" is also quite small.

Lastly, the measurements seem odd, what are your measurements of the lengths of each area and what is the cross sectional area  of each area?

Hi Annie;
The biggest issue I see is that your bricks are not hot yet.  
Currently, most of the heat generated is warming the bricks rather than going up the riser.

Next, your measurements have me a bit confused.
The feed tube should be evenly square  8"x8", 7" x7". Typically, a feed tube is 16" deep; yours looks to be two bricks deep, 8"?
The burn tunnel size should match the feed tube size, so 8x8 or 7x7. I measure burn tunnel length by roof length only, 10"- no more than 12"
The riser should be the same size, 8x8, 7x7. With a cook stove rather than a mass heater, it can be shorter than the suggested length.

Get your rocket roaring hot, and then try boiling water

.

Daniel Ray wrote:It will be hard to get good temps when there isn't any insulation. I'm not sure what those bricks you used are made from but they will grab a lot of the heat from the burn without insulation. They also look to be quite thick and will take up even more heat. 5" is also quite small.

Lastly, the measurements seem odd, what are your measurements of the lengths of each area and what is the cross sectional area  of each area?

Thanks Daniel. I was trying to create a prototype with what I had lying around, but i'll see if I can get some fire bricks.

The feed tunnel (A) is approx 5"x 5". The burn tunnel (B) is approximately 5" x 9" and the riser is 20" x 5"

I realise now I've messed up the measurements. I will rejig things tomorrow and report back. Thanks for your help

thomas rubino wrote:Hi Annie;
The biggest issue I see is that your bricks are not hot yet.  
Currently, most of the heat generated is warming the bricks rather than going up the riser.

Next, your measurements have me a bit confused.
The feed tube should be evenly square  8"x8", 7" x7". Typically, a feed tube is 16" deep; yours looks to be two bricks deep, 8"?
The burn tunnel size should match the feed tube size, so 8x8 or 7x7. I measure burn tunnel length by roof length only, 10"- no more than 12"
The riser should be the same size, 8x8, 7x7. With a cook stove rather than a mass heater, it can be shorter than the suggested length.

Get your rocket roaring hot, and then try boiling water

.

Thanks for your help Thomas! Whilst replying to another message in this thread, I realised my measurements are wrong.

that's a good point about the bricks taking up the heat. I'll try to redesign tomorrow and this time I'll give it longer to warm up and get hot.

Hi Annie;
Firebricks are ideal, but solid clay bricks will work, particularly in a cook stove
Your cement riser will likely crack when you reach dragon temperatures.  

The feed tunnel (A) is approx 5"x 5". The burn tunnel (B) is approximately 5" x 9" and the riser is 20" x 5"

Yes, try to keep it consistent throughout.  Once the gases get to the riser they are moving too slow. It should speed up and rocket much more once the burn tunnel and riser are down to 5x5"--or better yet--get it all to 8" and you'll have no issues boiling the water.