How much space between riser and hot water tank

So I finished my rocket core and now I'm going to install the hot water heater on top. How much space should I leave between the bottom of the steel water tank and top of the riser?

Hi Yen;
I forget is this a J-tube or a batch box?
If it is a batch then no less than 12".
For a J-tube, I would think 6" would be enough.
Of course, your water tank must never go empty while a fire is going.

EDIT) Now I remember this is the Tim Barker J tube water heater design.
Yes, 6" should be good.

Hi Thomas

Thank you for your valuable input.

I will put the water tank in a barrel and put the chimney vertically on top of the barrel. Do you think it's optimal to put the chimney at the top center? How much room do I need at the top between the water tank and the top of the barrel? Also 15 cm?

As for safety measures, the water tank will be open to the atmosphere via 2 pipes. I will have two pipes just incase one may get clogged etc. Also I will use the second pipe as fill up. The coil inside has an expansion tank and a blow off valve, also just incase. But technically the temperature within the coil cannot reach over 100c since the water in the tank is open to the atmosphere. Thanks for the heads up.

So I was doing some reading on stratification chambers and how hot gasses accumulate at the top and the cold gasses at the bottom. And I was thinking would it be beneficial to do the flu at the bottom like in the picture rather than at the very top. Or would it not work in this case as the barrel is directly on top of the riser?

In order to prevent short-cutting from the inlet to the outlet, the inlet should be a somewhat higher. In my experience, the exhaust out the bottom  doesn't work that well, better out the side. As low as practical and the inlet pipe ending higher.

Hi Peter

Thank you for your kind response.

I have sketched my understanding of your explanation.

The only concern for me would be that I'm not sure I'd have enough space left in the barrel so I'd have to extend the barrel.

How high would the input have to be from the output ?

Also how can I seal around the barrel opening and the riser in your opinion?

So I ended up putting the flu pipe about 8 cm below the riser. The water tank 15 cm above the riser.

İt's not finished yet as I will add a barrel over the water tank and insulate it with 4 cm rockwool.

With the water tank exposed I managed to raise the temperature by 12 C in 1 hour for 130 liters of water.

Hi Yen Yus, great to see your experiment coming together.
What are the dimensions of your J tube core and what did you make it from?

Hey Fox

Thank you, I'm taking it slow and steady.

I made the core out of fire bricks. The inside measurements are 20 cm wide X 70 cm long . 20 X 20 feed and 20 X 20 cm riser with a 30 cm burn chamber.

Ok, fire bricks and clay will take sometime to dry out and yes defenatily best to take it slow.
A lot of the drying will take place as the mass cools down so lots of small fires is the best way forward.

Thanks for the tip Fox.

I cobbed and then lime rendered it. I think it's been about 2 weeks since I cobbed it. I had it lit when I was cobbing it and hadn't lit it until today. I wanted the lime render to dry slowly.

I lit it today and to my suprise it started drawing as soon as I lit it . I thought it would take some time to heat up and need a bit of fanning before the rocket started drawing but it drew straight away.

Next I want to install the top barrel. Not sure how I will fit the bottom barrel inside the top one yet though. Also I have to open up some holes for the pipes at the top which looks like it's going to be a challenge.

Looking Great Yen!
Keep us posted with plenty of photos and temperature measurements.

Thanks Thomas

I managed to get the top barrel on today and insulate it.

I did another test fire. This time it took half an hour to start drawing and even then there was smoke coming out of the feeder. There is a gap between the bottom barrel and riser. I feel the warm air coming out. I will seal it and retest. Just not sure how to seal it yet. I think this may solve the problem somewhat.

After it started drawing it took 40 minutes for the top of the water tank to reach 50 C from 25 C . I filled up the feeder once.

I measured the top of the flu and it was 90 C . Not sure if this is high or not.

Hi Yen;
What size is your exhaust stack?
It appears to be 6" when horizontal but looks to be smaller when you go vertical.

thomas rubino wrote:Hi Yen;
What size is your exhaust stack?
It appears to be 6" when horizontal but looks to be smaller when you go vertical.

İt's 8 cm (3.15 inches)

Now that you point it out, I'm guessing it may be small?

Yes, I'm afraid so Yen.  Much too small.
Is your J-tube a 6"?  whatever size it is your chimney needs to match.

thomas rubino wrote:Yes, I'm afraid so Yen.  Much too small.
Is your J-tube a 6"?  whatever size it is your chimney needs to match.

Yeah, I recycled the 3 inch pipe you see.


My J is 20x20 cm . The problem is even if I put in 6 inch the top of pipe would line up with the riser . Would this be a problem? Would the air take a shortcut from the riser to the exhaust pipe? What if I put two exhaust pipes on each side?

Well spotted Thomas!
Unfortunately with a big J tube you need a big chimney, it need to be at least system size but the transition or exit hole really needs to be 25cm so as not to cause any restrictsion.
Or in other works the short bit of pipe that come dorectly out of the barrel need to be bigger than the chimney.
What diameter are the inside and outside barrels?

The water tank is 50 cm and the outside barrel is 61 cm. There's about 10 cm space on top and 15 cm between the riser and water tank.

Not sure how to solve this problem except make the flu at the top maybe. Or at the bottom. İf I make a bigger exhaust hole at the side bottom the heat out of the riser may take the shortcut out.

What do you guys think?

Yen;
Hot air rises, it will not shortcut thru the outlet.
The hottest air goes to the top and the cooler air falls to the bottom and out of your stack.
As Fox suggested funnel the outlet from 25 cm to 20 cm and then keep 20 cm all the way up.

Fox, I must give credit where it is due.
My fellow rocket scientist Jordan from BC pointed out the exhaust size to me.

One way would be to use a much bigger outer barrel and take it right down to the floor.

Thanks guys.

Now my next problem is where to find it. All our exhaust pipes here in Cyprus are 150 mm or smaller . I have never seen a bigger size. Hopefully I can find one or I will roll it from sheet metal.

There should be plenty of 8” air con pipe in Cyprus?

Fox James wrote:There should be plenty of 8” air con pipe in Cyprus?

You mean the flexible ducting?

No it is spiral pipe made by twisting flat sheet and is pretty standard for air con in large buildings hotels etc….
You really want a smooth inside to the pipe not flexible ribbed pipe.
I have been using air con pipe with car exhaust wrap for many years but it is thin wall and wont last forever.
Some air con pipe is galvanised  and some folk worry about that if it gets to hot, I am not sure if that is a real issue or not?

Most aircons are split units here but they must have specialized installers for hotels etc. I'll look into it and post once I've made some progress. Thank you again.

Hello everyone

I installed the 200 mm exhaust pipe over the weekend and fired it up.

The draw was instant. The paper I put in to light the fire was drawing towards the riser. I lit it for an hour and in that hour the water temperature went from 35 C to 55 C. I filled up the feeder 4 times with roughly 2 cm X 3 cm sticks. Not densely, I could have fit more.

The only problem I had was that I measured 200 C at the top inside of the exhaust pipe. This seems like a lot of heat energy to be losing out on. I'm thinking of blocking off the exhaust pipe inside the barrel at the top to prevent the heat from shortcutting from the riser, but I'm not sure. What do you think?

Other than that the 4 cm rockwool is doing a great job. I stopped the fire at 1430 yesterday and this morning it was 50 C ambient temperature is around 18 C .

Next I will connect it to my underfloor heating.

Oh nice shinny (expensive) chimney!
I think you could improve the overall performance buy making the exhaust outlet lower, as low as you can.
Maybe an easy way would be to fix metal around the support frame and exit the chimney lower.
That way a lot more of the hot gas will stay under the water barrel.
Possibly the best long term would be a bigger outside barrel going right to the floor, larger diameter and longer in length?

İt was expensive, but worth it. Now the system is working. I like your idea about putting the exhaust lower. I also thought of making an oven, not sure how that would work though. As is it will do the job but I just feel bad knowing I can do more with the energy from the wood. Thank you for all your help and input.

I have a j tube in my party house, I built it with a hollow mass under the barrel, you might be able to do something similar to make use of the excess heat.