Help! My Rocket doesn't heat

This summer I built a Rocket that also heats hot water with the aim of heating my house. Now that winter is coming I have started using it, but I am not satisfied with the performance. I will try to explain the typology of the house and the Rocket

House: divided into 2 floors, lower floor about 30m2 and upper floor 45m² for a total volume of about 170m³. The house has 50 cm thick stone walls, the windows are old with drafts. In winter the temperatures can reach around 0C.

Rocket: 21 cm style in refractory bricks with the riser not insulated. The barrel with the j is on the lower floor while the heated bench (mass) is on the upper floor. In the space between the barrel and the riser there is a copper coil (outer diameter of the tube 10mm, total length of the coil 40m) that heats a 500l water accumulation, to this accumulation are connected the radiators that heat the rooms on the upper floor where the mass is not located.

Circular chimney of 18 cm high 7 m that comes out at the highest point of the roof I use chestnut wood of various diameters from 5 to 15cm, dried correctly

Current situation: after a burn of about 5 HOURS at full load (so much so that sometimes the fire goes up the sticks), boiling barrel and bench, water accumulation temperature increased by just over 40 degrees from 20 to 60, I turn on the 3 radiators and use the water for a shower, in the house there are about 20 degrees with an outside temperature of 13 degrees. After 8 hours from turning off, cold bench, warm barrel, water in the accumulation at 35 or too cold to circulate in the radiators. I wonder why I have to do such prolonged ignitions (I use a lot of wood) and after 8 hours everything is already cold!!! Where am I going wrong? Undersized machine? Coil does not absorb enough heat

Another thing, as soon as I finished the work I tried the Rocket, in a few hours the water in the accumulation had reached 80 C°, why can't I get this great performance anymore? Was the mass humid, were the internal surfaces smooth? Wasn't the coil dirty?

I attach photos and drawings

Cedric Noussan wrote:

.. with the riser not insulated.

That might be the main problem right there...

Burra Maluca wrote:

Cedric Noussan wrote:

.. with the riser not insulated.

That might be the main problem right there...

How much does insulation affect combustion efficiency?

Do you think this is the only cause?

It looks like you are asking a huge amount from such a tiny fire!
I have never seen a design like that before, where did you get the idea from?
Apart from the uninsulated riser (insulation is essential) the pipe running up stairs must also be shedding a large percentage  of heat!
It may be possible to get some of the heat to warm the upstairs mass but it looks like a huge ask to me.
You could try adding insulation to  the fire box, burn tunnel and riser, then heavily insulate the pipe going up stairs.

I can't quite make out the positioning of the exit flue from the bench. I'm also assuming it's a hollow stratification chamber? If so and the exit is set too high, the heat might just be passing straight through the bench and up the flue instead of hanging around long enough to heat the bench.

Fox James wrote:It looks like you are asking a huge amount from such a tiny fire!
I have never seen a design like that before, where did you get the idea from?
Apart from the uninsulated riser (insulation is essential) the pipe running up stairs must also be shedding a large percentage  of heat!
It may be possible to get some of the heat to warm the upstairs mass but it looks like a huge ask to me.
You could try adding insulation to  the fire box, burn tunnel and riser, then heavily insulate the pipe going up stairs.

Can you by any chance tell me what the average power of a 8" (21cm) j is?

The project is from an Italian Rocket builder

Ok, it seems reasonable to isolate the combustion unit to direct more color into the bench, even if this requires a lot of work! While isolating only the riser and the barrel before the bench using ceramic fiber seems simpler to me, what do you think?

Hi Cedric;
I have not yet had time to read your post properly.
But regarding your last question.
Insulate the riser for sure.
I'm not sure what you mean by the barrel, but if it is easy then give it a try.

Burra Maluca wrote:I can't quite make out the positioning of the exit flue from the bench. I'm also assuming it's a hollow stratification chamber? If so and the exit is set too high, the heat might just be passing straight through the bench and up the flue instead of hanging around long enough to heat the bench.

The outlet is at the end of the bench, the pipe comes out directly from the bench, the bench is a brick duct and Adobe floor.
20cm (8") before the outlet a metal piece is positioned so as to block the duct by about half, this does not seem to affect the draft

The bench cross section is not large enough for stratification, it will all move through the channel as one flow. This is essentially a square brick duct. What is the surface temperature of the brick channel, both from the barrel exit and upstairs? For such a long burn, there ought to be a ton of heat being released somewhere from the system, and not much could be getting up the chimney. An 8" J-tube should produce enough heat to bake a space in 5 hours with temperatures above freezing outside. Also, the bench being just maybe 10-12 cm thick, it does not have much mass and will cool quickly.

I agree that the absence of insulation will allow too much heat to be lost from the core, lowering combustion efficiency. It would be interesting to know how much soot there is in the channels...

The very small copper tubing would be an obvious culprit for lack of water heating, except for the pump in the circuit. Tubing of that size would not thermosiphon well by itself. How hot is water coming out of the coil when the fire is going well? Is the coil one run, or multiple in parallel? That length of small tubing would have a lot of friction in one run.

Consideration: since I do not have an insulated riser, the combustion is not completely complete, it produces a bit of ash and creosote that over time (it will have burned for 80 hours now) have accumulated on the copper coil (I read in a book that 1mm of soot reduces efficiency by 6%) this would explain why during the first ignitions the water accumulation heated up more quickly in addition to reaching higher temperatures (80° maximum while now I reach a maximum of 60°)
What do you think?
Here is a video recorded with the phone entering from the inspection vent
https://youtube.com/shorts/Gx8m9X4I_XA?si=_kaqQgXymiA-HCMH

thomas rubino wrote:Hi Cedric;
I have not yet had time to read your post properly.
But regarding your last question.
Insulate the riser for sure.
I'm not sure what you mean by the barrel, but if it is easy then give it a try.

I mean, to isolate the riser I would just have to open the barrel from the top, while to isolate the whole j I would have to disassemble the whole rocket

Sorry to hear that it's not performing to your expectations. I don't really have any answers, just some observations and questions:

The firebrick riser will mean that it takes longer to reach optimum temperature from startup, but once you get there it should be fine. Is there insulation around the riser?

Have you opened it up to inspect things? Any deposits of ash indicating constrictions in the flow path? Dark stains, soot or creosote would be a definite sign of trouble.

The exhaust path is quite long and friction may be impeding the draw through the system. You mention flames back burning up the fuel in the feed tube...this is not good and sounds to me like it's not pulling well.

The heat exchanger for the hot water will pull a lot of energy out of the system before the exhaust gases reach the mass, especially after you take a shower. This may also slow down the draft and inhibit performance.

We have some senior rocket scientists in the crowd and I'm sure they'll be along shortly to help diagnose the trouble.

Even just insulating the riser will improve combustion significantly. If you do that and it makes a difference, great. If not, then you will need to take off the shell around the feed and burn tunnel.

A well-burning J-tube should emit no significant amount of creosote, so you definitely need to let it get hotter through insulating the outside. You might also want to partly cover the feed tube to restrict airflow, allowing the combusting gases to stay in the core longer and burn more completely. Your feed should in any case be tall enough that the fuel can be covered and airflow closed completely, for safety reasons in case something goes wrong. This will take care of the issue with the fire climbing the fuel and smoking into the room.

Glenn Herbert wrote:The bench cross section is not large enough for stratification, it will all move through the channel as one flow. This is essentially a square brick duct. What is the surface temperature of the brick channel, both from the barrel exit and upstairs?
For such a long burn, there ought to be a ton of heat being released somewhere from the system, and not much could be getting up the chimney. An 8" J-tube should produce enough heat to bake a space in 5 hours with temperatures above freezing outside. Also, the bench being just maybe 10-12 cm thick, it does not have much mass and will cool quickly.
.

unfortunately I don't have a thermometer at the moment, I'll try to explain my feelings, after 3 hours of ignition the bench is pleasantly warm, after 5 hours you can still sit down, the exit pipe of the bench after 5 hours you can still touch it with your hands for a few seconds. instead the lower level becomes nice and warm, you can wear a t-shirt with an outside temperature of 10 degrees

Glenn Herbert wrote:I agree that the absence of insulation will allow too much heat to be lost from the core, lowering combustion efficiency. It would be interesting to know how much soot there is in the channels... .

On my YouTube channel I posted a video of what you can see from the inspection port near the barrel and another video of the inside of the bench in the last section

Glenn Herbert wrote:The very small copper tubing would be an obvious culprit for lack of water heating, except for the pump in the circuit. Tubing of that size would not thermosiphon well by itself. How hot is water coming out of the coil when the fire is going well? Is the coil one run, or multiple in parallel? That length of small tubing would have a lot of friction in one run.

the coil is made up of 4 pipes in parallel.
when turned on about 30° after 5 hours the temperature of the water exiting the coil was 80°.

Glenn Herbert wrote:

A well-burning J-tube should emit no significant amount of creosote, so you definitely need to let it get hotter through insulating the outside. You might also want to partly cover the feed tube to restrict airflow, allowing the combusting gases to stay in the core longer and burn more completely. Your feed should in any case be tall enough that the fuel can be covered and airflow closed completely, for safety reasons in case something goes wrong. This will take care of the issue with the fire climbing the fuel and smoking into the room.

I am currently burning wood with double the length of the feed, I will cut the wood and do some tests by partially closing the entrance with a brick.
today I also added a p channel

Phil Stevens wrote:

The firebrick riser will mean that it takes longer to reach optimum temperature from startup, but once you get there it should be fine. Is there insulation around the riser?

Riser not insulated

Phil Stevens wrote:Have you opened it up to inspect things? Any deposits of ash indicating constrictions in the flow path? Dark stains, soot or creosote would be a definite sign of trouble.[/quote

The Wall of bench are black, see the video on my YouTube Channel
https://youtube.com/@cedricnoussan4893?si=-f-OAzjMYP1UxkoE

It sounds like you have good strong draft, so I think you may be able to make the feed a bit taller, maybe 5 or 10 cm. I would try that with some bricks set in place with mud to stabilize them. If it still works and you can partially cover the feed, you will get much better combustion. I run my J-tube with the feed around 80% covered all the time. The whole feed and burn tunnel becomes an inferno.

My armchair diagnosis is the uninsulated riser is the biggest issue. Incomplete combustion is hampering the efficiency of the system and the soot on the interior is an indicator.