Need help - very low temperatures on new heater

Hi everyone,

Me and friend built last weekend new heater. We made it old school way - 20cm/8'' system, firebricks, perlite/cement (1:10 ratio) insulation, heat riser made of firebricks pipes with perlite/cement (1:10) insulation, 2'' distance from riser to the top, no experimenting. WE connected it to the exisiting house chimney which is 17x30cm (which is cca 500cm2). We didn't put thermal mass yet, we wanted to try it first. First fire started without problem, draft is very strong. But soon we noticed that temperature is very low - you could almost hold your hand on the side of the barrel. From the chimney first was going lyellow-white smoke, than white. Temperature on the barrel is ridicoulous - it is about 130°C - 266°F. With totally dry wood. Then we tried to burn thinly chopped hornbeam (it's most caloric wood in our area), and we got 250°C barely, on the top of the barrel.
We burn this rocket for three and half day,and situation is almost the same.
We checked everything, every measure is correct. It is my third RMH I built, so I have some experience.

Is it maybe because the perlite/cement mass isn't dry yet? Here's a picture.

Thanks!

Jacob;
 White smoke is steam , your wood is very dry so no moisture from it.  I can tell you it takes a long while to dry out your cob. The condition and temp. of the home's chimney will have an effect as well.
You are an  experienced rmh builder.  You built to proper specifications. You did no "experimenting" I think your dragon just needs to warm up and dry out, I think she will roar just fine when all is warm.

Thank you very much Thomas! In the meantime, experienced people on the forum in my country said the same :)). I'll report when changes start to occure.
Also, one guy told me that it is possible that firebricks drank a lot of water and need time to vapor. I didn't think about the bricks at all. And they drank a lot of water in our case :).

thomas rubino wrote:Jacob;
 White smoke is steam , your wood is very dry so no moisture from it.  I can tell you it takes a long while to dry out your cob. The condition and temp. of the home's chimney will have an effect as well.
You are an  experienced rmh builder.  You built to proper specifications. You did no "experimenting" I think your dragon just needs to warm up and dry out, I think she will roar just fine when all is warm.

Jacob, two details.

First, 250C° is not that low a temp. Your fire, in a 8 inch is small. Compared to a box stove. And a barrel is 1.86m² of ISA (internal surface area)  This sheds a lot of heat. Plus your heater being wet. I don't think you should worry too much. What is interesting in a rocket. Is the amount of heat recovery. Compared to a normal box stove.  When you have the mass installed.

Then, about your top gap.

You have a CSA (cross sectional area) of the heat riser of about 64 sqin. If i understood well, you made a square heat riser. Then your top gap is  4x8" X2" which is 64 sqin again. Or, due to the boundary layer, your heat riser must be closer to 50 sqin. But still, you are under the recommended 1.5x csa usually accepted. You would be better off "freeing" your draft, i think. by raising your barrel a bit more. At 2.5 or 3 inches.

Thanks for answer, Satamax.
250°C we got barely by burning small chopped hornbeam which would at normal working RMH give 500-800°C. By burning normal pieces of wood (also hornbeam and pine, spruce) on the top of the barrel is 130°C, which is very low. Our heat riser is rounded - made of firebrick 20cm duct and perlite/cement. I'd rather wait for a week of burning, than I'll consider to raise a barrel.

Satamax Antone wrote:Jacob, two details.

First, 250C° is not that low a temp. Your fire, in a 8 inch is small. Compared to a box stove. And a barrel is 1.86m² of ISA (internal surface area)  This sheds a lot of heat. Plus your heater being wet. I don't think you should worry too much. What is interesting in a rocket. Is the amount of heat recovery. Compared to a normal box stove.  When you have the mass installed.

Then, about your top gap.

You have a CSA (cross sectional area) of the heat riser of about 64 sqin. If i understood well, you made a square heat riser. Then your top gap is  4x8" X2" which is 64 sqin again. Or, due to the boundary layer, your heat riser must be closer to 50 sqin. But still, you are under the recommended 1.5x csa usually accepted. You would be better off "freeing" your draft, i think. by raising your barrel a bit more. At 2.5 or 3 inches.

Jacob, trust me, your top gap is too small.

So it's round, then your CSA is 50.26 sqin.

8 inch round circumference, 25.13 inches. X2 for your top gap. 50.26 sqin again; So, considering the direction change, the turbulence created by the direction change, and the friction resulting of this, your top gap is too small.

Remember, the figure is 1.5 times the CSA for the top gap, at least.  So that's 75.39. /25.13  = 3, you need three inches top gap.

To check: 8'' system – P=r2x3,14  =50,24sqin (314cm2);  M=2x3,14rh  so  50,24=2x3,14x4xh  so 50,24/25,12=2''. I don't remember 1.5x factor from the book of Ianto. Is it something new? We were very precise by measuring this gap, it is 2''. It is no problem to expand it. W will do it in a few days. Just tell me please where 1,5 factor comes from, I'm interested. Now I checked, Ianto says it is something between 2'' and 3''.

Satamax Antone wrote:Jacob, trust me, your top gap is too small.

So it's round, then your CSA is 50.26 sqin.

8 inch round circumference, 25.13 inches. X2 for your top gap. 50.26 sqin again; So, considering the direction change, the turbulence created by the direction change, and the friction resulting of this, your top gap is too small.

Remember, the figure is 1.5 times the CSA for the top gap, at least.  So that's 75.39. /25.13  = 3, you need three inches top gap.

Hi Jacob.

Well, that number has been devised using guesswork and experience.

I don't know if you are familiar with fluid dynamics.

But what happens when your gases are exiting your heat riser, is pretty much what happens against one half of this cube.



Plus a sharp direction change, squashing the gases.

You can see the boundary layer here



Here is the turbulence behind a port.



What happens at the top of the  heat riser is as turbulent as this.  Plus the top plate of the barrel squashes everything.

Well, i thought, i have a kind of real life example. Tho, with a 20cm gap.



See the radius of the gases, when these hit the barrel top? That's the reason you need a bigger top gap. If you are really set on keeping it at two inches. There is a trick,  forming the top of the riser, like a trumpet endbell. So your circumference is larger than the one of the heat riser, allowing your top gap to be smaller. But there isn't many people who have tried this. Two or three i am aware of.

I have searched a bit on permies and donkey's forums. I can't find the first instances of the 1.5 figure.

Jacob;  I run mine at 2.5",   there is flex with the removable lid so gap can be 2.25" to 2.75"

I've watched videos. I think I'll widen the gap and on my own heater. It isn't problem because I have removable lid and is perlite-cement only. Maybe this is the cause that my own heater sometimes dones't draw as I wish :).

Satamax Antone wrote:Hi Jacob.

Well, that number has been devised using guesswork and experience.

I don't know if you are familiar with fluid dynamics.

But what happens when your gases are exiting your heat riser, is pretty much what happens against one half of this cube.

Plus a sharp direction change, squashing the gases.

You can see the boundary layer here

Here is the turbulence behind a port.

What happens at the top of the  heat riser is as turbulent as this.  Plus the top plate of the barrel squashes everything.

Well, i thought, i have a kind of real life example. Tho, with a 20cm gap.

See the radius of the gases, when these hit the barrel top? That's the reason you need a bigger top gap. If you are really set on keeping it at two inches. There is a trick,  forming the top of the riser, like a trumpet endbell. So your circumference is larger than the one of the heat riser, allowing your top gap to be smaller. But there isn't many people who have tried this. Two or three i am aware of.

I have searched a bit on permies and donkey's forums. I can't find the first instances of the 1.5 figure.

Thanks Tomas, I'll make it these days :). Also have removable lid so it will be finished in minutes :).

thomas rubino wrote:Jacob;  I run mine at 2.5",   there is flex with the removable lid so gap can be 2.25" to 2.75"

So -  we have been burning two more days, and temperature on the lid of the barrel now is 735°F (390°C), and we are satisfied with it. Obviously it was water.
We didn't catch up to cut riser, but we will.
Thanks a lot lot everyone for a help, I'll post when we make some changes!

It's been cca one month since I shortened my heat riser, I heat for three weeks, and can tell my heater works much better, draft seems stronger, sound is more even, and there's no 'pprrr...prrr' sound.
Thanks a lot! :)

Cool then!