# Log burner on Gravity

The horizontals can be ignored as they do not contribute to the convection.

They would not be horizontal, everything contributes toward convection.

How would the top 1/3rd of the system get hot if the water is not leaving the stove?
If its leaving the stove what replaces it?
Water being heated by the boiler.
Where is that water going?
Into the circuit and starts cooling.
This will continue, it wont just stop
Why have dunsley put that vertical loop in?
To start convection even in the event of a powerfailure, not to see how long the aav will last before it turns into a steam whistle.

As soon as the water leaves the stove it is cooling.

The hottest part of a system is not the top pipework, its in the boiler.

As I posted before, why do you drop a return pipe down low before you connect to a solid fuel appliance?
To help convection.

They would not be horizontal, everything contributes toward convection.

If the 'horizontals' were in fact on a tilt, only the vertical distance would contribute to the convection. Since this distance would have to be 'taken off' one of the verticals, it makes no difference to the overall effect.

How would the top 1/3rd of the system get hot if the water is not leaving the stove?
If its leaving the stove what replaces it?

My drawing assumed that the system was already hot. If starting from cold, the hot water would rise from the stove and flow across to the 'downward' column, until the weight of the columns is equal. At this point convection would cease and the water in the top loop would continue to rise in temperature until it boiled.
Water being heated by the boiler.
Where is that water going?
Into the circuit and starts cooling.
This will continue, it wont just stop

It will continue until the weight of the columns is equal. Then it will stop.

If you put a huge heat leak near the top of the circuit, you might be able to get some heat into the cylinder. It would be hugely inefficient though.

It will continue until the weight of the columns is equal. Then it will stop.

But it will never be equal because the water on the up leg of the flow will always be lighter then the water on the down leg.

You need to consider the return as well

IThe left hand vertical is 2/3 hot and 1/3 cool. The right hand vertical is 1/3rd hot and 2/3 cool. Therefore the net temperature on the left is more than the net temperature on the right. In this instance the convection would be clockwise in my diagram, contrary to the intended outcome. It won't work. All that will happen is the water in the upper loop will eventually boil.

If you replaced the stove by a pump you would be quite happy that the water would circulate. The pump providing the energy. If you have the stove there, and it is hot, it will provide the energy to cause circulation. I agree, the driving effort would be larger if the stove was at the bottom of the leg, but the water will still circulate in the position you have shown it.

The hot water only rises becase the cold water is denser and muscles its way in at the bottom. Once the 'downward' column becomes hot, its weight will decrease until it is in equilibrium with the 'upward' column (there still being a load of cold and heavier water in the upward return pipe to the stove). The thermosyphon will cease at this point.

I would suggest that the water will (slowly) circulate, but the heat will not.

mogget wrote

The thermosyphon will cease at this point

If it ceases then why has Dunsley used a similar arrangement as a safety feature to dissipate heat? I suspect the decending pipe will be left uninsulated to decrease the retarding influence this arrangement will have on gravity circulation.

That's just where the circulation theory falls down - the stove is NOT A PUMP - it does not impart any motive force to the water.
If you weigh up the water in the up and down pipes as someone said, circulation will not happen - colder water will not move up to replace hotter water in the stove.

If it ceases then why has Dunsley used a similar arrangement as a safety feature to dissipate heat?

I'm not familiar with the Dunsley products - could you post a link so I could have a look?

It would appear that the loop into the loft space kick-starts gravity circulation through the radiators in case of a power cut etc.

However the emitters (rads) are still above the source of the heat (solid fuel stove). If this weren't the case then there would be no circulation.

mogget wrote

If this weren't the case then there would be no circulation

How do you know?

Does plumbing break the laws of physics?

Probably not but then you probably don't have a full understanding of what is happening within the system to begin with so are unqualified to comment.

There seems to be a lot of misunderstanding of gravity circulation going on here!

Expressed simply there will be some gravity circulation BUT its only caused by the loop from the stove to the top of the circuit.

The lower floor below the boiler will not contribute positively to the gravity circulation and will significantly reduce the gravity flow by adding considerable flow resistance.

When I tried some calculations to see exactly what the effective gravity head was on a two story building it was just a head of about 10-20 cm.

Its that very small head that makes the gravity circuits on old cylinders 28 mm with a heat output of just about 3 kW.

My conclusion is that there will be some gravity circulation but probably not enough to transfer much usable heat.

Tony

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