I have noticed that when the boiler fires up, the vent pipe from the top of the HW cylinder gets hot most of the way back to the header tank in the attic. I'm surprised that around 90 or 100 litres of cool water can expand enough to fill a 15mm pipe for three or four metres. What is the coefficient of expansion of water?
How much expansion to expect when water heats up?
- Thread starter RigidRaider
- Start date
About 4% of the volume ISTR. It expands up the cold feed into the expansion tank, the level in the tank rises and the level in the open vent will be roughly the same (excluding any difference in static head between cold feed and open vent due to frictional resistance &/or the pump head).
The heating is due to convection within the vent pipe.
The pipe could be getting hot due to conduction. not because of water in it.
17 % is what I recall from my apprentice days but that was a long time agoThinking about it think that is the figure for raising the temp of water to steam,i.e. the amount of expansion.
Nope, the expansion as a result of water turning to steam is 1600% (one thousand six hundred - that's not a typo). That is why it is vitally important that pressurised systems have the necessary safety devices fitted, and why unvented hot water cylinders without them demolish houses.
Did nobody notice this?
A 100 litre cylinder of cold water, heated to about 60 degrees, will expand about 3%. If all this expansion was contained within a 15mm vent pipe, the pipe would have to be about 19 metres long, or 9.5 metres if a 22mm pipe is used! Now, as vent pipes are not usually this long, the expansion must be contained within the cylinder and the cold capacity will be approx 97 litres. Presumably the water level in the vent pipe will rise until it is the same as that in the cold water tank feeding the HW cylinder - like the water level in the CH vent is the same as the water level in the F/E tank.
He is not talking about the vent pipe for the CH system.
A 100 litre cylinder of cold water, heated to about 60 degrees, will expand about 3%. If all this expansion was contained within a 15mm vent pipe, the pipe would have to be about 19 metres long, or 9.5 metres if a 22mm pipe is used! Now, as vent pipes are not usually this long, the expansion must be contained within the cylinder and the cold capacity will be approx 97 litres. Presumably the water level in the vent pipe will rise until it is the same as that in the cold water tank feeding the HW cylinder - like the water level in the CH vent is the same as the water level in the F/E tank.
The vent pipe will be full of water before you start anyway!
The water level in the vent will be the same as the level in the cold water storage tank. The cold feed, hot cylinder and it's vent pipe behave just like a 'U' tube.
The vent is raised above the cold storage water level to stop any thermosyphonic circulation. This would be driven by the decrease in density of heated water, which would allow the water to rise up the vent a little higher than the stored water level.
Most of the water expansion normally finds it's way back up the cold feed to the HW cylinder into the CWST. The cold feed pipe remains cold, because the bottom few inches of the hot cylinder usually stays quite cool.
Single pipe convection can occur, where hot water rises up the centre of a straight vent pipe, and falls back down again against the pipe walls as it cools slightly. A horizontal offset at the bottom of the vent is usually enough to stop this happening.
Then we are left with conduction. Water is quite a poor heat conductor, but copper pipe is a good conductor, so eventually the pipe will be warmed up by conduction because one end is fixed onto to the hot cylinder.
The water level in the vent will be the same as the level in the cold water storage tank. The cold feed, hot cylinder and it's vent pipe behave just like a 'U' tube.
The vent is raised above the cold storage water level to stop any thermosyphonic circulation. This would be driven by the decrease in density of heated water, which would allow the water to rise up the vent a little higher than the stored water level.
Most of the water expansion normally finds it's way back up the cold feed to the HW cylinder into the CWST. The cold feed pipe remains cold, because the bottom few inches of the hot cylinder usually stays quite cool.
Single pipe convection can occur, where hot water rises up the centre of a straight vent pipe, and falls back down again against the pipe walls as it cools slightly. A horizontal offset at the bottom of the vent is usually enough to stop this happening.
Then we are left with conduction. Water is quite a poor heat conductor, but copper pipe is a good conductor, so eventually the pipe will be warmed up by conduction because one end is fixed onto to the hot cylinder.
Of course it is - now why didn't I think of that?
Can't follow that argument! Hot water rises, not falls. So why would it expand back into the CWST?You said:
Which is what I said in my first post!
Going back to the 'U' tube model.... Both ends of the 'U' are open to the atmosphere. For the water to rise up the expansion pipe, there would need to be a corresponding increase in pressure in the cylinder to support the column of water in the expansion pipe. The pressure in the cylinder would then be higher than the static head from the CWST, so there must be a flow from the cylinder back towards the CWST to balance the pressures. As the cross-sectional area of the expansion pipe is only a small fraction of the area of the CWST, most of the expansion will go back up the cold feed.
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