Like you, just scratching around for an explanation of that part.
Willis Water Heating System
- Thread starter JohnW2
- Start date
Like you, just scratching around for an explanation of that part.
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It does, but 'so what'?
That's actually very analogous to what we're talking about. Consider a switched-off CH system, with the water all cold and no pumps running. Feed a Willis heater with a source of cold water at the same pressure as that feeding the CH system (i.e. same 'head of water') and connect the output of the Willis heater (which itself was below the top of the radiator) to the top of the radiator. Would any hot water get into the top of the radiator (other than a slight amount by conduction) - and, if so, why?
Kind Regards, John
It's obviously very dangerous to stick my neck out to this extent (since I clearly must be 'missing something') but my inclination is to say that the system (as depicted in the diagram we've been looking at) simply cannot work as described, beyond supplying the small amount of hot water contained within the external heater.
Kind Regards, John
Yes, by simple convention currents.
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Why should 'convection currents' cause hot water in the feed from the heater to 'fall' through the colder water in the radiator?
Kind Regards, John
In the case of the radiator, because of the tubular nature of a radiator, water rising up one tube, will push water down the others.
We agree that the warmer a liquid or gas becomes, the more it will try to rise to the top of the medium. It will therefore rise up from the Willis, then cool some as it get to the pipe leading to the cylinder, then drop down into the cylinder. That process will continue whilst ever the Willis is heated and the cylinder contains cooler water than the Willis. No doubt the process will be assisted, by any localised boiling around the Willis element.
In the situation I described, the entire radiator is filled with cold water, all at the same temperature. Why should any of that cold water 'rise up'?
Only if it cools down to below the temperature of the (cold) water in the cylinder, which is not going to happen. A warm liquid does not 'fall down' through a cooler one.
I'm not just trying to be awkward. It seems to me that neither of us has a clue as to how this could possibly work, so it's natural that we are both trying to find straws to cling onto
Kind Regards, John
I can perfectly and fully understand how heat is passed from the Willis, into the cylinder - that part is quite obvious to me. I am less sure how it works when water is drawn off.
Maybe it's obvious to you - but, as I've said, it remains anything but obvious to me, to the extent that, as I've said, I'm currently very inclined to say that (with an arrangement per the diagram eric posted) "it can't possibly happen".
We seem to be 'opposites', since I have no problem in understanding that, provided that hot water has (somehow) got into the top of the cylinder.
As I said before, with the arrangement we're looking at, if one turns on a tap what 'flows' will be a mixture of what is coming from (the top of) the cylinder and what is coming directly from the Willis heater. IF the former has, somehow, come to be hot water, then what goes to the tap will be a mixture of hot water and hot water - hence hot water
However, as above, I've yet to understand how the water at the top of the cylinder can/does become hot.Kind Regards, John
When the water is heated in the Willis it will become lighter and try to rise - like a bubble.
Assuming the pipe is small enough not to allow convection in it then the only way the hot water can rise is if cold water enters the bottom.
It could come from the storage tank but this would mean the water level in the overflow pipe would have to rise. As this is not possible, then the cold water will have to come from the cylinder - hence the heated water will make its way to the top of the cylinder.
This is a better diagram as the overflow pipe actually looks correct and higher than the water level.
Assuming the pipe is small enough not to allow convection in it then the only way the hot water can rise is if cold water enters the bottom.
It could come from the storage tank but this would mean the water level in the overflow pipe would have to rise. As this is not possible, then the cold water will have to come from the cylinder - hence the heated water will make its way to the top of the cylinder.
This is a better diagram as the overflow pipe actually looks correct and higher than the water level.
The very same water convection system was used by unpumped back boiler systems.
I'm not quite sure what you mean by 'like a bubble' but, yes, it will rise.
I'm not so sure about that assumption. I've just climbed up into my roof space, and the expansion pipe near the tank, 2 floors above my HW cylinder, is almost too hot to touch - presumably due to a combination of convection and conduction.
I don't really get that.
Apart from anything else, if the hot water at the junction between pipe from heater and expansion pipe were to 'rise', it would have to 'rise up' the expansion pipe (probably causing water at the top to overflow into the storage tank), wouldn't it?
Kind Regards, John
Well, a part that is lighter.
I'll wait until you are sure.
It would, but it can't, so it won't.
Seriously, how could it do that without drawing water in at the bottom? Gravity would then maintain the level in the storage tank and overflow pipe.
You have acknowledged that with a simple loop the water would circulate so what difference does the overflow pipe make?
Indeed - I actually once had one. However, the water in the HW cyylinder will only get appreciably hot if the 'hot' feed from the back boiler enters the HW cylinder appreciably below its top, like this
However, if (as in diagram below) the feed from the boiler went to the outlet pipe at the top of the cylinder, although the taps would be able to draw some hot water directly from the boiler (mixed with cold from the cylinder), the water in the cylinder would not get significantly hot, other than (very slowly) by conduction ...
Kind Regards, John
I don't know where the heating pipe enters, because I have never had one, but I'm puzzled as to why it would enter halfway up. I cannot see the purpose at all, but it does demonstrate the principle. You said 'conduction', it is not conduction, rather it is convection - a convection current in water carry the heat to the cylinder.
Your back boiler would none the less, collect the hotter water at the top of the cylinder.
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