Why do gravity fed central heating systems need a header tank ?
(i.e. why can't the cold main be plumbed direct to the cylinder)
The only thing I can think of is that maybe the rads are kept topped up from it, but don't some gravity fed systems have a dedicated header tank for the rads ?
I'm sure i'm missing something bleedin' obvious, just can't figure out what it is at the moment :-}
Somewhere for water to go when it gets hot and expands. A method for stopping the water in the heating circuit mixing with the cold water supply.
"Colin Wilson" wrote in message news: snipped-for-privacy@news.motzarella.org...
To keep the boiler and the radiators full of water
They can if you have a filling loop and a proper pressure tank in the boiler.
Yes they do.
Also, things like radiators and most boilers are not designed for the water pressure mains water can go up to.
Colin Wilson coughed up some electrons that declared:
The hot water in the cylinder needs to expand somewhere whilst heating, and with a vented system (which I assume is what you mean) this is the header tank.
Cheers
Tim
When water heats up it expands - so needs a header tank to catch the water which does this. Obviously the water in your heating system isn't potable so needs a dedicated tank for this.
The modern way is no header tank but a sealed expansion vessel.
We do things different here in NZ. The tiny amount of water from the expansion just goes through a pressure valve and to a pipe poking out the wall to the outside, where it drips on the ground. No header tank needed up in the ceiling where it goes wrong and leaks through the ceiling. No cockroaches and dead rats or mosquito larvae in the cistern.
I'd guess you have an expansion vessel, then, somewhere - often inside the boiler casing. So the same as is done here.
You can't afford a cover for a tank? And allow rats in your house?
That seems to be common in much of the world, but the water companies objected to it here on the grounds of wasted water. The fact that it's probably 0.000001% of the water they waste in leaks didn't seem to be a factor in the reasoning. (There is another factor -- the continued gradual replacement of the water in the system is a cause of internal corrosion, although most sealed systems here don't seemed to be sealed well enough that they don't need occasional topping up.)
Colin Wilson submitted this idea :
Think 'pressure' and 'segregation'. You should have an header for the radiators, because of both the pressure and to keep the heavily contaminated water separate. You should also have an header for the HW system - this again is so that it can run at lower pressure. Your hot water cylinder will not be capable of coping with mains pressure.
Your actually missing quite a lot!
You talk about gravity fed central heating systems, and hot water cylinders in more or less the same breath - whereas they are, fact TWO separate (albeit linked) systems.
The PRIMARY system consists of the water which is heated by the boiler and goes to the radiators - and which also heats the domestic hot water by means of a heat exchanger (coil of pipe) INSIDE the hot water cylinder - so that it never mixes with the water which comes out of the hot taps.
The SECONDARY system is the domestic water which is heated INDIRECTLY by the primary system, and which feeds the hot taps.
Conventionally, they each have a separate header tank - a large one for the DHW and a small one for the primary circuit to allow for expansion and to top up to compensate for evaporation and small leaks.
The primary system is - in effect - a sealed system with the same water going round and round all the time. It usually has inhibitor in it to prevent corrosion. It can actually be gravity fed - with a small fill and expansion tank in the loft - or it can be truly a sealed system, in which case there's an expansion vessel rather than a F&E tank.
The secondary system is *usually* gravity fed - and has a large header tank which feeds cold water in to expel the hot water when a tap is opened, and which also caters for expansion as the water heats up. An ordinary cylinder would burst if subjected to mains pressure.
There are, however, cylinders[1] which are specially designed to withstand mains pressure and which do indeed have a direct mains-pressure cold feed, and no header tank. These have to be installed by people who are suitably qualified (not a DIY job!) and contain an air bubble which allows the water to expand when it gets hot.
HTH!
[1] Usually referred to as "unvented cylinders"
What happens when the sysem cools? Does it go to below atmospheric pressure inside the pipes and rads?
Robert
It does, thanks :-)
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