CH expansion numpty questions...

JOOI, what sort of arrangement does central heating normally have to allow for expansion of heated water? I keep finding CH diagrams that show no expansion system at all, but I don't see how it doesn't need *something* (either an open tank or perhaps some kind of sealed tank with a diaphragm and vent to atmosphere) - I'm not sure if they're just bad diagrams or I'm missing something :-)
For open tanks, is it just the height that keeps the heated water from overflowing the tank rather than flowing around the rad circuit?
cheers
Jules
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In an earlier contribution to this discussion,

CH systems are either vented or unvented (pressurised).
Vented systems have a fill and expansion tank *higher* than the highest part of the circulation circuit. The level is maintained by a ball valve - which is set to give a static level of only 2 or 3 inches in the bottom of the tank, so that there's plenty of room for expansion.
Unvented systems have an expansion/pressure vessel with (as you suggest) a diaphragm - with air on one side and water on the other. As the water in the CH system expands as it gets hot, the air in the expansion vessel is compressed a bit - increasing the system pressure, but not by anything like as much as it would increase if there were no resilience.
Simple schematic diagrams tend to omit gory details like this!
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Cheers,
Roger
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Since the advent of system boilers - ie everything including pressure vessel in one case - they may just show it as a box. Since what's inside it may not be of interest - merely the connections to it. But an open vent diagram usually shows the header tank since that has to be installed along with the pipes and rads.

Yes. Unless the circulation pump is set to too high a speed. But the tank won't overflow since hardly any water is being added to the circuit.
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*When cheese gets its picture taken, what does it say? *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Thu, 10 Dec 2009 18:44:40 +0000, Dave Plowman (News) wrote:

Aha, OK... I have an (_extremely_ theoretical at this stage!) plan to see if I can couple some rads up to an outdoor wood-burning furnace.
Very early days, because it seems hard to get details of such furnaces* - but they all appear to operate by heating water which then gets pumped through a heat exchanger to then heat air in a forced-air system. We have forced-air (propane furnace) for the ground floor of our house, but it'd be almost impossible to add ducts upstairs - hence radiators would be a lot easier up there, and I figure it might be possible to tap into the furnace's water circuit).
* what sort of expansion system they use in the water circuit, what size pipework / flow rates they use, typical water temps at the input and outlet of the heat exchanger etc.
Lots of CH diagrams do seem to show an expansion tank on the hot-water side, but not on the rad circuit (unless they're always coupled within the boiler?) so I wasn't sure how they were coping with thermal expansion.
cheers
Jules
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On Thu, 10 Dec 2009 13:22:31 -0600, Jules wrote:

With a wood burner, or any other solid fuel furnace, the primary loop must run under gravity and not have any valves that can be closed. Otherwise when the power goes you have to shut down the furnace and possibly very quickly to prevent it boiling. There also needs to be some form of heat dump system as well.
With your rads upstairs and the boiler (sorry I just can't type furnace in this context, a furnace is a thing for making iron or steel in...) downstairs you have a nice gravity loop...

I'd go for open vented system with a header tank, it may get noisey if it boils but there is a reduced risk of it exploding. Garvity primarys are normally done in 28mm copper or larger depending on how much heat has to be shifted, flow rate is up to gravity and the pipework layout. Water temps, anything up to boiling point...
Sizing of the boilers output in relation to that required to keep the rooms warm is important. To big a boiler compared to the rads an it'll overheat, to small and it won't keep the rooms warm. Snag is of course that the amount of heat required varies with the weather it takes a lot more to keep a place warm when it's -20C outside and howling gale than just 0C and still.
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Cheers
Dave.




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On Thu, 10 Dec 2009 19:54:37 +0000, Dave Liquorice wrote:

Interesting - I'd got the impression that some of them are pumped (and that some of them have no heat-dump mechanism, some don't have blower fans etc. - it all seems very variable. They seem fiendishly expensive, too, for what they are)

Yeah, US-parlance :-) I think of a furnace as being something large used for heating, a stove for cooking, and the piddly little things some people have in the corners of their rooms are just fireplaces/fires...

Well I did a bit more googling - seems that some of them just have a vent pipe, and you're supposed to top them up with water yourself. Others have an expansion tank - but you're *still* supposed to top them up yourself. I've not found any mention of one with automated top-up from domestic water supply (i.e. float valve in the tank), even though it'd be trivial to do.
I'm not sure what the implications would be of ditching any tank local to the furnace and moving it into the house side (where it's a lot easier to get a water feed to it). Even if it'd be safe to do so, that might have an impact on insurance (insurance companies are a bit wibbly about wodd-derived heat here anyway, and would be even more so with a 'modified' system)

Well, all Imperial this side of the Pond still, so I suspect 3/4" - I just wondered if they used something ridiculously oddball (I'm not sure if one pays for a complete system including furnace, pipework and heat exchanger, or if the latter two are normallyconsidered optional extras)

Yeah, it's been -15F outside here most of the day, which is what, -26C? It ain't very warm, the 'leccy heaters have all been off and the propane setup's been struggling to keep me from freezing...
I suspect if pondering more about a wood furnace route I need to find a few people with setups where I can check what the temperature is downstream of the heat exchanger; I'm not sure if there'd be enough heat left once it's been through there to even run some radiators too.
cheers
Jules
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