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?
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!
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.
*When cheese gets its picture taken, what does it say? *
Dave Plowman email@example.com London SW
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.
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.
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
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.
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