No idea. It's one of the great mysteries of life, right up there with
where do all the missing socks go....
I mean, all such systems have backflow preventer, so even if the
boiler pressure were higher than the feed pressure the water could not
go out that way. And even if it did, how does air get in to take its
place? And leaks usually make themselves known, unless you've got
underground piping. There is dissolved air in the water that probably
separates out over time, but I find it hard to imagine that it is
enough to explain the quantity of air that seems to be in there each
Now if the expansion tank is the type that doesn't have a bladder
(most older ones don't), the air in there can dissolve slowly into the
water, but with no circulation going on, I don't see how it would end
up migrating elsewhere in the system.
So, I got nothin...maybe someone smarter than me knows the answer.
On Sun, 09 Oct 2005 20:06:22 -0400, Paul Franklin wrote:
Crap! We know where they go; into the washer zone. ;-)
.... and why does it stay there? This year I did considerable work on the
plumbing (a leak and rotted subflooring in the upstairs bathroom convinced
me to take a goo look), but amazingly the air wasn't as bad as it has been
in the past. ...search me! ;-\\
Exactly. My house usually sounds like Niagra Falls the first time I
request heat. I replaced a bleeder underneath this desk last year because
it was noisy here (and I stripped the damned thing trying to open it).
Hmm, I wonder where I put the key?
The house/furnace is ~20YO, and the expansion tank, perhaps 10 (I replaced
it since I moved here in '93).
Yeah, I'm stumped too. ...although I did turn the heat on tonight
(it's been in the 30s-50s for several days) and it doesn't seem so bad.
I'll let it gurgle a while and then bleed the upstairs, if I can find the
silly key. ;-)
Not full of air sure, but it sounds like a waterfall and there is considerable
air in there. Why would the expansion tank allow air in? The water that's
there would have to be displaced. Where?
I'm not sure there is a back-flow preventer, but there is a pressure reduction
fill valve. Since the municipal pressure is above 60psi and the furnace at
14psi, I can't imagine water flowing up that "hill".
Were is the energy coming from to separate the oxygen from the hydrogen. It
would truly be scarry if there was free hydrogen and oxygen in the furnace!
I believe that getting all the air out requires some sort of black
magic. One good source of information is here:
which has a forum for posting. Dan is a proponent of the "pumping
away" design, which you can probably find among the many books he has
for sale. In essence, this design says that most older systems are
installed wrong with the circ pump in the wrong place (i.e. pushing
water into the boiler, vice pulling it out). Bladder-lined tanks are
a must, and perhaps a Spirotherm vent
Got an estimate to converrt my very old, cast iron ("it will be there
when the house falls down") Brunham boiler to this design of about
$1500, including the tank, rerouting the piping and plus the
My major concern is when you start wrenching on the major arteries of
a system that old that something is likely to let go in the innards.
We are going to start with the vent and the tank to see what happens.
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