On Sunday, June 26, 2022 at 7:27:21 PM UTC-4, Peabody wrote:
> Marilyn Manson says...
> > As far as the "numbers" inside the tank, assuming that
> > you have closed system (check valve at meter, perhaps)
> > the pressure will basically be the same as the
> > then-current system pressure.
> Well, I'm not sure about that. I found this link that does
> go through the numbers.
>
>
https://rogergriffith.com/thermal-expansion-tanks-part-2/
>
> The size of the expansion tank affects how much the pressure
> increases when the water is fully hot. But it's not going
> to be the same as normal system pressure. It will be higher
> that that, depending on the precharge pressure and the size
> of the expansion tank.
I didn't say that it would be the same as the "normal system
pressure". I said the "then-current system pressure".
You seemed to have been wondering what the internal pressure of
the tank might be after the WH recovers. You said "If the water
heating up causes the tank to become half filled with water,
what is the pressure at that point? Is it 120psi because the
air now occupies half the volume it did before?"
What I am saying is that the pressure in the tank will always be the
same as the *current* pressure throughout the system. You can test
that at any fixture within the system. I test at my utility sink since it
has faucet with hose threads, making it easy to put a pressure tester
on it.
That's the whole point of an expansion tank and the importance of
proper pre-charging. The expanding water causes the baffle to
compress the air, thereby essentially "enlarging" the system. The
same amount of water in a larger system means lower pressure
throughout.
Can I tell you how/if it will work with your system? No. All I can tell
you that the addition of the tank in my house reduced the increase
in pressure from 90+ after recovery to about 75. That's good enough
for me.
>
> The thing is, in an open system with no regulator, an
> expansion tank provides no benefit. The pressure will
> always be at the City pressure. I think the regulator
> changes that, and I suspect requires an expansion tank.
It may or it may not. You can only determine that by testing the
pressure. There are other ways to eliminate excess pressure, such
as air chambers - vertical stubs of standard copper pipe that are
filled with air. Unfortunately, they do not have baffles and can get
filled with water over time, eliminating their effectiveness. Draining
and refilling the entire system can often solve the waterlogging
issue.
BTW...just as an aside, the Watts gauge that is shown in the expansion
tank video that I linked to can be somewhat deceiving. You'll notice the
red needle that is supposed to retain the highest pressure that the system
reached. That needle can show a ridiculously high false reading if you
have any water hammer. Here's the issue:
The red needle is "freewheeling" meaning that it is pretty loose on the shaft.
That allows it to be pushed up by the black needle. There is just enough
resistance to hold it in position after the black needle "recedes". However,
there is not enough resistance to stop it from being *thrown* upwards
when water hammer causes the black needle to swing violently. IOW, the
black needle may jump from 60 to 80 when water hammer occurs. When
the black needle hits the red needle, the red needle keeps going past 80
like a ball off of a bat. I watched my black needle jump to 80 psi and the
red needle shot to over 110. That can be very deceiving if you aren't
watching (like I wasn't) when the water hammer occurs. You walk up to
the gauge and see a max pressure of 110+ and think that system is
as spiking to 110+ which can be quite concerning.