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Posted by RayV on November 3, 2006, 6:18 am
Steve Scott wrote:
>
> >
> >Steve Scott wrote:
> >> Where are the fill valves located on the systems you see?
> >
> >At the same level as the boiler inlet or lower.
>
> On the boiler return side? Same side as a lot of circs you're seeing?
> Is the fill valve before the circ? Replace many expansion tanks?
>
> >> You're right about less air problems with the circ on the supply side.
> >> Do you know why that is?
> >
> >Higher pressure from the hotter water raising the boiling point?
>
> The expansion tank is the point of no pressure change in the system.
> The circ causes pressure differential. So, the expansion tank is the
> point of no pressure change and the suction side of the circ is close
> to the tank the circ adds to the static system pressure. That causes
> the pressure to be as high as possible at the highest point in the
> system.
>
> If the circ is on the return side and the expansion tank is on the
> supply side the highest points of the loops can be under negative
> pressure causing air to migrate into the water...even from water tight
> solder joints.
>
> >> Where should the fill valve be?
> >
> >Close to the boiler inlet.
> >
> >> Why?
> >
> >Because it is easier to run the piping?
>
> The fill valve should be near the expansion tank because it's the
> point of no pressure change.
>
> What we all commonly see is the expansion tank of the supply side, the
> circ on the return side (cause the boiler manufacturer stuck it there)
> and the fill on the suction side of the circ. This is a guaranteed
> revenue stream as you will be replacing the expansion tanks regularly
> and maybe the relief and fill valves trying to figure out what the
> heck is going on.
>
> The problem is this:
> Fill valve and tank factory set at 12psi. Who changes them?
> Virtually no one.
>
> So the circ is off and the system is full there's 12psi at the fill
> and at the expansion tank.
>
> Let's assume the circ creates 6psi differential. The expansion tank
> is the point of no pressure change so that stays at 12psi. There's
> maybe 1psi loss through the boiler so the outlet side of the circ is
> at 13psi (12psi at the tank + 1psi through the boiler). The circ
> creates 6 psi differential so the suction side is 7psi. Which is what
> the fill valve is seeing cause it's on the suction side of the circ.
>
> Wait a minute. The fill valve is set at 12psi and now it's seeing
> 7psi? What's it gonna do?
>
> Add water?
>
> Yep.
>
> Where's the water go?
>
> The only place it can go is the expansion tank. The rest of the
> system is already full.
>
> What happens when the expansion tank has too much water in it?
>
> The diaphragm ruptures.
>
> And that's the short version of why in a correctly piped system the
> expansion tank goes in the supply near the boiler with the feed near
> the expansion tank and the circ right after the tank. With very few
> exceptions.
>
Thanks for the explanation. Easy to understand and makes perfect sense
the way you explained it. A welcome difference from the way most posts
are written.
I'm going to disconnect the pump on the return side of his system and
run the zones to see if that cures the problem of non-calling zones
always getting hot.
Thanks again.
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