Posted by Solar Flare on November 3, 2005, 10:46 pm
Grundfos model number?
How is the speed controlled? I thought most pumps were synchronous.
Posted by daestrom on November 4, 2005, 11:37 pm
As I understand them, a typical drain-back system has a check-valve at the
top that acts as a vacumn breaker. It isn't in the direct flow path, but is
on a tee at the high point to let air in when the pump is off, and stop
water from flowing out the high-point vent when the pump is running.
In such a system, the pressure at the top of the system is above atmospheric
when the pump is running. If the flow drops because of a clogged inlet
strainer, then the pressure at the top falls to atmospheric and you get
water/air combination in the return line.
So the return line can't siphon water 'up' the supply line, it just sucks
air in via the check-valve. The pump is always pumping water up the full
height to the collector outlet.
Posted by Iain McClatchie on November 5, 2005, 3:51 am
Daestrom> As I understand them, a typical drain-back system has
Daestrom> a check-valve at the top that acts as a vacumn breaker.
Does anyone bother with solenoid-actuated vacuum breakers, or
something else that might allow siphon action during normal
operation but still get good drainback?
Posted by Duncan McNiven on November 5, 2005, 6:56 am
On Fri, 04 Nov 2005 23:37:43 GMT, "daestrom"
When the pump starts again, how is the air removed from the system? I
would expect it to accumulate at the top, where the check-valve is,
and the valve would not let it out. Can you get an automatic bleed
valve that lets out air but not water?
Posted by daestrom on November 5, 2005, 1:47 pm
Well, one 'can' get a lot of different things. A trap system with a
ball-float would work well to let the air out at the top of the unit, but
that adds a second valve in a difficult-to-service location.
If the pump sends water into the collector from the bottom, then any air
will be forced up through the collector and down the return line. In a pool
system, that just means a few bubbles from the jet when the pump first
starts up. It is important though that the collectors be on the 'riser'
side, not the 'return' side, so that any small air pockets will not
interfere with them staying flooded when the pump runs.