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SDHW Pump Selection - Page 7

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Posted by DJ on February 4, 2005, 3:13 pm
 
The TD pump I have runs pretty much 24/7, keeping my hydronic system
running.

But yeah, they are pricey. I needed energy efficiency, so I went with
that pump. If electricity isn't an issue, certainly, AC is cheaper.

DJ


Posted by DJ on February 4, 2005, 3:28 pm
 
Sorry, Brian, didn't answer your question. For your purpose, you'd
probably be ok if:

You mount the recirc pump, at the very least, at the bottom of the
supply tank. They will prime if the outlet is non-pressurized, ie, no
"air-lock" in the pump head.
You use the absolute largest inlet pipe size you can put on the pump.
This will help prevent cavitation. And make sure the outlet side, at
least for a short distance, is a pipe size smaller than the inlet size.
Usually the pump outlet fittings force you to do this anyway.
Try to avoid elbows on the inlet side of the pump, recipes for
cavitation. If you must bend, use double 45s.
To avoid cavitation, it is highly advisable to maintain ten pipe
diameter's worth of straight pipe on the inlet of the pipe, ie, if the
inlet is 1", there must be 10 inches of straight pipe going into it. I
commonly see nineties right on the inlet, to keep systems compact.
Don't do this. People that do this think that Reynold's Numbers refer
to Burt's salary ;-).

So yes, that Grundfos would probably work for you.

DJ


Posted by Brian Graham on February 4, 2005, 3:38 pm
 
supply tank. They will prime if the outlet is non-pressurized, ie, no
"air-lock" in the pump head. <<

I can certainly have the line filled to start with. But its obviously not a good
pump then for a drainback system. Any (AC) suggestions?


This will help prevent cavitation. And make sure the outlet side, at
least for a short distance, is a pipe size smaller than the inlet size.
Usually the pump outlet fittings force you to do this anyway.
Try to avoid elbows on the inlet side of the pump, recipes for
cavitation. If you must bend, use double 45s.
To avoid cavitation, it is highly advisable to maintain ten pipe
diameter's worth of straight pipe on the inlet of the pipe, ie, if the
inlet is 1", there must be 10 inches of straight pipe going into it. I
commonly see nineties right on the inlet, to keep systems compact.
Don't do this. People that do this think that Reynold's Numbers refer
to Burt's salary ;-). <<

Thanks for the tip on the pipe sizes, etc. I hadn't heard of Reynold's Numbers.
Time to Google some more!


Again, thanks for responding.
--
Brian

Posted by DJ on February 4, 2005, 4:03 pm
 The drive doesn't matter much, just the head configuration. What you're
looking for is a disc style centripital (erroneously called a
centrifugal) pump.
The reason for that is the flow backwards through the pump has to be
unimpeded when the pump is off.
So nothing with lobes, or rubber fins, or stuff like that.

Alternatively, you can run the drainback with a solenoid on a pump
bypass. Again, not a bad idea. You could plumb in something like a
RedHat solenoid that works on the same voltage as the pump, so that
when the pump ran, the solenoid would be closed, but when the pump
stopped, the solenoid would open the bypass loop, allowing the system
to drain back. I'm sure some drainback system manufacturer must do that
already. Then you could use any pump you wanted, and just chuck a few
more bucks at the solenoid.

DJ


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