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pipe size and flow rate.

#1
12-07-08, 06:38 PM
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Join Date: Sep 2008
Location: new york
Posts: 109
pipe size and flow rate.

Probably a stupid question but how would I go about figuring out flow rate and pipe size(near boiler piping) for my new BB sys? I have 45' of 3/4 fin tube baseboard on one zone and 14' on the second zone. There will also be, in the near future, a 40-50 gal. indirect. Been crunching numbers like Xiph suggested in regards to outside temps and supply temps and have used 1 gpm flow in the BB. Is this correct or should I be using 4 gpm?
Been reading primary and secondary piping by Dan Holohan but cant seem to find anything on determining flow rate, which in turn would dictate pipe size. He talks about it but doesnt address how to get it. I'm sure that odr may effect flow and/or pipe size also?

#2
12-07-08, 09:10 PM
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Join Date: Jun 2008
Location: S.E. Texas
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Here is a page that might be of some help to you.

Pipe flow stuff.

Travis

#3
12-08-08, 01:26 AM
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Join Date: Jan 2006
Location: San Jose
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Not sure that you don't already know this...

The flow should be typically 2 to 4 fps. If faster the flowing water will make noise and if slower it won't clear air bubbles where pipes travel downwards. The GPM is a function of the piping size and velocity.

#4
12-08-08, 06:02 AM
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Join Date: Nov 2005
Location: USA
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If you're going with p/s, the boiler loop is generally full-sized to match the tappings on the boiler (say 1 - 1.25"). The flow rate is whatever the boiler manufacturer says it should be, with a circulator sized accordingly.

The baseboard will be 3/4", typically. Conventional wisdom is a flow rate of 2-4 ft/sec (which in 3/4" is 3.2-6.5 gpm) to keep entrained air moving to the elimination point. However...

You can play with the design temperature drop of the circuit and go higher or lower than the standard 20F, with slower flow resulting in more temp drop, obviously. With good piping practices (e.g., pumping away) you can have an air-free system that flows slowly if that's what the design calls for. Or if you size the radiation on the end(s) of the circuit(s) with more finned element to compensate for the cooler water.

#5
12-09-08, 09:19 AM
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Ok, so if this is the case what happens to the btu ratings listed at 1gpm and 4gpm for the bb? Do I need to recalculate supply temps? Do I just guesstimate what it is and hope for the best. Is the flow dependent on circ. size or vice versa and how do I get what rate it should be at?
Originally Posted by Who
Not sure that you don't already know this...

The flow should be typically 2 to 4 fps. If faster the flowing water will make noise and if slower it won't clear air bubbles where pipes travel downwards. The GPM is a function of the piping size and velocity.

#6
12-09-08, 11:09 AM
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Join Date: Nov 2005
Location: USA
Posts: 2,458
Originally Posted by johndl58
Ok, so if this is the case what happens to the btu ratings listed at 1gpm and 4gpm for the bb?
Very little happens. Flow rate does not exert a strong influence on output. Take Slantfin FineLine 30, for example. Rating at 180F water, 1gpm is 580, 4gpm is 610. That's only 5% difference in output, but a 4x greater flow rate.

See the link in a post above to the Taco Selecting Circulators tech doc. Most of what you need to know to figure out head, flow, what circ, etc.

#7
12-09-08, 01:01 PM
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Join Date: Sep 2008
Location: new york
Posts: 109
I gotta search around, that link isnt working