circulator sizing questions

Old 07-27-07, 11:16 AM
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circulator sizing questions

I am looking for advice on circulator sizing.

System description: I will be adding 4 radiant floor zones (1/2" pex in wirsbo's joist trak) to an existing piping system with 9 cast iron radiators (2-pipe 1.5" black that send 3/4" black to foirst floor and 1/2" black to second floor.) I intend to install a condensing boiler with outdoor reset. Water supply temp 150degrees at design temp (19degrees).

I intend to zone each radiant floor room separately with their own circulator. I intend to zone the existing piping / radiators with a single circulator.

My calculations show that for the radiant zones, I need the following flow rates (20 degree temp drop): zone1- 1.45 gpm (3 circuits at .48gpm per circuit, each circuit 235' of 1/2" pex). zone2- .415gpm (1 circuit, 120' tubing). zone3- .420gpm (1 circuit, 200' tubing). zone4- .064gpm (1 circuit, 55' tubing).

Question- Zone 4 is a small room with small BTU/hr req'd. That flow rate seems awfully small. Is it acceptable?

Question: How do I properly size the circulator for the existing piping / radiator zone. There are 9 radiators in a two-pipe system with 1.5" black supply and return main lines with branch lines of 3/4" black to first floor radiators and 1/2"black to second floor radiators.

Question: I am guessing that there might be the potential for a large pressure differential between the circulator feeding the existing radiator zone and the radiant floor zones. When the return lines join-up prior to the boiler, will I have a problem with the return pressure of the radiator zone exceeding the maximum head of the smaller circulators? OR, will the 1.5" return line on the radiator zone (which necks down to 1" just prior to the boiler) mean that the return line resistance will be low enough to prevent that problem? I'm guessing that I need to know the answer to the previous question before this one can be answered, unless there are some rules of thumbs that the pros use that I'm not aware of.

Any other recommendations you guys might have from design to execution would be greatly appreciated.

Old 07-28-07, 08:57 PM
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Circulator Sizing

Taco has a lot of very good information on circulators on their web site.

I would suspect both Grundfos & B&G would as well but good luck finding it on B&G's site. Don't know how Grundfos's site is when trying to find charts, etc.
Old 08-08-07, 07:52 AM
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Pump sizing.

Grunfos and Taco make 3 speed circulators ideal for your application. Get the pump, install it and dial to the speed that suits your application. A 10 to 40 degree temperature differential is usually acceptable with 20 being the norm.

Last edited by twelvepole; 08-09-07 at 01:51 PM. Reason: Company name and state edited
Old 08-19-07, 12:00 PM
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Thanks Grady and Badger.

As I continue to do more research, the problem I seem to be having is that the resistance of my zones is so small that the resulting flow rates become really high. The end result being that I don't get the planned temperature drop (and will probably also have flow noise issues as well).

The resistances I have calculated are (actually the Taco rep helped me with) are:

Zone 1- 3 loops at 235ft each at .48 GPM is .0095ft./ft
resistance= 2.23ft resistance at 1.44GPM.

Zone 2- .415gpm at .0063 ft/ft
X 120ft =.756ft. Resistance.

Zone 3- .420GPM at .0063 ft./ft. X 200 ft.=

Zone 4- .06GPM at .0005 ft./ft.X 55 Ft.= .003 ft.

Looking at the Taco and Grundfos charts, it appears to me that ALL of their pumps provide significantly more water velocity than required given the low head resistances calculated. The Taco 003 appears to be the closest.

What am I missing? It is also ENTIRELY possible that I'm making a mountain out of a mole hill.......which is pretty standard for me.

Thanks guys,

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