Pumps.......er, circulators.

Alan (California Radiant) Forbes

I ran into my first relatively large pump on a residential hydronic system; a Grundfos UPS 43-44 BFC. It's on a Hydrotherm boiler with 135K BTU output serving finned tube recessed convectors. The pump head has got to be relatively low and all that's necessary on the flow is 13.5 gpm at a 20°ΔT.

So, why this pump when a 15-58 would do?

I also have another question. If you have two pumps; one is a high head pump and the other is a high volume pump, how are they different? The high head pump has a larger motor? The larger volume pump has a larger impeller to throw more water?

hot_rod

good question on the pump sizing. Does that model have gauge taps that you could determine what is actually doing?
The flanges are larger on the 43 series than the 15-58 so it would not be easy to size down.

High head have thin, large diameter impellers. Low head wide, small diameter.

RPM might be different also.

jumper

Not even on big jobs do you see a contractor determining system curve. At best designer calculates head loss for desired flow and "chooses" circulator. Usually fudge factor leads to way over powered. Better to err, in my opinion, the other way for heating. For chilled water system situation is more difficult.

Jamie Hall

On the difference in high head vs. high volume -- it's in the impeller and diffuser geometry. Quite possible to have two very different pumps in terms of head/flow relationships operating with the same horsepower motor. The diffuser -- in the body of the pump -- is often overlooked, but it is the section of the pump which trades velocity for pressure, so it's critical.

Jamie Hall

jumper said:

Not even on big jobs do you see a contractor determining system curve. At best designer calculates head loss for desired flow and "chooses" circulator. Usually fudge factor leads to way over powered. Better to err, in my opinion, the other way for heating. For chilled water system situation is more difficult.

Um. Well, I can't count the number of jobs I've had to determine the head demand curve of a system, and then select a pump or pumps appropriate to that. But that was bigger jobs...

EBEBRATT-Ed

@Alan (California Radiant) Forbes

Increasing the width (thickness) of the impeller gives you more water flow but at the same head as the skinnier (thinner) impeller if they are the same OD.

Increasing the diameter of the impeller (same thickness) gives you more water flow and more head

jumper

Jamie Hall said:

jumper said:

Not even on big jobs do you see a contractor determining system curve. At best designer calculates head loss for desired flow and "chooses" circulator. Usually fudge factor leads to way over powered. Better to err, in my opinion, the other way for heating. For chilled water system situation is more difficult.

Um. Well, I can't count the number of jobs I've had to determine the head demand curve of a system, and then select a pump or pumps appropriate to that. But that was bigger jobs...

I'm pleased to hear that, misterJamie. Pump salespersons tell me that many customers do not, even for bigger sizes. They replace pump with previous size that probably (in my opinion) destroyed itself from being oversized. Even worse, some technicians suppose that pumps fail from being too small (maybe mix up with motors?).

Task is not so difficult that one should forgo even for smaller sizes. If existing pump is working one can measure pressure difference across pump and read flow from manufacturer's pump curve. A single point on head demand curve indicates entire curve (a second point is zero zero).