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# Will pumps in parallel really double the flow?

Member Posts: 13
If you open the Grundfos pump example attached by Rich, it looks like two UP 26-96 pumps in parallel will pump at double the flow for the same head.

Open up the example and look at the data.

I am confused if people think this is a myth, it looks like it is true to me based on the Grundfos example.

## Comments

• Member Posts: 19
parallel pumping question

I was always under the impression that identical pumps in parallel will double the flow rate. I recently had a discussion (argument) with an engineer who insisted that this is only partially true and only when the identical pumps have flatter curves.

So for example 2 Grundfofos 26-96s (which have a steep curve of 26 gpm at 31 ft head) will only increase the gpm marginally and definitely will not double the flow. I realize that this is probably a poor example because if you needed only 50 gpm you could just buy the right circulator, however this discussion came about because a snowmelt project with a very high flow rate requirement and I was trying to avoid getting into three-phase pump. Is the engineer right? Will the flow rate double only if I have two flat curve pumps in parallel?? Thanks!
• Member Posts: 998

Hi,

With identical head, two pumps will flow almost twice what one will but for a given fixed piping, the point where you will be on the combined pump curve will not be twice the flow.

Multiply the flow axis of the single pump curve by about 1.9 and that will give you what two pumps in paralles will flow.

Ron

BTW, the 1.9 factor rather than 2.0 is an aproximation to take into account the extra elbows and T's to combine the pumps.
• Member Posts: 1,790
Series and Parallel

This explains it better than I can.

-Andrew
• Member Posts: 104
PUMPS

You just shook my world! I was also told the same as deltat.
• Member Posts: 2
Pumps in parrallel

Great posting. Grundfos' WinCAPS program allows you to see the pump curves with pumps in parrellel or series. You can request the software at www.grundfos.com, select the US website and contact us.

I've attached an example.
• Member Posts: 2
Oops, forgot the atachment!

• Member Posts: 19
I'm amazed!

Doubling the flow must be a common myth because I cant remember anyone ever saying anything different to me. I appreciate the input and will now start to factor the system curve into my designs. Also have to apologize to that engineer!
• Member Posts: 2,322
Simply put...........

They will..............PROVIDED, you have enough pipe to handle the higher flow rate without raising the head past the point of no return.
• Member Posts: 1,790
Double

It's only true if the system curve is flat (no head loss), and there is always head loss.

-Andrew
• Member Posts: 140
In any given piping system

to double the flow, in return you get 4 times the head loss.
It's math. Can't change that. Well maybe with a sharp pencil on paper only.
• Member Posts: 998

If you are designing a system for a particular flow and head, then 2 pumps in parallel will be about the equivilent of a single pump with twice the flow at the same head that the individual pumps were rated at. For example, if your piping needs 30 gpm and has a head of 6 feet at that flow, you could use 2 pumps that flow 15gpm at 6 foot head in parallel.
• Member, Moderator, Administrator Posts: 16,487
This is correct.

And when you shut off one of the circulators, the other will drift to the right on its curve and move about 80% of the total flow. The key is to size the pipe for the total flow, and then divide that flow between the two circulators.

This can often move you from commercial circulators into residential-size circulators, while giving you the benfit of having a standby circulator (at least one that will move about 80% of the total flow).
Retired and loving it.
• Member Posts: 122
This also means

That in big circs and small circs alike. The energy consuption of that second pump to gain 20% comes at a electrical consumption cost of greater that one circ to handle 100% capacity. Then have a standby circ if needed.
• Member Posts: 998

If one pump fails, the flow will be about 70% of when both are running. Often the watts draw of two small pumps is less than one large pump. You have to compare on a case by case basis.

Ron Schroeder
• Member Posts: 122
I'll agree to that.

• Member Posts: 1,790
Options

It is also nice to keep things smaller than 50mm pex if possible. Maybe multiple circuits would be easier to install and circulate if the distances are not too great.

Ron and Dan are correct that you can buy a bigger pump, multiple pumps,and/or you can buy a different system curve. 50 gpm would buy you relatively large pipe to keep the head loss low enough to double the flow with smaller circulators.

-Andrew
This discussion has been closed.