Primary Pump

Don \"Grumpy\" Walsh

Does the primary loop pump gpm need to be sized to the total gpm of all the secondary pumps?

Brad White_203

No but

it all depends...

First of all, I assume that by primary you mean the boiler-side pump and by secondary do you mean the system-side pumps. Is this correct? Either way, just to define the terms and what they are doing.

Here is the "it depends" part (given as an example):

1) Say you are running a high-temperature cast iron boiler at 180F, and your secondary circuits are radiant, panel radiators, even fin-tube with outdoor reset, whatever. Each of those circuits may take in a varying flow rate from the boiler that is much less than what each circulates, in order to mix-down to the desired temperatures.

The total BTU transfer is the same, but the overall temperatures and sub-zone flow rates may well exceed the total boiler flow rate.

Remember that whatever goes into a tee must come out of a tee.

Mark Hunt_6

Siggy's book


gives a GREAT explanation and diagram of this.

Currently residing at #1 in my library is Modern Hydronics.

Absolutely a must study!

Mark H

Don \"Grumpy\" Walsh

Further explanation

I have two mod cons piped into a 2" primary loop (1.6 ft head) all as per the manufacturers requirements. Off of this primary loop there are three secondary loops. 2 are 1 1/4" and one is 1 1/2". These are distribution mains throughout the building that serve the 23 injection pumps which are located at the individual radiant manifolds. The combined gpm flow through the three secondary loops totals 42 gpm @6.25 ft head. The designer says that a Grundfos 15-58 on the primary loop is all that is required. I seem to think That the primary pump should be capable of delivering at least the 42 total GPM. Am I wrong? The three secondary pumps will run 24/7 unless the system is turned off when the warm weather shutdown setting is reached. Your input would be really appreciated.

Mark Custis

The primary loop

pump only needs to pump the primary loop.

A 30 foot long primary loop of 2 inch pipe only needs a pump that will run the water around that loop. Thats all. Each secondary pump either load or boiler needs only to do that secondary loop.

Keep reading.

The fewer or smaller pumps the better. Think loops not BTUH.

Brad White_191

It is less about flow

than it is about flow at temperature.

If you know what each of your loops require for flow rate and BTU delivery (at what temperature), you can figure out what primary flow rate is required for each.

I can take 10 GPM at 180 degrees and mix it down to a much greater flow at lower temperatures.

Take Mark's advice and get Siggy's book. After 30+ years in the business, I still find it a handy reference.