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have to work backwards to see if original circ pump was sized properly. have 2 fancoils of unknown output and perimeter baseboard.can only guesstimate total length of piping. bottom line, what is the formula for selecing a pump?

• Member Posts: 6,877
edited March 2011
Pump Selection ...

Is based on total feet of head (resistance to flow) and required gpm from the manufacturer's pump curve chart.

Feet of head is determined by size and length of piping and fittings and equipment in the line that produce resistance to flow. There are general rules of thumb, but it's best to use the flow chart for the type and size of piping being used. You will first need to know how many gpm are required for the btu's you need the line(s) to carry.

Gpm is determined by the btu's required and the delta T (temp drop) designed into the system. In high temp hydronics, a 20 deg delta T is the norm for design. In a low temp radiant floor, a 10 deg delta T is the normal design.

To determine the required gpm we use the universal hydroncs formula:  Btu's =  Delta T x 500 x gpm. In our scenario, we would have determined btu's from a heat loss calculation. Let's take an easy example with a round number like 100k btu's. The formula would look like this:  100,000 / (20 x 500) = 10 gpm. 100k was our btu requirement, 20 deg was the Delta T, and 500 is a constant for the heat carrying capacity of water.

Designing with a 20 deg delta T makes the calculation very simple and easy to remember: 1 gpm carries 10k btu's @ 20 deg delta T. Thus, 40k btu's requires 4 gpm; 100k btu's requires 10 gpm, etc.

As you can see, I've taken the steps backward to the beginning of determining the required gpm because you'll need to know this before you can size your piping and then size your circ. And you'll need to know your btu requirement first in order to determine the gpm requirement.

One other note of importance: keep the velocity of flow @ 4 feet per sec. or less when sizing your piping or you'll have noise and wear issues. Piping charts will also caution about this.

Find your btu requirement first. Then determine your gpm requirement. Then select the correct size line. Then select your circ from the manufacturer's pump curves. Simple as 1, 2, 3, 4 (almost).

Some basic rules of thumb on pipe sizing in hydronics:

A 1/2 in line will carry 1.5 gpm @ 4ft. pr sec.

A 3/4 in line will carry 4 gpm @ 4ft. pr sec.

A 1 in line will carry 8 gpm @ 4ft. pr sec.

A 1.25 in line will carry 16 gpm @ 4 ft. pr sec.

A 1.5 in line will carry 22 gpm @ 4ft. pr sec.

These figures will allow for an average amount of fittings and valves and are based on each 100ft of piping.

One other thing. You mentioned fan coils to deal with. These may very well be oversized for your job. If you select your gpm for these based on your heat loss calc. and they are oversized, your delta T will be  higher and the discharge air temp from these will be lower than expected. You may need to slow the blower down to compensate for this and / or increase the flow (gpm) to the coil.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 4