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This article shows you how to select a Taco circulator that lets the hydronic system you’re designing perform as expected.

The process requires that you first define the flow performance of the piping system.After this, you will match the sys-

tem’s performance requirements with a circulator that can supply them. It’s much like defining the performance require-

ments of a car and then selecting an engine capable of delivering that performance.

STEP 1: ESTABLISH THE TARGET FLOW RATE FOR THE SYSTEM

The word “target” means the flow rate you expect under design load conditions. This flow rate depends on both the

rate of heat transfer needed and the temperature drop of the piping system as it delivers this heat transfer.

Keep in mind that the actual flow rate at which the system operates may not be exactly equal to the target flow. It should

however be relatively close (within plus or minus 10 percent of the target flow rate).

For systems using water, the target flow rate can be calculated using the following formula.

Where:

f=flow rate (gpm)

Q=rate of heat transfer (Btu/hr)

∆T= temperature drop of circuit (supply temperature – return temperature) (ºF)

If the fluid is a 30% glycol solution, change the 500 in the formula to 479. If the fluid is a 50% glycol solution, change the

500 to 450.

Example 1: A hydronic distribution system is being designed to deliver 100,000 Btu/hr under design load conditions.The

system fluid is water, and the expected temperature drop at design load is 20ºF.What flow rate is required?

Answer: Putting the numbers into Formula 1 yields:

Although it’s customary to design hydronic circuits with target temperature drops of 20ºF under design load conditions,

there is nothing magical about the number 20. Many systems can be designed to operate with higher temperature drops.

The greater the circuit temperature drop, the lower the flow rate needed for a given rate of heat transport. Lowered flow

rates often lead to reduced tube sizes and smaller circulators.

STEP 2: SELECT THE TUBE SIZE

Once the target flow rate has been established, the tube size can be selected based on flow velocity limitations.

Select a tube size that keeps the flow velocity in the tube in the range of two to four feet/second.The lower end of this

range provides sufficient velocity to entrain air bubbles and carry them along until the flow passes through an air sepa-

rator. Flow velocities lower than two feet/second may not entrain larger air bubbles, especially in downward flow through

a vertical pipe.The upper end of the velocity range keeps flow

noise at acceptable levels for tubing passing through, above, or

below occupied space.

Table 1 can be used to select sizes of type M copper tubing as

well as PEX and PEX-AL-PEX tubing based on these limiting

velocities.

Example 2: Assume the target flow rate of 10 gpm from exam-

ple 1, and that copper tubing is used for the circuit.What is the

proper tube size based on keeping the flow velocity between

two and four feet per second?

Taco Radiant Made Easy Application Guide

Selecting Circulators

EFFECTIVE:August 15, 2005 SUPERSEDES: New

Technical Documents

TD10

Formula 1: f =

500 x (∆T)

f = = = 10 gpm

500 x (∆T)

100,000

500 x (20)

Supporting Technical Documents:

TD09 – Understanding Pump Curves

Taco Product Catalogs:

100-8.5 – “00” Series Cartridge Circulators

100-34 – 1400 Series High Capacity Circulators

Summary of content (6 pages)

PAGE 1
Taco Radiant Made Easy Application Guide Technical Documents Selecting Circulators TD10 EFFECTIVE:August 15, 2005 SUPERSEDES: New This article shows you how to select a Taco circulator that lets the hydronic system you’re designing perform as expected. The process requires that you first define the flow performance of the piping system. After this, you will match the system’s performance requirements with a circulator that can supply them.
PAGE 2
Answer: Look at the copper tube sizes in Table 1. Find a size (or sizes) for which the target flow rate of 10 gpm falls within the values in the second and third column. In this case, a 1-inch copper tube meets this condition and so does a 1.25-inch copper tube. Either tube size is thus a possibility for the system based on the conditions stated. The 1.25inch tube would generate less head loss than the 1-inch tube. This might mean that a smaller less expensive circulator could be used.
PAGE 3
STEP 4: ESTABLISH THE HEAD LOSS OF THE PIPING SYSTEM AT THE TARGET FLOW RATE In a system with multiple zones, the head loss needs to be calculated for each parallel loop. The loop with the highest head loss will be used in figuring the total equivalent length in Example 4. The formula for estimating the head loss for piping circuits constructed of smooth tubing such as copper, PEX, or PEXAL-PEX is: HL = k x c x L x (f1.75) Where: HL = k = c = L = f1.
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STEP 5: SELECT A SUITABLE TACO CIRCULATOR Plot the target operating point just established on a graph showing the pump curves of several “candidate” circulators as shown in Figure 1. 3 7 8 9 10 10 30 25 9 target operating point 009 8 7 20 0014 6 5 15 4 10 3 TOTAL HEAD - METERS Because the curve for the 0012 is slightly above the target operating point the flow rate in the system will be slightly higher than the target value. This is not necessarily a problem.
PAGE 5
SOME FINAL POINTS In addition to selecting a circulator that produces a flow equal to or slightly above the target value, other factors should be considered: 1. To achieve reasonable wire-to-water efficiency the target operating point should fall within the middle third of the flow rate range covered by pump curve for the selected circulator.
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Radiant Made Easy™ TACO, INC., 1160 Cranston Street, Cranston, RI 02920 Telephone: (401) 942-8000 TACO (Canada), Ltd., 6180 Ordan Drive, Mississauga, Ontario L5T 2B3. Visit our web site at: http://www.taco-hvac.com TD10 – 6 FAX: (401) 942-2360. Telephone: 905/564-9422. FAX: 905/564-9436. Printed in USA Copyright 2005 TACO, Inc.