Recommendation 1; Recommendation 2; 24-03 THRU 24-11 (W/20°F DROP)- Page 6

Series 24 Minimum Piping Recommendations – Water Boiler

Recommendation 1

 is used  

when the load is constant and  
not varied due to mixing or 
multiple zones. Use when:

■

   system return water 

is not  

less

 than 135° F for  

prolonged periods of time

■

   system flow 

does not impact

flow through the boiler

DRAIN VALVE

(SEE NOTE 4)

24-03 THRU 24-11 (W/20°F DROP)
24-03 THRU 24-12 (W/40°F DROP)

24-12 (W/20°F DROP)

DRAIN VALVE

(SEE NOTE 4)

REAR OF 

BOILER

REAR OF 

BOILER

3” x 12” NIPPLE
AND BELL REDUCER

(SEE NOTE 3)

SUPPLY

RETURN

2

RETURN

RETURN

HEADER

RETURN

BRANCH

2

2A

23 2

1

2B

1

SUPPLY

1

Recommendation 2

 is a primary-secondary piping method 

that maintains a constant flow through the boiler by using a 
secondary boiler circulator. This arrangement isolates the 
boiler from flow variations but does not safe-guard against 
cold return water temperatures. Use when:

■

   system return water 

is not less

 than 135° F  

for prolonged periods of time

■

   system flow 

does not impact

 flow through the  

boiler (i.e. zoning, mixing)

BOILER CIRCULATOR

(BY OTHERS - SEE NOTE 5)

DRAIN VALVE

(SEE NOTE 4)

SUPPLY

SYSTEM PUMP

CLOSELY
SPACED

TEES

MAX

4D

BOILER CIRCULATOR

(BY OTHERS - SEE NOTE 5)

DRAIN VALVE

(SEE NOTE 4)

SUPPLY

SYSTEM PUMP

CLOSELY
SPACED

TEES

MAX

4D

D

REAR OF 

BOILER

RETURN

3” x 12” NIPPLE

AND BELL REDUCER

(SEE NOTE 4)

REAR OF 

BOILER

RETURN

RETURN HEADER

RETURN BRANCH

2

2

2A

2B

1

23 2

1

24-03 THRU 24-11 (W/20°F DROP)
24-03 THRU 24-12 (W/40°F DROP)

24-12 (W/20°F DROP)

 RETURN PIPING SIZE (IN)

SUPPLY 

RETURN 

RETURN 

PIPING 

HEADER 

BRANCH 

SIZE (IN) (1) 

RETURN (2) 

(2A) 

(QTY.) SIZE (2B)

  BOILER 

20°F 

40°F 

20°F 

40°F 

20°F 

20°F 

   MODEL 

DROP 

DROP 

DROP 

DROP 

DROP 

DROP

24-03 

2 

1-1/2 

2 

1-1/2 

– 

– 

24-04 

2 

1-1/2 

2 

1-1/2 

– 

–

24-05 

2 

1-1/2 

2 

1-1/2 

– 

–

24-06 

2-1/2 

1-1/2 

2-1/2 

1-1/2 

– 

–

24-07 

2-1/2 

2 

2-1/2 

2 

– 

–

24-08 

2-1/2 

2 

2-1/2 

2 

– 

–

24-09 

3 

2 

3 

2 

– 

–

24-10 

3 

2-1/2 

3 

2-1/2 

– 

–

24-11 

3 

2-1/2 

3 

2-1/2 

– 

–

24-12 

4 

2-1/2 

4 

2-1/2 

3 

(2) 3

Notes:

1.  All piping is schedule 40.
2.  Pipe sizes listed are based on a 20° F or 40°F differential (temperature drop).  

Select one to match application.

3.  When specified return piping is less than  3”, install 3” x 12” nipple and appropriate  

size bell reducer directly into boiler return tapping as shown.

4.  Drain valve—ball valve preferable, gate valve acceptable alternative  

(supplied by others).—Minimum valve size per ASME code is 3/4 ” NPT.

BOILER CIRCULATOR

(BY OTHERS - SEE NOTE 5)

DRAIN VALVE

(SEE NOTE 4)

SUPPLY

SYSTEM PUMP

CLOSELY
SPACED

TEES

MAX

4D

BOILER CIRCULATOR

(BY OTHERS - SEE NOTE 5)

DRAIN VALVE

(SEE NOTE 4)

SUPPLY

SYSTEM PUMP

CLOSELY
SPACED

TEES

MAX

4D

D

REAR OF 

BOILER

RETURN

3” x 12” NIPPLE

AND BELL REDUCER

(SEE NOTE 4)

REAR OF 

BOILER

RETURN

RETURN HEADER

RETURN BRANCH

2

2

2A

2B

1

23 2

1

24-03 THRU 24-11 (W/20°F DROP)
24-03 THRU 24-12 (W/40°F DROP)

24-12 (W/20°F DROP)

PIPE SIZING AND NOTES FOR RECOMMENDATIONS 1 AND 2