Weil-McLain SB0203 Referenzhandbuch

Subject: Water Boiler Installations with Three-Way Valves

Number: SB0203 (Page 1 of 2)

Date: November 22, 2002

Primary/Secondary pumping is the preferred method for system temperature modulation in

commercial hydronic heating installations. Three-way valves are also used on many

installations for this purpose. Without proper system design and control, three-way valves

can cause cold system water to be returned to a hot boiler. This condition could lead to

boiler thermal shock, creating a potential for section failure. With minimal equipment

investment, thermal shock conditions can be reduced on three-way valve installations.

Weil-McLain makes the following design recommendations.

Piping Selection:

1. Use by-pass piping arrangement – See Figure 1 or 2.

2. Follow basic boiler piping instructions in the boiler manual.

Three-Way Valve Selection:

1. Use valves with minimum timing of four minutes from one extreme position to the other.

2. Size to circuit design requirements. Follow manufacturer’s recommendations. Normally

valve will be one or two pipe sizes smaller than recommended boiler piping size.

3. Avoid oversizing to prevent poor control of mixed water.

4. Pressure drop across the valve should be high enough to provide accurate control

without “hunting”.

Circulator Selection:

1. Size system circulator to circuit flow characteristics. Specify gpm and head requirements.

2. Size each by-pass circulator as follows:

1 .

# of boilers used

[1/4 to 1/3 flow (gpm) of system circulator] X

3. All circulators must run at the same time.

EXAMPLE: Specify equipment to by-pass one 1,000,000 btuh Weil-McLain boiler.

Piping: (1) tee, (1) ell, (1) 45° tee (By-pass size and length will vary on each installation.)

Circulator:

1,000,000 btuh .

10,000 btuh

System flow rate with 20° ∆T through boiler = = 100 gpm

1 .

1 boiler

By-pass flow rate = 1/4 X 100 gpm X = 25 gpm

In most cases a standard booster pump can provide the necessary flow at by-pass gpm and

head requirements.

www.weil-mclain.com.

Technical Services Bulletin No: SB0203 (Page 2 of 2)

Subject: Water Boiler Installations with Three-Way Valves

Date: November 22, 2002

BOILER

SYSTEM

VALVE

3-WAY

PUMP

10 TO 20

PIPE

AT LEAST

DIAMETERS

PUMP

FROM SYSTEM

Figure 1

By-Pass Piping Using Three-Way Valve – Single Boiler

10 TO 20

AT LEAST

DIAMETERS

3-WAY

VALVE

SYSTEM

TO P1

BOILER

PUMP

PIPE

PUMP

BOILER

PUMP

AT LEAST

10 TO 20

DIAMETERS

PIPE

FROM SYSTEM

Figure 2

By-Pass Piping Using Three-Way Valve – Multiple Boilers

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