Number: SB0203 (Page 1 of 2)
Date: November 22, 2002
Subject: Water Boiler Installations with Three-Way Valves
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
.
[1/4 to 1/3 flow (gpm) of system circulator] X
# of boilers used
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 .
System flow rate with 20° ∆T through boiler =
= 100 gpm
10,000 btuh
1
.
By-pass flow rate = 1/4 X 100 gpm X
= 25 gpm
1 boiler
In most cases a standard booster pump can provide the necessary flow at by-pass gpm and
head requirements.
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