Weil-McLain SB0203 Merkblatt

Seite von 2
 
 
 
 
 
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
.