Mueller Model D Fre-Heater Installation and Operation Manual
Effective March 1, 1993
Part No. 8800430
Revised November 6, 2019
6.1 THERMAL EXPANSION TANK INSTALLATION FOR DAIRY FARM APPLICATIONS
Paul Mueller Company requires the installation of a thermal expansion tank on all Model “D” Fre-Heaters used on
dairy farm applications.
IMPORTANT: Without the installation of the thermal expansion tank, the Fre-Heater warranty may be
void at the discretion of the Mueller Dairy Farm Equipment Service Department.
Thermal expansion will occur in all closed tank water heating applications. For example, a closed tank with an
initial pressure of 50 psig will reach a pressure of 250 psig with a temperature rise of just 10°F, or 40 gallons of
water at 40°F will expand to 41 gallons at 160°F. The use of a thermal expansion tank will absorb these pressures
and help eliminate premature Fre-Heater failure. The temperature pressure relief valve is intended as a safety
device and should not be relied upon to perform this function.
The following diagram shows the proper installation method for a thermal expansion tank. A thermal expansion
tank, Mueller Part No. 8805175, is available and will work well for this application.
Please Note:
• One thermal expansion tank, Part No. 8805175, is required for every 120-gallon Fre-Heater and/or water
heater storage capacity. For example, a Model D-120 Fre-Heater installed in series with a 120-gallon water
heater would require two expansion tanks installed on the cold water supply line of the Model “D” Fre-Heater.
• The thermal expansion tank is shipped with 40 pounds of pressure on the diaphragm and must be pressurized
to the cut-out pressure of the well pump before it is installed. For example, if the water supply is a 30 psig
cut-in and 50 psig cut-out, then the proper pressure in the thermal expansion tank would be 50 psig.
FIGURE 21: THERMAL EXPANSION TANK DIAGRAM
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Section 6.0 – Appendix A
Cold
Water
Supply
Backflow Preventer, Check,
or Pressure Reducing Valve
Thermal
Expansion
Tank
Hot Water
Mueller
Fre-Heater
