Expansion tanks are used in many hydronic systems in order to absorb pressures that build up as a result of increasing temperatures. These devices safely compensate for increased systems pressures so that pressure relief valves do not have to open every time a high-pressure event occurs. In indirect glycol systems the loop must be manually re-pressurized after pressure relief valves open.

Expansion tanks used specifically on the solar side of an indirect glycol system absorb increasing system pressure by allowing some of the fluid to enter the tank. Tanks used in this scenario are exposed to different conditions than typical hydronic expansion tanks and must meet requirement over and above typical tanks. Expansion tanks used in this manner must be able to tolerate propylene glycol solutions and operate at elevated temperatures (> 200°F) & pressures (> 100 PSI). When specifying an expansion tank for use on the solar side of an indirect glycol system always verify that it can meet these requirements.

Expansion tanks are comprised of a steel tank shell that contains a butyl rubber bladder. A cushion of pressurized air exists between the steel tank shell and the bladder which can be changed, typically, via a Schrader valve. Once connected to the system fluid is able to enter the tank whenever the pressure in the solar loop exceeds that of the expansion tank’s charge. Because of this the expansion tank must be charged with enough pressure that fluid is not able to enter the tank unless there is a high-pressure event in the solar loop. When this occurs fluid is pushed out of the loop and into the tank to help regulate pressure. When the solar loop’s temperature and pressure return to operating levels the tank’s pressure compresses the bladder and pushes the fluid inside back into the system. This ensures stable pressures and no loss of fluid.

Typical Expansion Tank

Typical Expansion Tank

Solar Expansion Tank Operation

Generic Expansion Tank Operating Diagram

Because of this it is imperative that the expansion tank be sized and charged properly for the specific system being installed. Incorrect sizing and charging of the expansion tank can lead to problems that could cause the system to function improperly or even damage it. The method for sizing the expansion tank is detailed below.

Sizing the Expansion Tank

The first step to properly sizing the expansion tank is to determine the total fluid volume in the solar loop. This includes volumes of piping, collectors, heat exchangers, and other equipment that contains fluid. Table 1 displays the fluid volumes of the various Solene collector models. Table 2 lists the volume (per foot of length) of various sizes of copper pipe typically used in solar water heating systems.

Table 1 – Solene Collector Fluid Volumes
SLAR-240.86 gallons
SLAR-321.02 gallons
SLAR-401.18 gallons
SLSG-401.32 gallons
Table 2 – Copper Pipe Volumes (gallons/foot length)
Nominal DiameterType KType LType M

Once the total loop volume has been determined the acceptance volumes can be calculated. In doing the calculation the system height (vertical distance between the expansion tank and the top of the collectors) must be known and the maximum operating temperature and pressure must also be known. The International Mechanical Code (IMC) section 1009.2 provides Equation 10.1 for calculating the required acceptance volume for expansion tanks.

Note: Manufacturers of expansion tanks will list an acceptance volume and a tank volume in specification documents. Some tanks have a bladder that can contain the full volume of the tank (I.E. acceptance volume = tank volume) but many have only “partial acceptance bladders” that accept a volume less than the total tank volume. The acceptance volume listed should be matched to the result of the equation below.

\[Acceptance Volume = \frac{( 0.00041 * T_{op} – 0.0466 ) * V_{system}}{(P_a / P_f) – (P_a / P_{max})} + V_{steam} \]

Equation 10.1


  • Vsystem = The solar loop volume (gallons)
  • Top = Operating temperature of the system (°F)
  • Pa = Atmospheric pressure (PSI) – See Table 3
  • Pf = Fill pressure (PSI)
    • Convert the system height in feet to PSI and add 20 PSI to determine this value. (Hsystem * 0.4455) + 20 PSI
  • pmax = Maximum operating pressure of the system (PSI)
    • Corresponds to the pressure relief valve’s rated set pressure
  • Vsteam = Equivalent to the collector fluid volume. This term is added to the published equation because the fluid in the collectors could flash to steam pushing the remaining fluid out and increasing the pressure further.
Table 3 – Atmospheric Pressure at Various Elevations
Elevation Above Sea LevelPressure
0 ft14.7 PSIA
1000 ft14.2 PSIA
2000 ft13.7 PSIA
3000 ft13.2 PSIA
4000 ft12.7 PSIA
5000 ft12.2 PSIA
6000 ft11.8 PSIA
7000 ft11.3 PSIA
8000 ft10.9 PSIA

An indirect glycol system is to consist of (5) SLAR-40 collectors installed on a roof resulting in a 35 ft. height difference between the top of the collectors and the equipment. The solar loop will consist of 110 ft. of 1″ Type L copper pipe and a heat exchanger with an internal volume of 2.9 gallons. The system is estimated to operate at a maximum temperature of 200° F and the PRV is set to 90 PSI. The site is at an elevation of 3000 ft. above sea-level. Determine the minimum expansion tank acceptance volume and pre-charge pressure.

  • Vsystem = (5 * 1.2 gal.) + (110 ft. * 0.0429 gal. / ft. ) + 2.9 gal. = 13.62 gallons
  • Top = 200° F
  • Pa = 13.2 PSI
  • Pf = 35 ft. * 0.4455 PSI/ft. + 20 PSI = 35.593 PSI
  • Pmax = 90 PSI
  • Vsteam = 5 * 1.2 gallons = 6 gallons\[Acceptance Volume = \frac{( 0.00041 * 200 – 0.0466 ) * 13.62}{(13.2 / 36.593) – (13.2 / 90)} + 6 = 8.15 gallons \]
  • Pre-charge pressure = 35.593 PSI – 3 PSI = 32.593 PSI Use 33 PSI
  • Select a thermal expansion tank for use with solar thermal systems having a minimum acceptance volume of 8.15 gallons. Charge expansion tank to 33 PSI prior to charging system with heat transfer fluid.