Innovative Energy Engineering

Expansion Tanks and Relief Valves

Water or glycol in HVAC systems expands when heated. Piping system and equipment have very little flexibility and expansion tanks provide the required expansion volume for closed HVAC systems. If pressures increase beyond the design pressure (i.e. overheating boiler, closed-off expansion tank), a safety relief valve opens to relief pressure.

Gage and Absolute Pressure

Gage Pressure (Pg) is measured as a difference to a reference point. In most cases the reference is the atmosphere (101 kPA or 14.7 psi at sea level). At sea level an absolute pressure of 201 kPA (29.1 psi) would read as 100 kPA (14.5 psi). Absolute pressure (Pa) is important to determine boiling points for cavitation. HVAC systems only need to withstand the internal pressures against ambient pressure (gage pressure).

Pressure at different Heights

At high locations gage pressure could be below atmospheric pressure. This could lead to boiling and also would cause de-aerators to suck air into the system instead of de-aerating. At very low locations the pressure could be high enough to burst the device or cause leaking.
Care must be taken when evaluating height differences to know if pressures get subtracted or added. Water Pressure increases at a rate of 9.81 kPa per meter (0.43 psi / foot) below the reference point.

System Pressure and Point of no Pressure Change

Pumps add pressure at the discharge side. Over the length of the piping system all the way back to the suction side of the pump, this pressure is eliminated due to friction and does not add pressure to the suction side. Therefore no pressure is added to the pump suction side. This is referred to as the point-of-no-pressure-change. The expansion tank, de-aerator and system fill components should be located close to this point.
Closed loop water systems get filled and pressurized by a reduced pressure zone (RPZ) valve connected to the domestic water system. Glycol systems don't have an RPZ connected to the domestic water system because adding water would reduce freeze protection. Glycol systems employ a reservoir tank for pre-mixed glycol and a feed pump that is set to maintain a certain pressure. System pressure is derived from minimum pressure at high point (typically 70 kPa -10 psi) and static pressure of the fluid column:  
Example of system with RPZ 10 m below highest point:

In this case the fill system pressure needs to be set to at least 168.1 kPa (24.3 psi).

Determine Relief Valve Pressure

Determine the device with the lowest pressure rating and/or at low locations. For example, a radiator could be rated at 400 kPa (58 psi). Relief valves typically are located at the boiler or other heat-generating sources.

Example with device 10 m below relief valve:

231.1 kPa (33.6 psi) including safety factor would allow safety valves of ratings 207 kPa (30 psi) or LESS. Iterate with other devices with relatively low pressure ratings to find the LOWEST relief valve pressure.

Pressure at Expansion Tank

The relevant pressures for the expansion tank are the system pressure (low) and the relief valve pressure (high) at the tank height. The tank needs to be pressurized for the system pressure. Diaphragm tanks come factory-pre-charged (typically 80 kPa or 12 psi) and need to be adjusted accordingly.

The highest pressure the tank experiences depends on relief valve pressure adjusted for height.

Expansion Tank Sizing

Modern systems use diaphragm-type expansion tanks. Tanks with an air-water interface are outdated, introduce air into the system and require more maintenance. If a solar collector is part of the system its volume needs to be added since all collector fluid could boil – this is not relevant to most HVAC systems.

This equation differs from generally recommended ASHRAE tank sizing equation in that it does not account for expansion of the piping system and adds a safety factor of 10%. Both modifications increase the expansion tank size. It is good to not stretch the diaphragm to the maximum - a larger tank will last longer. For heating systems use the lowest assumed space temperature and the System pressure for lower temperature values. For the higher temperature values assume the maximum possible boiler temperature, which may be much higher than system design temperature. For high pressure use the height adjusted high temperature pressure calculated above. For chilled water systems space temperature will be the higher temperature and the lowest possible chilled water temperature will be the lower temperature. The system and tank need to be pressurized when chiller is operating at lowest setting.

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