Tanks & Air/Dirt Separators

Equipment Types at a Glance

Expansion Tanks

Expansion tanks accommodate the increase in fluid volume as the system heats up. Without adequate expansion volume, the pressure relief valve opens, fluid is lost, and the system loses pressure repeatedly.

Key sizing parameters

• System volume: total gallons of water (and glycol) in piping, equipment, and coils
• Minimum fill pressure (P₀): must be set equal to or above the static head pressure at the tank connection point; for a diaphragm tank, the pre-charge must equal P₀
• Maximum operating pressure (P₁): typically set 10 PSI below the relief valve setting to avoid nuisance trips
• Fluid and glycol concentration: glycol has different specific volume change per degree � all sizing calculations must use the correct fluid properties
• Temperature range: volume change from fill temperature (typically 40�60�F) to maximum operating temperature

Expansion tank connection point

The expansion tank must connect at the point of no pressure change � typically at the pump suction, near the system fill connection. This ensures:

• Pump operation does not cavitate by pulling pressure below atmospheric
• The suction pressure at the pump remains positive regardless of pump operation
• Air elimination is effective (air separators function best near the tank connection)

Buffer/Storage Tanks

Buffer/storage tanks provide thermal mass that prevents boiler, chiller, or heat pump short-cycling when the system load drops below the equipment's minimum output. They are not heat exchangers; both source and load circuits typically share the same fluid.

When a buffer tank is needed

• The boiler's minimum fire rate produces more BTU/hr than the minimum system load requires
• A mod-con boiler is expected to modulate below 10�15% of full fire regularly (short-cycling risk)
• A chilled water plant shows low-delta-T syndrome (supply and return temperatures collapse; chiller cycles on high-pressure cutout)
• An on/off boiler serves a zone-valve system where all zones could simultaneously close

Sizing

A common rule of thumb: minimum buffer volume = boiler minimum fire output (BTU/hr) � minimum cycle time (min) � (500 � allowable temperature swing in �F). Verify with the boiler manufacturer's minimum required volume specification when available � some list a required buffer volume in their engineering guide.

Piping configuration

• Buffer tanks are typically piped in a "reverse return" or "primary-secondary" arrangement to promote stratification
• The supply connection enters near the top; return exits near the bottom � preserves thermal stratification
• Internal baffles or dip tubes help maintain stratification in some designs
• Some manufacturers offer integrated connections; field-piped connections must avoid turbulent short-circuit between inlet and outlet

Air/Dirt Separators

Air and debris in a hydronic system cause corrosion, reduced heat transfer, pump noise, cavitation, fouled strainers, and sticking control valves. Air/dirt separators remove dissolved air, entrained air, particulate, and magnetite from the circulating fluid.

Types

• Air eliminators (coalescing): stainless steel mesh or coalescing medium causes micro-bubbles to merge and rise to an automatic float vent; most effective at low velocity and elevated temperature; install at the boiler outlet or near the point of highest temperature
• Air separators (centrifugal): use velocity change and centrifugal action to spin air to the center and float it to an auto vent; effective for higher flow rates and larger piping
• Dirt separators: capture pipe scale, construction debris, and particulate that would otherwise foul pumps, heat exchangers, and control valves
• Magnetic separators: capture magnetite from cast iron, steel, and legacy piping systems
• Combination air/dirt separators: perform air removal and dirt separation in one vessel; magnetic versions add ferrous debris capture

Installation location

Install at the point of lowest velocity and highest temperature � this is where dissolved air comes out of solution most readily. Typically the best location is at the boiler outlet on the supply before the main system header, co-located with the expansion tank connection.

Auto air vents

• All high points in the system piping must have manual or automatic air vents
• Auto vents are acceptable in accessible locations; manual vents preferred in concealed spaces (auto vents can leak if debris blocks the float seat)
• Coil high points require venting � this is a common source of capacity complaints

Dirt Separators

Particulate, magnetite (black iron oxide), and system debris cause erosion at pump impellers, fouling of heat exchanger tubes, and sticking control valves. Dirt separators protect downstream equipment.

Types

• Strainer separators: screen-based; require periodic blowdown; select mesh size based on application (coarser for first flush, finer for clean operation)
• Magnetic separators: capture magnetite and other ferrous particles using permanent magnets; effective in systems with cast iron, steel, or ferrous piping components; magnets must be cleaned periodically
• Centrifugal separators: use velocity and centrifugal force to drop particles out of solution; no screen to clog; effective for continuous particulate loads

System flushing

• All new systems generate construction debris, weld slag, pipe scale, and flux residue during first operation � install a temporary or permanent high-capacity dirt separator during startup and flush until effluent runs clean
• Check and clean dirt separator catches at 30, 90, and 180 days after commissioning, then annually

Selection Checklist

Common Selection Mistakes

• Setting expansion tank pre-charge too high � tank works in reverse and cannot absorb volume increase; system pressure rises until relief valve trips
• Installing expansion tank at pump discharge instead of pump suction � causes pressure variations that affect air elimination and can produce cavitation noise
• Undersizing buffer tanks � if the tank is too small, the boiler still cycles too fast; short-cycling causes premature heat exchanger failure on mod-con units
• Omitting glycol correction from expansion tank sizing � glycol expands differently than water; undersized tanks will overpressure regularly
• Locating air separator at a high-velocity pipe run � reduces effectiveness significantly; must be at the low-velocity, high-temperature point
• Skipping startup flush with an installed dirt separator � lets construction debris circulate through and foul equipment before being captured
• Specifying a non-magnetic dirt separator on a system with cast iron boilers, radiators, or legacy piping � magnetite will pass through
• Ignoring buffer tank piping orientation � connecting supply and return at the same height, or both at the bottom, destroys stratification and defeats the tank's purpose