Expansion tanks and pressure vessels that absorb the volume change of heated water and protect your hydronic system. AHHAC supplies the Varem (Hurlcon) expansion tank range.
A hydronic expansion tank is a sealed pressure vessel with an air cushion behind a rubber membrane that absorbs the extra volume water takes up as it heats, keeping pressure below the relief-valve setting, commonly 3 bar (300 kPa) on residential heating circuits. It must be sized to the system's water volume and temperature swing and pre-charged correctly. AHHAC supplies and installs Varem expansion vessels through Hurlcon for heating, cooling, potable and solar circuits.
Reviewed by the AHHAC Technical Team — licensed plumbers & hydronic engineers, NSW Lic 262528C.How we write & review
Water expands as it heats. In a sealed hydronic system that expansion has to go somewhere — and that's the job of the expansion vessel (pressure vessel). A correctly-sized vessel absorbs the volume change as the system heats from cold to operating temperature, holding system pressure within the safe band and stopping the relief valve from weeping.
AHHAC supplies the Varem expansion tank range (distributed through Hurlcon) — Italian-made pressure vessels for heating, cooling, potable water and solar applications. We size the vessel to the system's water content and temperature swing, because an undersized vessel causes pressure spikes and relief-valve discharge, while the wrong pre-charge causes premature failure.
Expansion vessel sizing depends on total system water volume, the cold-fill pressure, the maximum operating temperature and the system height. AHHAC calculates the required vessel volume and sets the correct air-side pre-charge pressure at commissioning. For larger systems, multiple vessels or a larger floor-standing unit are specified. We also fit potable-grade vessels (with a butyl bladder) for domestic hot water expansion.
An expansion vessel gives no warning when it starts to fail. It sits on the wall doing nothing visible until the system starts showing symptoms that look like other problems. These are the ones we see most:
The third symptom does the most damage. Every top-up brings in fresh oxygen and dilutes the corrosion inhibitor, so a failed vessel slowly turns into a corrosion problem in the radiators, pump and heat exchanger.
We isolate the vessel, release the water side to zero and read the air-side pre-charge at its valve. If the charge has leaked away, we re-pressurise it to suit the system's cold fill pressure and check it holds. If water comes out of the air valve, the membrane has ruptured: a fixed-membrane vessel is replaced, while a replaceable-bladder type can be rebuilt. With a relief valve commonly set around 3 bar (300 kPa), the vessel has to keep the hot system comfortably below it.
Systems that grow also outgrow their vessels. Adding radiators, a buffer tank or a second floor of fan coils increases the water volume, and a vessel sized for the original system can no longer absorb the expansion. We re-run the calculation whenever we extend a system. Solar and chilled-water circuits have their own temperature swings and fluids, so each sealed circuit gets its own vessel rather than sharing one.
A healthy vessel protects everything else. Pair it with the right hydronic water treatment and an annual check of pressure and pre-charge, and the circulating pump and heat source will see clean, stable conditions for years.
Written from on-site experience by the AHHAC Technical Team.
AHHAC's in-house design and service team — licensed plumbers and hydronic engineers (Lic 262528C) who have designed and installed 2,400+ hydronic heating, cooling and hot water systems across Australia since 1998.
We only specify equipment after a heat-loss and system-compatibility check, then supply, install and commission it with our own licensed plumbers to AS/NZS 3500 (plumbing) and AS/NZS 5601 (gas), register the manufacturer warranty and service it for the life of the system.
"AHHAC delivered our 22-zone underfloor system on time, on budget and to spec. The crew were tidy, the documentation was thorough, and the commissioning was textbook."
"We've worked with AHHAC for 17 years. They've installed and serviced our hot water plant across three commercial buildings — and they pick up the phone on weekends."
"Three mobs quoted us; AHHAC's design was the only one based on a proper heat-loss calculation. The system runs beautifully — the others would've been oversized and expensive."