HOT WATER COMPLIANCE

Thermal expansion control on hot water systems

Heat water in a closed system and it expands. With nowhere for that pressure to go, you get a relief valve that weeps, fittings under stress, and a callback. Expansion control is how you handle it.

Why expansion has to be managed

When a non-return or pressure control valve closes off the cold inlet, the system becomes closed. Heated water expands and the pressure climbs. Without somewhere to absorb it, the relief valve ends up doing the job on every cycle, which is both a nuisance and a sign the install is not right.

Expansion control valve or vessel

Two common approaches. An expansion control valve releases the small volume of expanded water to a drain. An expansion vessel absorbs it into a pre-charged bladder so nothing is wasted. The vessel needs its pre-charge set to match the incoming supply pressure, and that pre-charge documented.

Pre-charge and labelling

What to photograph

Show the expansion control device in place, its connection, and for a vessel, the pre-charge label. Elemetric keeps that with the rest of the job record.

Common questions

Why does a closed hot water system need expansion control?

Heated water expands. With nowhere for the pressure to go, the relief valve ends up venting on every cycle, which is a nuisance and a sign the install is not right.

What is the difference between an expansion control valve and a vessel?

A control valve releases the small volume of expanded water to a drain. A vessel absorbs it into a pre-charged bladder, with the pre-charge set to the supply pressure and documented.

General information for licensed tradespeople, not legal or regulatory advice. The licensed plumber remains solely responsible for compliance. Refer to the current AS/NZS 3500 standards and the Building and Plumbing Commission (formerly the VBA) for authoritative requirements.