Identify the system before borrowing a formula.

The heated volume must come from the actual arrangement

A storage-heater worksheet starts with a recorded stored-water volume and a verified isolated circuit. A tankless arrangement may include an exchanger, return pipework, internal or external check devices, a pump and sometimes a buffer. The product name alone cannot establish the heated trapped volume or the required expansion provision. Do not borrow a storage-tank answer without checking the actual unit’s drawing and method.

A return loop adds questions, not a universal vessel size

• Locate the model-specific diagram

  Which return arrangement is described by the manufacturer, and does the installed system match it?

• Identify the one-way devices

  Which checks or backflow arrangements isolate which parts of the heated circuit? Internal components may not be visible externally.

• Identify stored or trapped water

  Does the documented method count an internal buffer or the return loop, and under which temperature conditions?

• Find the actual expansion instruction

  If the manual specifies an expansion arrangement or vessel, retain its conditions and model scope. This site does not generalize a “2-gallon” requirement.

The document guide can help organize the record. This site has not independently verified every tankless model or recirculation configuration. If the relevant unit manual is missing, the correct output is a question for the manufacturer or installer, not an invented volume.

Storage expansion and heat-pump recovery are separate tasks

A heat-pump storage heater still contains stored water, so the recorded volume and isolated pressure boundary remain relevant to expansion review. Recovery, however, can depend on compressor, resistance elements and mode controls. DOE’s residential guidance (opens source in a new tab) distinguishes efficiency, hybrid and higher-demand operation. A single fixed resistance input should not be used to forecast every operating mode.

A change from 50 to 66 US gal increases the illustrative 40→140°F expanded volume from 0.75 to 0.99 US gal. That does not prove an existing expansion tank must be replaced: the previous tank’s actual acceptance, pressure conditions and available margin still need to be checked. Increased demand and insufficient capacity are different statements.

Take a scope record to the right source

Keep the heater and any buffer models, the recirculation mode, the drawing revision, the recorded water volume, pressure boundary and unresolved questions together. For a verified storage circuit within the worksheet assumptions, use the expansion record. For heat-pump recovery, use the model’s mode-specific performance rather than our gas/resistance cold-start worksheet.