Homeowners · For auckland homeowners
What size hot water cylinder do I need?
Choose cylinder size using a manufacturer’s household guidance, then check it against your shower routine, climate and heating arrangement. Headcount is a starting point, not the whole decision. The published Rheem examples below show why the exact model and its sizing assumptions belong in the quote. Ask our hot water team with the details.
Checked against official sources on by Stately Plumbing.
The short answer
- Start with a published model-specific sizing chart.
- Describe your busiest hot-water period.
- Check product fit as well as household demand.
Give the installer a picture of your busiest morning
A hot water cylinder sizing enquiry works best when you describe how the house uses hot water rather than asking for a tank by volume alone. Tell the installer who normally showers in succession, whether baths are part of the routine, and whether demand has changed since the existing system was installed. Include any proposed bathroom changes in the same brief.
Write down the sequence of your busiest ordinary day. A household that spreads showers across the day presents a different brief from one that needs everyone ready together. Describe the routine you want the new system to support, not just the compromises you currently make. If guests stay regularly, explain that pattern separately from the normal occupancy so it can be discussed explicitly.
Rheem’s published sizing assumptions use seven-minute showers at a mixed temperature of 42°C and a flow rate of 10 litres per minute. These are reference conditions for its guidance, not instructions to change your stored-water temperature. If your showers are longer or use a different flow, tell the installer. A headcount chosen from a chart is only meaningful alongside the conditions behind it.
If the current cylinder used to cope and now struggles despite an unchanged routine, investigate performance before buying more storage. Rheem identifies thermostats, elements and valves among possible contributors to reduced hot-water performance. The no-hot-water checks guide can help you describe the symptoms. That investigation keeps a repairable fault from being mistaken for evidence that the original size was wrong.
Read a published household sizing table in context
The table reproduces storage volumes and moderate-climate household ranges from Rheem’s current indoor stainless product pages. Each row identifies a particular model, not a universal capacity for every family. Read the linked product specifications with the installer and confirm that their shower assumptions match how your household uses hot water.
The overlap between household ranges shows why headcount alone cannot select a tank. Read across the row for the proposed model and ask why that capacity and heating arrangement suit your busiest period. Ask which climate and usage assumptions support the recommendation. Your quote should let you trace the choice back to the manufacturer’s published information rather than a general rule about family size.
The table applies to these Rheem products and the shower assumptions described above. It should not be transferred unchanged to a heat-pump system, a different manufacturer or another product family. If a quote proposes something else, ask for that product’s own guidance. The point is to make the selection transparent, not to turn these examples into a shopping list that bypasses design.
Ask how the recommended model will be heated in your installation. Have the contractor explain the heating arrangement, any electricity-supply constraints and how the system is intended to recover between busy periods. Request a model-specific explanation rather than a generic reheating-time estimate. Where electrical changes form part of the proposal, make sure their scope and coordination are recorded alongside the plumbing work.
| Rheem indoor stainless model | Storage capacity | People: moderate climate |
|---|---|---|
| 35513515 | 135 litres | 1 to 3 |
| 35518015 | 180 litres | 3 to 5 |
| 35525015 | 250 litres | 3 to 6 |
Check the existing pressure, supply and available space
Capacity and pressure are separate parts of the selection. Describe whether you want to retain the present system or change how the shower feels. The low-pressure versus mains-pressure guide explains the questions to raise before choosing equipment. Ask the plumber to confirm the existing arrangement and the compatibility of connected fittings, rather than inferring system pressure from the age or appearance of the cylinder.
Photograph the cupboard opening and the surrounding connections as well as the cylinder label. A replacement has to be brought into position and leave workable access for installation and future service. Ask the installer to check the route through the house, available space and relevant manufacturer requirements. Product dimensions alone are a poor basis for promising that a new unit will fit an old cupboard.
Water supply is another product-selection question. Rheem’s indoor stainless range says it is unsuitable for hard-water areas with high calcium carbonate. Tell the installer if you use a private supply or have water-quality information. Ask them to confirm material suitability before committing to a model. Choosing from a household table comes after confirming that the product is appropriate for the property.
Include future plans that are concrete enough to influence the job. A scheduled bathroom renovation is different from a vague possibility of extending the house. Ask which changes would affect the recommendation and which can be left for later. This helps keep the proposal focused on the household you expect to serve, while avoiding unnecessary equipment chosen for an imagined worst-case day.
Get the sizing reasoning written into the proposal
Ask the quote to identify the exact model, storage capacity, intended household routine and source used for the recommendation. Request an explanation of any departure from the manufacturer’s example conditions. A brief written rationale makes alternative proposals easier to compare because you can see whether they are solving the same demand problem, retaining the same pressure arrangement and allowing for the same work.
The cylinder replacement guide helps you check the wider installation scope. Compare removal, connections, access work, electrical coordination and any excluded building finishes as well as the tank itself. If a contractor proposes a different technology, ask them to explain why it suits your routine and what changes elsewhere in the property are included. Keep equipment selection and installation obligations visible in the same decision.
Before approval, read the model designation back against the quote and product sheet. Similar product names can hide differences that matter to the installation. Ask for any agreed alternatives to be recorded rather than treating an unspecified equivalent as interchangeable. Also clarify how changes will be approved if the site visit reveals an access or compatibility issue that was not apparent from photographs.
Send your busiest-day notes and cupboard photographs with your hot water cylinder request, and ask for both the selected model and the reasoning behind it. After installation, retain the product instructions and commissioning details with the invoice. If hot-water use changes later, that record gives the next adviser a clear starting point: what the system was selected to do, and which household conditions have changed.
Common questions
Can household headcount alone choose the cylinder?
No. Use headcount with the manufacturer’s assumptions, climate column and your busiest hot-water routine. Ask the installer to explain the heating arrangement and confirm that the selected product suits the property.
Do the table sizes apply to every brand?
No. The rows are specific Rheem indoor stainless models. A different product needs its own published guidance and a check of the installation requirements.
Should I buy a larger tank if hot water suddenly runs out?
Have the change investigated first if household use is unchanged. A heating or valve fault can affect performance; more storage should follow a demand assessment rather than replace diagnosis.
What should I send with a sizing enquiry?
Send the existing label, cupboard and connection photographs, normal occupancy, shower sequence, bath use and planned bathroom changes. Ask the installer to confirm the chosen model and sizing basis in writing.
Sources
We checked this guide against these official pages. Rules change, so confirm anything that affects a decision with the source.
Need a hand with this?
Stately Plumbing is Kiwi-owned and has worked across Auckland since 2012. Phone us or send the details and we will tell you what we see and give a clear quote before work starts.