What to work out first
- How many people live in the house, and whether that's about to change.
- How many showers or baths run one after another at the busiest time of day.
- Whether your hot water is low pressure or mains pressure, if you know.
- What heats it: electricity, gas, a heat pump, a wetback, solar, or a mix.
- Where the cylinder sits now and how much room there is around it.
- What the label on your current cylinder says. A photo of it helps the plumber.
How each answer moves the size
People in the house. More people means more hot water through the day. If someone is moving in or out soon, give the plumber the number you expect.
Showers back to back. A storage cylinder has to get through your busiest stretch without running cold, so a queue for the shower matters more than the daily total.
Mains or low pressure. This decides which cylinders suit your pipes in the first place, and whether moving to mains pressure is worth asking about.
The heat source. Electricity, gas, a heat pump, a wetback and solar each heat water their own way, which changes how quickly a cylinder catches up after a run of showers.
What you have now. If your current cylinder has never run out, say so: that's as useful as the headcount. A photo of its label helps too.
The size itself is the plumber's call, made once they've seen the house.
Things that change the size, or the choice
Pressure. Low pressure copper cylinders are built for working pressures of no more than 120 kPa. With mains pressure, Rheem says, hot and cold arrive at the same pressure. Whether you can move from one to the other is a plumber's call after a look at your pipes: see low to mains pressure.
A heat pump. It keeps a storage tank much like an electric cylinder, but Rheem notes it needs outdoor space, which can mean moving the cylinder.
A califont. Gas continuous flow heats water as you use it, so there's no tank to size. Rheem says it needs a larger gas pipe from the meter than most gas storage heaters.
A wetback. Replacing a cylinder connected to a wetback is exempt from building consent only if the new one is a comparable open-vented cylinder in the same position, on the same pipework. A different cylinder may fall outside that, so raise consent with the plumber before you choose.
Questions to ask the plumber
- Will this size cover our busiest stretch of showers?
- Will it fit where the old one stands, or does something need to move?
- Is our system low pressure or mains pressure, and does that limit the choice?
- Is anything else connected, like a wetback or solar, that the new cylinder has to work with?
- Does this replacement need a building consent, and who applies for it?
Once you've worked through the list, tell us about the job and we'll match you with a local plumber who does hot water cylinders to check your answers against the house.