Six ways to heat water, side by side
| How the water gets hot | Stores hot water? | What the house needs | |
|---|---|---|---|
| Electric cylinder | An electric element heats water stored in the cylinder | Yes, low pressure or mains pressure | Power to the cylinder and room for it, inside or out |
| Gas storage | A gas burner heats stored water, supplied at mains pressure | Yes | Natural gas or bottled LPG; Rheem says most can use an existing gas pipe |
| Gas continuous flow, often called a califont | Heats water only while a hot tap is running | No | Natural gas or LPG bottles, and a larger gas pipe from the meter, according to Rheem |
| Heat pump | Takes warmth from the surrounding air and moves it into the water | Yes, with an electric back-up element | Outdoor space, a plumber and an electrician |
| Solar | Roof collectors heat the water, with a gas or electric boost | Yes | A roof area facing north and out of shade, in Rheem's advice |
| Wetback | A wood or pellet fire heats water in pipework inside the firebox | Feeds a cylinder | A fire placed so it can work with the cylinder; a plumber decides |
Electric cylinders
The familiar set-up: a tank, an element and a thermostat. Rheem and Rinnai both sell them in low pressure and mains pressure versions.
Low pressure copper cylinders can be installed open vented or valve vented, so knowing a cylinder is low pressure doesn't tell you the whole set-up. Our page on converting to mains pressure covers the difference.
Gas: storage or continuous flow
A gas storage heater keeps a tank of hot water ready. A gas continuous flow heater, which many Kiwis call a califont, heats water only on demand, so there's no stored hot water to use up; the question is whether it keeps up with the taps you run together.
Where there's no piped natural gas, Rheem says a gas water heater can run from refillable LPG bottles.
All gasfitting is restricted work, so the gas side is for an authorised gasfitter. A gasfitter the Board accepts as competent can also fit a gas water heater's water pipes. A Gas Safety Certificate is needed for all gasfitting once it's connected, and a brand-new installation is general gasfitting, which also needs a gas certificate of compliance.
Heat pumps
A heat pump water heater draws warmth from the air around it into the water held in its tank. Rheem's models carry an electric back-up element for very cold weather.
The unit needs outdoor space, so switching to a heat pump can mean moving the cylinder. Rinnai's install guide lists a licensed plumber, an electrician and a suitable site. More on heat pump hot water.
Solar and wetbacks
Solar and wetbacks add heat to a cylinder rather than replacing it. Rheem's solar system is a closed-loop design with a gas or electric boost, and Rheem suggests collectors face north on a part of the roof that isn't shaded.
A wetback uses a wood or pellet fire. Water in pipework inside the firebox heats up and rises to the cylinder, so the element doesn't have to run while the fire's going. Where the fire sits relative to the cylinder is crucial, and Rheem says a local plumber has to say whether a wetback will work at all. See wetbacks.
Rules that apply to every type
- Authorised people only. Plumbing on a cylinder is sanitary plumbing, which only someone authorised under the Plumbers, Gasfitters, and Drainlayers Act 2006 may do. An authorised gasfitter handles gas, and an electrician makes the electrical connection.
- Building consent. Replacing a water heater is exempt only in the conditions in Schedule 1 of the Building Act, and a cylinder on a wetback has its own, tighter clause. Read do you need a building consent.
- The Building Code still applies. Every type must meet clause G12, consent or no consent: safety features that relieve excess pressure, the ability to control legionella, and water at personal washing fixtures delivered at a temperature that avoids scalding.
- Room to maintain it. G12 also asks for reasonable access to parts likely to need maintenance, and a way to isolate the system.
Questions to ask the plumber
- Which types will actually work at our place, and which have you ruled out?
- Do we need a bigger gas pipe, new wiring or a new spot outside?
- If we switch type, does the replacement still fit a consent exemption?
- Who else has to come: a gasfitter, an electrician?
- What paperwork will I get at the end?
What we would do
Rule types out before you compare them. No piped gas and no appetite for LPG bottles? Gas drops off the list. Nowhere outside for a unit? A heat pump may too. Once you're down to two, ask an authorised tradie who specialises in hot water to quote both, with the whole job written out, so you're comparing like with like.