Low pressure and mains pressure, explained
Low pressure copper cylinders are built to NZS 4602, a standard for working pressures of no more than 120 kPa (12 m head). Rinnai's low pressure copper range, for example, is rated to 76 kPa. Cylinders for higher working pressures fall under a different standard, NZS 4606.
Rheem describes mains pressure as water arriving from the street's water main, with hot and cold at the same pressure so they mix easily at the tap.
Low pressure doesn't always mean an open vent. A low pressure copper cylinder can be installed open vented, or valve vented, where a pressure relief valve takes the place of the vent.
What decides whether it works at your place
- Where your water comes from. If it's a tank or bore rather than a street main, say so early, because the pump and tank set-up becomes part of the question. See water pumps and tanks.
- The pipes and tapware you have. Ask whether the existing hot pipework and fittings suit mains pressure, and what would need replacing.
- Any wetback. If a fire heats the cylinder, the replacement has to work with it. Rheem and Rinnai both make cylinders that take wetback heat at mains pressure, but your layout decides.
- Where the cylinder sits. Moving it, or switching to a heat pump that needs outdoor space, changes the job.
- Temperature at the taps. Water at basins, showers and baths must arrive at a temperature that avoids the likelihood of scalding. Tempering valves are made for exactly that in homes; see tempering valves.
- Pressure safety. The Building Code requires safety features on any cylinder that relieve excessive pressure and limit temperatures. Rheem's All Pressure guide also asks for a cold water expansion valve.
Cylinders built to work either way
Some cylinders are made for both. Rheem says its All Pressure (APVE) cylinders can be installed at low or medium pressure and upgraded to mains pressure later with a valving change. Rinnai designed its EasySwap to replace existing 180 L top-port cylinders on mains or low pressure systems.
Those are the makers' own product descriptions. They don't tell you whether your house is ready for mains pressure, which only a plumber can judge on site.
What about adding a pump?
Some people ask whether a pump can lift the pressure without changing the cylinder. We couldn't read any official New Zealand guidance on this, so we won't call it a fix.
Keep the cylinder's rating in mind: one built to NZS 4602 is designed for working pressures of no more than 120 kPa. If a plumber suggests a pump, ask what it connects to, how it stays within that rating, and what noise and power use it brings.
Consent and the law
A conversion means replacing or altering the water heater, and the general plumbing exemptions in Schedule 1 of the Building Act leave water heaters out. Only clauses 36 to 38 deal with them.
Under clause 38, replacing a water heater is exempt when the new one is connected to, or incorporates, a controlled heat source, and an authorised person does the work. A cylinder on a wetback is covered by clause 37 only if the new one is a comparable open-vented cylinder in the same position on the same pipework, so on a wetback system, check with the plumber, and the council if in doubt, before work starts.
We couldn't read MBIE's guidance on how these clauses apply to a pressure conversion, so we won't tell you it never needs a consent. The council can also decide, case by case, that a consent isn't needed. More in our building consent guide.
Be wary of
- A promise that any house can go to mains pressure, sight unseen.
- A quote that's silent on the old pipework, the valves and tempering.
- Anyone who can't show a current PGDB practising licence, or name their certifying supervisor.
- Talk of consent never being needed for any cylinder job.
Questions to ask the plumber
- Is our supply and pipework suitable for mains pressure, and how did you check?
- Which valves will you fit, and will the bathroom hot water be tempered?
- Does this change fall under a consent exemption, or should we ask the council?
- We have a wetback. How would it connect to the new cylinder?
- What will the work and the paperwork include, in writing?
What we would do
Have the house looked at before you choose a cylinder. Ask an authorised plumber who specialises in hot water to check the supply, the pipework and the tapware, and to put in writing what has to change. If you run a wetback or are weighing up a heat pump, raise it at the same visit, because each one changes the answer on consent and on which cylinder makes sense.