Low pressure and mains pressure side by side
| Low pressure | Mains pressure | |
|---|---|---|
| Working pressure | Low pressure copper cylinders (NZS 4602) are designed for no more than 120 kPa, about 12 m of head | Higher working pressures, covered by a separate standard, NZS 4606 |
| Hot against cold | Hot water arrives at a lower pressure than the cold | Rheem describes hot and cold arriving at the same pressure, so they mix easily |
| How it's vented | Either open vented (NZS 4603) or valve vented (NZS 4607) | Relies on valves, which the plumber selects for the cylinder |
| Building Code safety | Must relieve excessive pressure and limit temperature | The same requirement |
Low pressure isn't just one thing
A low pressure copper cylinder can be open vented, with an open vent pipe, or valve vented, where a pressure relief valve takes the place of the open vent. So 'low pressure' doesn't automatically mean a vent pipe through the roof.
That matters when you talk to a plumber about upgrading, because the existing setup shapes what has to change. Our main hot water cylinders page covers the other cylinder types.
Signs it's worth asking about mains pressure
- Showers that dribble while the cold tap runs strong.
- Hot and cold that are hard to balance at a mixer.
- Your cylinder needs replacing anyway.
- A renovation is bringing new showers or tapware.
- Upstairs showers that are noticeably weaker than downstairs.
Switching is the plumber's call
Going from low to mains pressure is more than a different cylinder. A plumber will look at your pipework, tapware, valves and mixers before saying whether the house suits it, and what else would need to change. Some homes take it well. Others need more work, or are better left on low pressure.
Some cylinders are built to work either way. Rheem says its All Pressure cylinders can go in as low pressure and be upgraded to mains pressure later with a valving change. Rinnai's EasySwap is designed as a direct swap for existing 180 L top-port cylinders, on mains or low pressure. Those are the makers' product claims; whether one suits your house is still for the plumber to decide.
Our low to mains pressure guide goes deeper, including the option of a pump.
Consent, safety devices and who does it
Replacing the cylinder itself comes under the Building Act's water heater clauses (Schedule 1 clauses 36 to 38): exempt from consent when an authorised plumber does it and the new heater meets the conditions. We couldn't read official guidance on how a pressure conversion and its pipework are treated, so ask the plumber, and the council if there's any doubt.
Higher pressure makes a cylinder's safety devices more important, not less. Rheem's All Pressure guide lists a cold water expansion valve among the parts its cylinders need. The work is restricted plumbing, and the new cylinder's electrical connection is for an electrician.
Before the plumber visits
You can
- Check whether your cylinder label says low or mains pressure.
- Note which showers and taps are weak, and when.
- List any tapware and mixers you want to keep.
- Mention renovation plans, even rough ones.
Leave to an authorised tradie
- Judging whether pipes and fittings will cope with mains pressure.
- Choosing and fitting relief, expansion and pressure-limiting valves.
- Replacing the cylinder and any pipe that needs it.
- Telling you what else would have to change.
Ask before you commit
- Can our existing pipes and tapware handle mains pressure?
- What would need changing besides the cylinder?
- Would a pump be a better fix for our house?
- Does this job need a building consent?
- Which valves will you fit, and where will they drain?
What we would do
Don't buy a mains pressure cylinder until someone has looked at your pipes. Ask a plumber who does low to mains conversions to check the pipework and fittings, explain everything else that would change, and put it all in the quote. If the answer is no, a pump or another fix for weak flow may be the better spend.