If you've ever stepped into a shower that barely dribbles, or noticed one tap running hot while another runs cold, your hot water system is likely the culprit. In New Zealand, homes are fitted with one of two main types of hot water delivery: mains pressure or low pressure (also called header tank or open-vented systems). Understanding the difference helps you make smarter decisions about upgrades, renovations, and everyday comfort.
What is a low pressure hot water system?
Low pressure systems — common in New Zealand homes built before the 1990s — use a header tank (usually in the roof space) to gravity-feed cold water into the hot water cylinder. Because the pressure depends entirely on the height of the tank above your outlets, flow rates are modest and can vary significantly around the house. Showers on upper floors may feel particularly weak.
These systems use a vented cylinder, which has a vent pipe running back up to the header tank as a safety overflow. They are generally simpler and cheaper to maintain, but the trade-off is noticeably lower water pressure — typically well under 100 kPa at the tap.
What is a mains pressure hot water system?
Mains pressure systems connect the hot water cylinder directly to the town water supply (or a pressure-boosted tank supply), delivering hot water at the same pressure as your cold taps — usually between 350 kPa and 500 kPa in most NZ urban areas. The result is a strong, consistent flow at every outlet, including upstairs bathrooms and multiple simultaneous users.
These systems use a sealed, unvented cylinder fitted with a pressure relief valve (TPR valve) for safety. Modern mains pressure cylinders comply with NZ Building Code clause G12 (water supplies) and are the standard choice for new builds and renovations today.
Head-to-head comparison
| Feature | Low Pressure | Mains Pressure |
|---|---|---|
| Flow rate | Low (gravity-fed) | High (equal to cold supply) |
| Shower performance | Often weak, especially upstairs | Strong and consistent |
| Multiple outlets at once | Pressure drops noticeably | Handles simultaneous use well |
| Cylinder type | Vented (open) | Sealed (unvented) |
| Header tank required? | Yes (roof space) | No |
| Typical install cost (guide only) | Lower upfront | Higher upfront |
| Ongoing maintenance | Header tank needs periodic checks | TPR valve annual checks |
| Mixer tap / thermostatic shower compatibility | Limited — needs equalised pressure | Fully compatible |
| Suitability for modern bathrooms | Poor | Excellent |
| DIY-friendly? | Minor tasks only | Minor tasks only |
| NZ climate suitability | Header tank can freeze in cold regions | No exposed tank — better in cold climates |
Cost considerations
As a rough guide only, replacing a low pressure cylinder with a like-for-like unit is generally less expensive than upgrading to a full mains pressure system, which may also require new pipework, a pressure-limiting valve, and updated fixtures. However, when you factor in the cost of modern mixer showers, thermostatic valves, and multi-outlet bathrooms — none of which work properly on low pressure — the long-term value of mains pressure becomes clear.
BRANZ research consistently shows that bathroom upgrades are among the most value-adding renovations in NZ homes, and mains pressure is increasingly expected by buyers. Get at least two quotes from licensed plumbers before committing — you can find qualified local plumbers on tradedirectory.co.nz.
Mixer taps, showers, and compatibility
This is where low pressure systems catch many homeowners off guard. Modern mixer taps and thermostatic shower valves require balanced hot and cold pressures to function correctly. On a low pressure system, the cold supply (at mains pressure) will overpower the hot, making temperature control difficult or impossible.
If you're renovating a bathroom and want a rainfall shower head, a handheld combo, or a multi-function shower system, a mains pressure hot water system is essentially non-negotiable. Check product specifications at suppliers like Bunnings or Mitre 10 — most modern shower products will state their minimum pressure requirements.
What work is restricted — and what isn't?
Important: Hot water cylinder work in New Zealand is largely restricted plumbing work under the Plumbers, Gasfitters and Drainlayers Act 2006. This means:
- Connecting, disconnecting, or modifying a hot water cylinder must be carried out by a licensed plumber registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).
- Installing or replacing a cylinder, fitting a TPR valve, or altering pipework are all restricted tasks.
- Homeowners may do minor tasks like draining a cylinder for maintenance (if no pipework is altered) or replacing a showerhead — but when in doubt, call a pro.
If your system is also connected to a gas califont or heat pump, any gas work must be done by a licensed gasfitter, and electrical connections by a registered electrician registered with the EWRB.
Safety note: Never tamper with a TPR (temperature and pressure relief) valve or attempt to seal a vented cylinder. Faulty hot water cylinders can cause serious scalding injuries or, in extreme cases, catastrophic failure. Always use a licensed tradesperson.
Our recommendation
Here's practical guidance for the most common NZ scenarios:
- Older home, tight budget, no renovation planned: A low pressure system may be fine to maintain as-is. Keep the header tank clean and have a plumber inspect the cylinder every few years.
- Renovating a bathroom or adding a second bathroom: Upgrade to mains pressure. The performance difference is significant and modern fixtures demand it.
- New build or major renovation: Mains pressure is the standard — your builder and plumber will almost certainly spec this from the outset in line with current NZ Building Code requirements.
- Cold South Island or high-altitude location: Mains pressure is preferable — no exposed header tank means no freeze risk.
- Rental property: Check your obligations under the Healthy Homes Standards and ensure your hot water system delivers adequate flow for tenants.
If you're unsure which system you have, look in your roof space for a small tank (low pressure) or check whether your hot water cylinder has a pressure relief valve on the side (mains pressure). A licensed plumber can assess your setup and advise on the most cost-effective path forward.
Master Plumbers
