Hot water systems are one of those things most Kiwis don't think about until something goes wrong — or until they move into a new home and wonder why the shower is barely a trickle. Understanding the difference between mains pressure and low pressure hot water systems can help you make smarter decisions about your home's plumbing, comfort, and running costs.
What Is a Low Pressure Hot Water System?
A low pressure (LP) hot water system uses a small header (expansion) tank, usually installed in the ceiling or roof space, to feed cold water into the cylinder by gravity. Because the water relies on gravity rather than mains pressure, the flow rate and pressure at your taps and shower is limited — typically well below what you'd get from a mains pressure system.
Low pressure cylinders are common in older New Zealand homes, particularly those built before the 1990s. You can usually spot one by the small tank sitting above the hot water cylinder, or by noticing that your hot water pressure is noticeably weaker than your cold water pressure.
Pros of Low Pressure
- Generally cheaper to purchase and install initially
- Simpler system with fewer components
- Widely understood by plumbers working on older homes
Cons of Low Pressure
- Poor flow rates — frustrating for showers and filling baths
- Hot and cold pressure don't match, making tempering (mixing) taps trickier
- Not compatible with many modern tapware and shower fittings
- Header tank can be a source of contamination or frost damage
What Is a Mains Pressure Hot Water System?
A mains pressure (MP) hot water system connects directly to your home's cold water supply, so the hot water is delivered at the same pressure as your cold water. Modern mains pressure cylinders include a pressure limiting valve (PLV) and a temperature and pressure relief (TPR) valve to keep everything safe and within the requirements of the NZ Building Code.
Most new homes built in New Zealand today are fitted with mains pressure systems as standard. They work well with modern tapware, mixer showers, and multi-outlet systems.
Pros of Mains Pressure
- Strong, consistent hot water flow
- Hot and cold pressures are balanced — great for mixer taps and showers
- Compatible with modern tapware and appliances
- No header tank required, freeing up ceiling space
Cons of Mains Pressure
- Higher upfront cost than low pressure systems
- Requires correct pressure limiting and relief valves (mandatory under NZ regulations)
- If the cylinder fails, leaks can be more significant
Side-by-Side Comparison
| Feature | Low Pressure | Mains Pressure |
|---|---|---|
| Water pressure | Low (gravity-fed) | High (matches cold supply) |
| Header tank needed | Yes | No |
| Modern tapware compatible | Limited | Yes |
| Typical install cost | Lower | Higher |
| Common in NZ homes | Older homes | New builds & upgrades |
| Shower performance | Poor to moderate | Good to excellent |
| Balanced hot/cold pressure | No | Yes |
Which System Do You Have?
The easiest way to check is to look for a small tank (usually 10–20 litres) mounted above your hot water cylinder or in the ceiling space — that's a header tank, which means you have a low pressure system. If there's no header tank and the cylinder connects directly to the cold water pipes with a pressure limiting valve on the inlet, you have a mains pressure system.
You can also simply compare the flow from your hot and cold taps. If the cold is noticeably stronger, you're likely on low pressure hot water.
Can You Upgrade from Low Pressure to Mains Pressure?
Yes — and many Kiwi homeowners choose to do this when their old cylinder reaches the end of its life. Upgrading typically involves:
- Removing the old cylinder and header tank
- Installing a new mains pressure cylinder (electric, heat pump, or gas)
- Updating pipework and installing the required safety valves
- Checking that your existing tapware is rated for mains pressure
This is a great opportunity to consider a heat pump hot water cylinder, which can significantly reduce your energy bills. Energywise has useful guidance on energy-efficient hot water options for NZ homes.
BRANZ research also highlights that hot water heating is one of the largest energy costs in a typical NZ household, so upgrading to an efficient mains pressure system can pay off over time.
Healthy Homes and Rental Properties
If you're a landlord, it's worth knowing that the Healthy Homes Standards set minimum requirements for heating and insulation, and tenants increasingly expect reliable hot water. While the standards don't mandate mains pressure specifically, poor hot water performance can affect tenant satisfaction and your obligations under the Tenancy Services framework.
When to Call a Professional
All hot water cylinder installation, replacement, and modification work is restricted plumbing work in New Zealand and must be carried out by a licensed plumber registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). This includes:
- Installing or replacing any hot water cylinder (mains or low pressure)
- Fitting pressure limiting valves and TPR valves
- Altering pipework connected to the cylinder
- Upgrading from low pressure to mains pressure
Attempting this work yourself is illegal and could void your home insurance. A licensed plumber will also ensure the work complies with NZ Building Code clause G12 (water supplies) and any relevant local authority requirements.
If your system uses a gas califont or gas hot water unit, a licensed gasfitter must carry out any gas-related work — again, check the PGDB register to confirm your tradesperson's licence.
You can find qualified, local plumbers in your area through tradedirectory.co.nz.