Plenty of older North Shore homes still run a low pressure cylinder: hot water that dribbles while the cold runs hard, and a shower that goes cold when someone turns on the kitchen tap. A mains pressure cylinder fixes that. Whether it’s worth doing in your house depends less on the cylinder than on everything around it: the pipes, the taps, the cupboard and how the hot water is heated.
This guide weighs those, so you know before anyone quotes whether yours is a straight swap or a bigger job.
What is the difference between low pressure and mains pressure?
The pressure your hot water arrives at. BRANZ puts low pressure systems at 30 to 120 kPa, fed from a header tank in the roof or through a pressure-reducing valve. A mains pressure cylinder runs at the incoming supply pressure, typically 350 to 700 kPa, so hot and cold arrive at about the same pressure.
Low pressure systems are what BRANZ calls unequal: low pressure hot water and high pressure cold water at the same tap. Header tank systems are “still found in older homes but not normally installed in new homes today”, and mains pressure cylinders are now the main type of new installation in New Zealand.
If you’re not sure which you have, the label on the cylinder usually says. A copper vent pipe through the roof, a tank in the roof space or a large valve on the cylinder’s cold pipe all point to low pressure. Our guide to low water pressure shows each one.
What changes in the shower?
Flow and balance. BRANZ puts a low pressure shower at about 7 litres a minute and a mains pressure shower at 12 to 20. With hot and cold at the same pressure, a mixer holds its temperature when another tap opens; BRANZ says balanced flow is difficult on an unequal system. Two showers can run at once.
There’s a trade-off. A shower running at twice the flow uses about twice the hot water per minute, so a household that never ran out on low pressure can start to. Watercare calls 9 litres a minute or less ideal, and a flow-limited shower head keeps most of the feel while using less. If your cylinder was sized for low pressure showers, ask whether the new one should be larger.
Can your pipes take mains pressure?
This is what decides most upgrades. Copper pipework generally can. Dux Quest plastic, from the late 1970s to about 1987, should be replaced first, and galvanised steel that has corroded inside is a poor candidate. A Licensed Plumber checks what the pipes are made of, especially on the hot side, before quoting.
Why the hot side matters. BRANZ notes that plastic pipes “lose strength and ability to withstand the pressure with increasing temperature”. A low pressure system kept old hot water pipe at a fraction of the pressure it now faces at 60°C or more. G12/AS1 also requires the first metre of pipe from a water heater to be copper, so that section is replaced if it isn’t.
Dux Quest. Grey or black plastic marked “Dux Qest” fails at its fittings, and a pressure upgrade puts four-decade-old pipe under far more load than it was installed for. Our Dux Quest guide shows how to check the cylinder cupboard in five minutes.
Galvanised steel. It corrodes from the inside and narrows the bore. Rusty water first thing in the morning is the usual sign.
Will your existing taps and shower still work?
Most mixers and shower heads carry a pressure rating, and that tells you. Methven, for example, labels its mains-only products for installations above 150 kPa and its all-pressure products for anything above 35 kPa. Fittings made for low pressure usually still work on mains pressure, though mixers may need adjusting after the change.
The reverse is the common problem: a new mains-only mixer or rain head fitted to a low pressure system trickles. If a bathroom renovation is planned, the pressure decision comes first, because it decides which tapware you can buy.
What valves and changes does a mains pressure cylinder bring?
More valves and a pipe to the outside. A mains pressure cylinder has no vent through the roof, so it controls its own pressure: a limiting valve on the way in, an expansion valve, and a temperature and pressure relief valve on top, each draining outside. The header tank and vent pipe are no longer needed.
| Low pressure cylinder | Mains pressure cylinder | |
|---|---|---|
| Hot water pressure | Well below the cold | About the same as the cold |
| How pressure is controlled | Height of the header tank or vent pipe, or a pressure-reducing valve | Pressure limiting valve: 500 kPa or less on Rinnai’s cylinders |
| Where expansion goes | Open vent pipe through the roof, on open-vented systems | Expansion control valve and a relief valve rated at 850 kPa and 90°C (Rinnai), draining to a visible point outside |
| Tempering valve to bathrooms | Often absent in older systems | Required on a standard home installation |
| Where it can go | Indoors | Indoors, or outdoors with a model built for it |
| Wetback | Traditional open copper wetback | Coil cylinder, with relief valves that need replacing more often |
| Ongoing servicing | Few valves | Relief valve checked or replaced at least every five years (Rinnai) |
The relief drain. Rinnai’s installation guide requires the relief valve’s drain to run with a continuous fall to a visible point outside. In practice that’s a copper pipe from the cupboard to an outside wall, along a wall or under a timber floor. Where the route is long or the floor space tight, the job takes longer.
Normal drips. BRANZ says an expansion control valve normally drips about 5 litres a day, so a damp patch under the new drain is expected. Our guide to the overflow pipe dripping explains when a drip is a fault.
Scald protection. Rinnai’s guide calls for a tempering valve on a standard home installation, to keep bathroom water within the Building Code’s limit. Many older low pressure systems have none. Our article on the 60°C rule and tempering valves covers why.
Higher pressure has one other side effect. BRANZ notes that water hammer, the bang when a tap snaps shut, generally occurs in high pressure systems, and quick lever mixers make it more likely. It has straightforward fixes.
What does the upgrade involve?
A new cylinder, not a converted one, because a low pressure tank isn’t built for mains pressure. Most of the work is around the cylinder: the pipes, the valves, the drain to outside and the code items an older system usually lacks.
- A look first. Pipe materials, the cylinder cupboard, the floor under it, and the route for the relief drain.
- Old system out. The water goes off, and the cylinder, vent and header tank feed are removed.
- New cylinder in. On a drained tray, which Rinnai requires, restrained against earthquakes as G12/AS1 sets out.
- Valves and drains. The limiting, expansion and relief valves, the drain to outside, an isolating valve and the tempering valve.
- Pipe changes. The first metre of copper, and any runs the check found unfit for the pressure.
- Power and commissioning. An electrician connects the cylinder; it’s filled, tested and heated to at least 60°C.
Same cupboard or outside? Rheem sizes its All Pressure range to match popular low pressure copper cylinders, so the old cupboard often still works. Moving it outside frees the cupboard and means a future leak lands on the ground rather than the floor, but needs a model built for outdoors.
Is there an in-between option?
Yes, two. Rheem’s All Pressure enamel cylinder can go in as low or medium pressure now and be changed to mains pressure later “with a valving change”. Rinnai’s low-medium pressure stainless cylinders give a better shower without going to full mains pressure. Both suit a house whose pipes or taps aren’t ready yet.
The in-between makes sense when the cylinder has failed but the bathroom renovation is a year or two away, or when part of the pipework needs replacing first. You get a new cylinder now and the full upgrade once the pipes and tapware are ready.
Replacing the cylinder altogether is the other route. A gas califont and a hot water heat pump both deliver mains pressure hot water. Our guides to califont or cylinder and gas vs electric hot water compare them.
What about a wetback?
You can keep it. Rinnai says a mains pressure stainless cylinder with a thermosyphon coil is now the most common new wetback installation: the fire heats water in a closed circuit through a coil inside the cylinder. It gives mains pressure hot water, but its relief and expansion valves operate more often and need replacing sooner than on a standard cylinder.
Like the traditional copper wetback, a thermosyphon cylinder needs to sit close to the fire and higher than it. Where that isn’t possible, a pumped coil cylinder can go elsewhere, with a circulation pump to maintain. BRANZ also notes that plastic pipe shouldn’t be used with an uncontrolled heat source such as a wetback, which makes the pipework check stricter.
When is staying on low pressure the better call?
In three cases: the pipework can’t take the pressure and no renovation is planned; the cylinder is sound and the shower suits you; or it’s a bach or low-use property where one or two people share the hot water. A like-for-like replacement is the simplest job and keeps the house working as it always has.
The first case is common in houses that still have their original separate hot and cold taps and untouched pipework: there, the pipes become a bigger job than the cylinder.
Timing matters as much as the decision. The best moment to change systems is when the cylinder is due anyway or the bathroom is being redone, because the work overlaps. Our guide to how long hot water cylinders last helps you judge how close yours is.
For landlords, between tenancies is the easiest time, with the house empty and access simple.
What we do when you ask us about a mains pressure upgrade
We look at the cylinder, the pipework and the tapware first, and tell you whether your house suits a mains pressure upgrade, a like-for-like replacement, or something in between. If the old unit no longer meets current requirements, you hear that too. Every option comes with its price before any work starts.
When we fit a mains pressure cylinder, it’s done to code: seismic restraint, a tempering valve, a tray and drain, and the right relief valves. Where old galvanised or Dux Quest pipe can’t take the pressure, we replace the sections that need it or repipe the house, and you decide with the price in front of you. Our plumbers are licensed, so the person doing the job is the person who signs it off. If a gas califont suits better, that work is done and certified by our own licensed gasfitter.
Our hot water cylinders page covers repairs, replacements and upgrades in more detail.