Do you find yourself standing around waiting for the shower or tap to get hot?

You might not think much of the few seconds it takes for hot water to arrive, but if you have a long run of hot water pipe, litres of water can go straight down the drain every time you turn on the tap.

And that wasted water isn’t the only issue. The water sitting in your hot water pipes cools down between uses, so every time you turn on the tap, your hot water system has to heat more water before you get the hot water you’re actually waiting for.

If you’re looking for ways to save money on your electricity bill and reduce hot water waste, the size and layout of your hot water pipes are worth considering.

Why does hot water take so long to arrive?

Traditional plumbing systems often use larger hot water pipes, such as 15mm or 20mm, with one main pipe branching off to several taps and appliances.

The problem is that a larger pipe contains more water.

For example, a 20mm diameter pipe holds approximately four times as much water per metre as a 10mm diameter pipe, assuming the same internal diameter. So if you have several metres of larger pipe between your hot water cylinder and a tap, there can be a surprising amount of water sitting in the pipe waiting to be used.

That water cools between uses.

When you turn on the tap, the cooled water has to leave the pipe before the fresh hot water from the cylinder reaches you.

This is sometimes referred to as dead water or dead-leg water.

How can 10mm hot water pipe help?

Using appropriately sized 10mm hot water pipe can significantly reduce the volume of water sitting in the pipe between uses.

Less water in the pipe means:

• Less cold water wasted while you’re waiting for hot water
• Hot water reaches the tap sooner
• Less cooled water needs to be reheated
• Less heat is lost from water sitting in the pipe
• Less water is sent down the drain

For households that use a particular bathroom or kitchen tap frequently, these small savings can add up over time.

It is not simply the diameter of the pipe that matters, however. The length and layout of the pipework are also important.

The plumbing layout matters too

Imagine you have one hot water pipe running through your house, with several tee-offs supplying different taps.

The hot water might have to travel several metres along the main pipe before it reaches your bathroom. Every time you use that bathroom tap, all the cooled water sitting in that section of pipe has to be displaced first.

A more efficient approach is to design the system so that individual outlets have shorter, appropriately sized runs of hot water pipe.

This can be particularly effective in a new build or during a renovation, when pipework is accessible and the hot water system can be designed around the layout of the house.

Can existing hot water pipes be replaced with 10mm pipe?

Sometimes, yes.

If your existing hot water pipes are accessible in the ceiling, under the floor or through other accessible areas, a plumber may be able to replace larger sections of pipe with smaller-diameter pipe.

Renovations can provide a particularly good opportunity to do this because walls and ceilings may already be open.

However, 10mm pipe isn’t suitable for every situation. Hot water pipework needs to be correctly sized for the length of the run, water pressure, flow requirements and the fixtures being supplied. A smaller pipe can reduce the volume of water sitting in the pipe, but it also has less capacity to deliver water.

Your plumber should therefore assess the whole system rather than simply replacing every hot water pipe with 10mm pipe.

New Zealand’s Building Code requires water supply pipework to be appropriately sized and suitable for the temperatures and pressures of the system.

Don’t forget to insulate your hot water pipes

If you’re replacing hot water pipework, pipe insulation (lagging) is another simple way to improve efficiency.

Insulating hot water pipes helps slow down heat loss while hot water is sitting in the pipe. This is particularly useful for longer pipe runs and pipework passing through colder areas such as ceilings, under floors or garages.

New Zealand’s Building Code includes energy-efficiency requirements for hot water systems, and hot water pipe insulation forms part of the overall approach to reducing heat loss.

What about water pressure?

You may have heard that smaller hot water pipes can reduce water pressure.

This is why pipe sizing needs to be considered as part of the overall plumbing design. Mains-pressure systems can be suitable for smaller-diameter hot water pipework in the right circumstances, but the length of the pipe run, available pressure, number of outlets and expected flow all need to be taken into account.

We wouldn’t recommend changing your hot water pipe size without having a plumber assess your particular system first.

Is 10mm hot water pipe worth considering?

If you’re building a new home, renovating, or already have a long wait for hot water at certain taps, it’s worth asking your plumber about 10mm hot water pipe and a more efficient hot water pipe layout.

The biggest benefits are likely to come from reducing unnecessarily long pipe runs and reducing the amount of water sitting in the pipe between the hot water cylinder and the outlet.

And if your goal is to reduce hot water waste and save money on your electricity bill, it’s worth looking at the whole hot water system—not just the cylinder itself.

A well-designed system can help you get hot water to the tap faster, waste less water while you’re waiting, and reduce heat loss from your hot water pipework.

Want to reduce hot water waste in your home?

If you’re in Wellington and would like to know whether your existing hot water pipework could be improved, contact us, Plumb Plumbing and Drainage.

We can assess your existing hot water pipe layout and let you know whether replacing accessible pipework with appropriately sized 10mm hot water pipe could be worthwhile for your home.

If you’re renovating or building, it’s even easier to incorporate an efficient hot water pipe layout from the beginning.