How to Properly Size a New Unvented Cylinder So You Never Run Out of Hot Water

23 April 2025

Is it time to replace that aging unvented cylinder? Perhaps you’re finally upgrading from a vented cylinder in hopes of enjoying mains pressure hot water at every tap in the home.

Whatever the case, there’s an unvented model for every home, family and budget. The trick is getting the right one.

No one likes running out of hot water, especially when you’re in mid-shower. But go too big, and you’re wasting energy (and money) heating water you don’t use. The challenge is getting the right size unvented cylinder for your household.

Here’s how to make sure you never run out of hot water again without having to pay for heating water that you don’t use.

1. Know Your Household’s Hot Water Demand

The size of your unvented cylinder depends on how many people live in your home and how much hot water you use daily. Here’s a rough guide:

1–2 people: 120–150 litres
3–4 people: 180–210 litres
5+ people: 250+ litres

Of course, there are a few mitigating circumstances. Frequent long showers? A big bathtub? A power shower? These all increase your hot water needs, so size up accordingly.

2. Match Your Cylinder to Your Heat Source

Your hot water supply depends on how quickly your system can reheat water. Faster recovery means you can get away with a smaller cylinder. Not all heat sources are created equal when it comes to water heating, either.

Gas or oil boilers: Heat water quickly, so you may not need an oversized tank.
Electric systems: Slower recovery times mean you might want a slightly larger cylinder.
Heat pumps: Require a cylinder with a larger coil for efficient heating.

Pro tip: If you have an off-peak electricity tariff (like Economy 7), you’ll want a larger cylinder to store enough hot water for the day. Make sure it’s very well insulated so that you don’t have to worry about heating water over and over again.

3. Consider Future Needs

Are you planning to expand your household, add bathrooms, or install a high-flow shower? If so, sizing up your cylinder now could save you from an upgrade later. Remember – a good unvented water heater will last you between 10 and 15 years, so you don’t want to replace it before it’s time.

4. Don’t Forget Water Pressure and Flow Rate

Your cylinder’s size needs to match your home’s mains water pressure and flow rate. A large cylinder won’t help if your water supply can’t keep up. Not sure what your flow rate is? Get a two-litre jug and a stopwatch. Put the jug under the tap, open it all the way, and time it for six seconds, then turn off the tap. Multiple the amount of water in the jug in litres by 10, which will be your flow rate in metres per minute. Check your mains pressure before upgrading; a plumber can help with this.

5. Insulation Matters

A well-insulated cylinder reduces heat loss and keeps your water hot longer. Look for cylinders with thick factory-applied insulation to save on energy bills.

Get It Right the First Time

Choosing the right unvented cylinder isn’t just about guessing—it’s about balancing hot water demand, heat source efficiency, and future-proofing your system.