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Choosing the Right Size Unvented Cylinder

22 Jul 2022
Water heaters provide you with the ability to enjoy hot water right at the tap. They make washing up the dishes and bathing far less problematic than they once were. However, choosing the right size unvented cylinder can be challenging. In this post, we’ll explore what you need to know to ensure you get the perfect size for your water needs.

Why Does Size Matter?

First, let’s consider why the size matters in the first place. The size of your unvented cylinder determines how much heated water it can hold. Simply put, an unvented cylinder that’s too small might leave you without hot water when you need it most, while one that’s too large could end up costing you more in energy usage. You need to find the middle path between too small and too large.

How to Gauge Size

The best gauge of whether a particular unvented cylinder will work for your needs is to choose based on the number of people in your household. Each person will use hot water every day, so it’s important to choose a unit capable of providing ample heated water for all requirements, from bathing to washing dishes.

For a one-bedroom apartment you should choose an unvented cylinder of between 120 – 150 litres. For two bedrooms increase the size to between 150 – 180 litres. For larger properties cylinders typically range from 200 – 300 litres.

In addition to size, you’ll want to consider other factors, too.

First-Hour Rating

An unvented cylinder’s volume is an important consideration, but you also need to compare different options to get the right first-hour rating. Basically, this is a measurement of how much hot water the unit can produce in a single hour. It’s a measure of efficiency, but also gives you an idea of how much heated water you can expect. Often, this is higher than the volume of the tank. For instance, you’ll find that some unvented cylinders might have a total volume of 175 liters, but a first-hour rating of 230 or higher.

Reheat Time from Cold

How long does it take for the unvented cylinder to reheat water from cold? This is the length of time you’ll need to wait if the cylinder is completely emptied before you’ll have full-temp hot water once more. Today’s unvented cylinders are very efficient and can offer reheat times as low as 30 to 35 minutes. If you choose an indirectly heated model, you can cut that time down even more, although you’ll need an external boiler.


A new unvented cylinder can be quite an investment and you want to make certain that it’s protected over time. Pay close attention to the warranties offered by each manufacturer. Some companies might offer only 90 days, while others could offer two years or more. Others have much more extensive coverage. For example, Gledhill offers 25 years on some of their unvented cylinders.

Ultimately, you need to choose your unvented cylinder based on the number of occupants in your home. Next, compare warranties, efficiency ratings, and other performance metrics to ensure that you make an informed decision.


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