The size of a hot water cylinder is vital to many aspects of your home – including your energy bill and your family’s daily schedules. Using a hot water cylinder that is the wrong size can lead to problems – including:

• The cylinder may wear out faster, because it is working harder than intended.
• Your electricity bill will be larger, since the cylinder will have to be re-heated more frequently.
• More maintenance may be required, since the cylinder may break down more frequently.

Rules of Thumb for Sizing Your Cylinder

When determining the correct size of your hot water cylinder, there is a basic “rule of thumb” that is commonly used. To determine how much hot water a family needs, you multiply the number of people in the house by 50. Therefore, if your household consists of 6 people, you will need a 300 liter cylinder; if your household consists of 4 people, you will need a 200 liter cylinder; and if your household consists of 2 people, you will need a 135 liter cylinder.

However, this is a general guideline and does not fit every family.

Variables That Can Affect Sizing

There are many variables that affect how much hot water is required in your home. Each family has different hot water usage requirements. For example, some families take shorter showers than others; some families have young children and teenagers that spend a lot of time in the bathroom; some families wash dishes in the morning while everyone is getting ready; and some families take care of elderly parents that take baths instead of showers.

All of these and other factors can affect the sizing of your cylinder and thus you will need to adjust your initial sizing estimate accordingly.

Where Is My Hot Water Going?

It is crucial to know where your hot water is going before you can properly size your system. The average shower uses 10-15 liters of hot water per minute. So, if your average shower lasts 10 minutes then you are using 100-150 liters of hot water. Bathing uses approximately 80-100 liters of hot water depending on how full the bath is. Dishwashers use 15-25 liters of hot water per cycle. Washing machines vary in their hot water usage – anywhere from 20 liters to 100 liters – depending on the type of washer and cycle used.

As you can tell, hot water usage can add up quickly when multiple appliances are operating at once. For example, during the morning rush in a family home, you may have two showers operating at the same time, someone doing the dishes, and a load of laundry running. During this time, a minimum of 200 liters of hot water would be used.

Because of this, the “per person” formula does not always accurately depict the hot water needs of a family. Peak usage periods – when multiple faucets are pulling hot water at the same time – greatly exceed the level of usage that occurs during normal, non-peak periods.

Getting Help From a Professional

Determining the right size for your Electric Hot Water Cylinders in Auckland is quite complex and you should consider getting help from a professional. Those professionals that install hot water cylinders can assess your household’s usage patterns and recommend the best size of the cylinder for your specific household usage, rather than recommending a size based solely on a generic estimate.

They will inquire about the size of your household, the number of bathrooms, your typical household routines, and whether you anticipate any changes to your usage (e.g., as children grow and begin taking longer showers, or as an additional bathroom is added).

Obtaining a quote for installing a new cylinder typically does not cost anything for the consultation. Incorrectly sizing a cylinder, either too small, or too large, can result in unnecessary expenditures – e.g., replacing a cylinder due to insufficient capacity, or purchasing an excess of capacity and increasing your utility bills.

What Are the Consequences of Incorrect Sizing?

Incorrectly sizing a cylinder presents serious consequences. Under-sizing a cylinder creates a lack of hot water during peak usage periods, leading to cold showers and heated discussions over shower timing. In addition, an undersized cylinder runs hotter and more efficiently – requiring more frequent heating cycles to maintain a steady supply of hot water – thereby shortening its life and increasing power consumption.

Although it may appear that an oversized cylinder would prevent these issues from arising, it has its own drawbacks. An oversized cylinder costs more than a standard cylinder – prices can range by $500-$1,000 – and occupies more space, especially in homes with limited area available for a hot water cylinder.

Furthermore, oversized cylinders operate continuously and use more power than standard cylinders – even though they are only needed intermittently. Over time, the increased energy consumption translates into real dollars spent on power to heat unused water.

Other Usage Scenarios

Sometimes, a homeowner must account for less common usage scenarios. For example, businesses operated from a residence – i.e., hair salons, food preparation, cleaning services – require more capacity than residential-only usage estimates.

Households with spa baths or large soaking tubs also require additional capacity. The amount of hot water needed to fill these types of baths can be substantial – up to 200 liters or more – quickly depleting the capacity of a standard residential cylinder.

Bathrooms vs. Bedrooms

Having multiple bathrooms does not automatically mean that you need a large cylinder. However, if the bathrooms are used at the same time, you may require more capacity.

Example: If a 4-bedroom home has 3 family members taking a shower within 30 minutes of one another, you will require more capacity than a home with the same number of bedrooms but with separate bathroom usage patterns.

Guest Rooms and Vacation Rentals

If you have guest rooms that are used frequently, you should also take this into consideration when sizing a cylinder. Although infrequent guests will not have a significant impact on cylinder sizing, you should consider the additional hot water demand created by regular guests or those renting a vacation property.

Low Flow Fixtures and Appliances

Many modern homes have been installed with low-flow fixtures, such as low-flow shower heads that produce good pressure while using 7-9 liters per minute compared to 15 liters per minute for standard shower heads, and low-flow dishwashers and washing machines. All of these low-flow appliances will lower the total hot water usage of a home. When receiving a quote for a new cylinder, let the installer know that your home has been installed with low-flow fixtures and appliances as this will effect the recommended capacity of the cylinder.

Recovery Time

Electrically heated cylinders require a certain amount of time to recharge after high usage, and the recovery time depends on the size and wattage of the elements. Generally, electrically heated cylinders will take 1-2 hours to fully recover from high usage.

Recovery time can affect the size of the cylinder

If the household has a pattern of peak usage separated by time, a smaller cylinder may suffice because it will have enough time to recover between peak usage periods. Conversely, if the household has a pattern of continuous usage or consecutive peak usage periods, a larger capacity will be required as the cylinder will have little to no recovery time.

Examples of differing usage patterns include weekday morning usage versus weekend usage. Families typically have a pattern of concentrated usage in the morning before work and school, followed by light usage throughout the rest of the day. Weekend usage is usually spread out, allowing the cylinder to recover between usage peaks.

Final Decision

Start with the “per person” baseline, and make adjustments upwards or downwards based on your individual circumstances. Consider your peak usage times, how many appliances are being used at the same time, if anyone takes baths, and if you expect any changes to your usage in the foreseeable future.

Talk to a professional that installs cylinders about your household’s situation and what has worked for households with similar characteristics to yours. They install cylinders regularly and understand the size of cylinder that works well for various usage scenarios.

As stated above, you will likely notice if the size of your hot water cylinder is incorrect – but only when it is incorrect. By taking the time to determine the correct size for your cylinder, you will benefit from reliable performance and reduced operating costs for many years to come.