Heater Capacity: What Size Hot Water System Do I Need

Heater Capacity: What Size Hot Water System Do I Need

What size water system do I need is a question everybody asks when they need to decide on new hot water system heater capacity. One of the other things to consider is a water saving shower head, to save water and money on energy.

You can save a substantial amount of money by buying the correct size water heater capacity.

You may save on the initial cost plus the ongoing running cost.

No wonder “What size hot water system do I need?” is one of the most common questions we hear.

And a water heater capacity makes a huge difference.

Learn more…

How Much Water Does a Shower Use Per Minute: Shower Heads Matter

Let’s address the question “How much water does a shower use per minute?” It’s much less with a water saving shower head.

If you have one water saving shower head, you will need a ≥7L per minute tankless unit.

With 2 shower heads you need ≥14L per minute unit.

With 3 shower heads you need ≥21L per minute unit.

With 4 shower heads you need ≥28L per minute unit*.

As an example, an instant hot water heater size for family of 5 would be a 21 litres per minute flow rate vs a hot water tank size for family of 5 would be 230-300L (60-80 gallons)*. This is assuming 3 showers running simultaneously with 3 star water saving shower head.

What Size Hot Water System for Family of 4

The size hot water heater capacity for a family of 4 needs to supply 190-230L (50-60 gallons) assuming a water saving shower head and an 8 minute shower.

Some more examples:

For  ≤2 people: 115-150L (30-40 gallons).

For ≤3 people: 150-190L (40-50 gallons).

For ≤4 people: 190-230L (50-60 gallons)

For ≥5 people: 230-300L (60-80 gallons)*.

As an example, a hot water tank size for family of 5 would be 230-300L (60-80 gallons)*.

*Assumes a 3 Star water saving shower head and you mix in some cold water with the hot, the typical hot water consumption would probably be in the 7 litres per minute range and assumes an 8 minute shower.

Hot Water System Size Calculator and Simultaneous Use

The hot water system size calculator determines by counting the number of simultaneously used shower heads and whether or not you have a water saving shower head, as your hot water tank sizes determining factors.

The hot water heater capacity is best determined by counting the number of simultaneously used showerheads and whether or not you have a water saving shower head.

As an approximation of what size hot water system do I need, you would want a 16L/min water heater capacity for 2 water saving shower heads, 24L/min heater capacity for 3 water saving shower heads and 32L/min heater capacity for 4 water saving shower heads.

This hot water heater capacity is based on having 3 Star water saving shower head with continuous flow (instantaneous) gas hot water heaters and using all of the showers simultaneously.

With tankless gas hot water heaters, the number of people doesn’t really matter, just the simultaneous use of the showers affects what size hot water heater capacity you need.

The use of other hot water taps, such as for a washing machine, will decrease the number of showers that can be used at the same time.

Hot Water Heater Capacity Factors

A key indicator of a hot water heater capacity is simultaneous use and water saving shower heads as two of the fundamental determining factors of what size hot water system do I need.

The number of people in your home combined with how and when they use hot water will affect the heater capacity sizing.

Below are the various water heater capacity factors:

• Do they all take showers at the same general time of day?
• Do they take showers simultaneously in different bathrooms?
• Do the showers have WELS 3 Star Rated Water Saving Shower Head (maximum 9L/Min) or standard 18 to 25L/Min shower heads?
• Do some prefer baths, which use more hot water? Is the bath or spa oversized, requiring even more hot water?
• Do you run the washing machine, using hot or warm water, while people are showering?
• Is your dishwasher connected to the cold or hot water? If hot, do you use it while hot water is being used elsewhere?
• Do you live in a cold climate with cold incoming water?

Calculating Hot Water Heater Capacity – Hot Water Tank Sizes

Calculating hot water heater capacity or hot water tank sizes comes down to the number of people, length of showers and water saving shower heads. Hot water capacity must equal hot water consumption.

To calculate what size hot water system do I need, evaluate capacity versus consumption.

Compare water tank capacity or continuous flow rate to peak usage rate.

Multiply the shower head flow rate by the average length and number of showers in a given period of time.

Choose the water heater capacity to match.

How Much Hot Water Does a Shower Use?

How much hot water does a shower uses depends on the duration and the average flow rate of the shower head.

An average shower uses 56 litres (15 gallons) of hot water based on a water saving shower head using 7 litres/min (1.8 gallons/min) and a shower duration of 8 minutes.

So, the average family of four would use 224 litres (59 gallons) of hot water to take showers.

This is based on the assumption that the average hot water shower lasts 5 to 10 minutes.

7 litres (1.8 gallons) per minute of hot water use also assumes that some cold water is mixed in when you take a shower.

Correct Water Heater Capacity Helps You Save Money

When it’s time to get a new hot water system, one of the key questions relates to the heater capacity of the unit.

Hot water accounts for about 25% of the average home energy costs, so it’s important to get this right.

Calculating Heater Capacity by Number of People or Bathrooms?

The sizing information available is very confusing because manufacturers, and others, are trying to provide a simplified answer to a complicated question.

Some suggest that you calculate by the number of bathrooms in your home while others say to go by the number of people in your family.

Also, in many cases, their recommendations are based on the worst case scenarios, assuming higher consumption than your actual water use.

Hot Water Heater Capacity for Family of 5

A hot water tank size for family of 5 would be in the 230-300L range (60-80 gallons)*.

For an instantaneous hot water unit servicing two water saving shower heads, you would need a hot water heater capacity with a flow rate of at least 16L/min.

To accurately calculate the hot water heater capacity for a family of 5, you need to determine the shower head flow rates and the average duration of their showers.

Assuming you have a 3 Star water saving shower head and you mix in some cold water with the hot, the typical hot water consumption would probably be in the 7 litres per minute range.

If you also assume an 8 minute shower, water saving shower head will consume about 56 litres of hot water per shower.

Therefore, a hot water tank size for family of 5 would need to supply 280 litres in the first hour.

Of course, a single teenager taking hour long showers ruins the whole calculation!

*Assumes a 3 Star water saving shower head and you mix in some cold water with the hot, the typical hot water consumption would probably be in the 7 litres per minute range and assumes an 8 minute shower.

Tankless Instantaneous Hot Water Heater Capacity for Family of 5

In this case, a teenager taking an hour long shower doesn’t matter, except for water and energy used.

For tankless instantaneous units you also need to determine the flow rates for the shower heads for the family of 5.

Once again, hopefully they are all water saving 3 Star water saving shower head with a maximum flow rate of 9L/min.

Then you just select the continuous flow (instant) unit with the appropriate flow rate.

As everyone mixes in some cold water, you can conservatively use 8L/min to calculate this.

As the average shower temperature is 38-42°C and the typical hot water temperature at the tap is 50°C, cold water is mixed in to achieve the desired temperature.

So, as an example, if you have two water saving shower heads you would need a hot water heater with a flow rate of at least 16L/min.

As you can see from the flow rate calculator table below, you would need multiple instantaneous units for all but single showers, when using a non-water saving shower head.

However, a single large instantaneous unit will handle 4 water saving shower head when you calculate the maximum flow rate, including the typical amount of cold water mixed in.

Instantaneous Flow Rate Calculator

Buying Too Large Heater Capacity Wastes Money

The result is that many people end up buying a larger heater capacity than they actually need.

This costs them extra money when they buy the hot water system and will increase the ongoing operating costs, in many instances.

Showers Are the Key

Showers use the most hot water in a home.

The use of WELS (Water Efficiency Labelling and Standards) 3 star rated water saving shower head is an important factor in saving water and energy.

Water saving shower head also reduce the size and cost of the hot water heater capacity required.

Storage Hot Water Tank Sizes

In the above example, if all four people take their showers in the morning, you would only need a storage hot water tank sizes of at least 224 litre first hour heater capacity, regardless of how many take showers at exactly the same time.  

This all presumes that you are not using any other hot water devices at the time.

The use of additional hot water, after the shower period, would depend on the hot water system’s recovery time required to achieve full temperature.

So, with a larger family or house guests, a bit of scheduling may be required to avoid the need for a larger unit.

Off Peak Electric

Off peak electric storage units are a completely different story.

Inasmuch as they only heat water at night, they need to have a quite large heater capacity to make sure you don’t run out of water during the day.

So, some of the electric savings is offset by the fact that you need to heat a larger heater capacity.

The bigger tank would also cost more to purchase.

Tankless Continuous Flow Hot Water Systems

For a tankless continuous flow unit, it would depend on how many showers are being used simultaneously.

These are powered by LPG or natural gas.

Using the example above, if it’s two showers, then you only need a unit with a minimum flow rate of 14L/min.

If you have 3 people showering at exactly the same time, you would need a minimum 21L/min.

This does assume that you are not using other hot water devices simultaneously.

Recovery time is not an issue with continuous flow units, so even a large family with guests would not run out of hot water.

Summary

In summary, the heater capacity of the hot water system you need primarily depends upon on your family’s peak usage period and when you use additional hot water, as opposed to the number of bathrooms or people.

Taking the time to consider your actual usage, before buying, can save you energy and money.

For those who live in cold weather areas…

If you live in a cold climate, you should read the following special section!

Choosing Hot Water Heater Capacity for Cold Climates

There is less hot water output when the incoming water supply gets colder.

If you live in a cold climate, there are special factors to consider when you purchase a new hot water system.

Hot water accounts for about 25% of the average home energy costs, so it’s important to get this right.

Let’s look at these cold weather factors and how you can avoid making an expensive or uncomfortable mistake.

The 25°C (Often False) Assumption

Flow rates and recovery time calculations, for hot water heaters, are generally based on the assumption that the incoming cold water is 25°C.

This may be a reasonable supposition in the warmer months or in a temperate climate, but not in an area that gets very cold winters.

Colder Incoming Water Affects Heater Capacity

Ground temperatures drop during the winter months.

If you live in a cold climate area, that drop can be in the 15-20°C range.

This reduces the temperature of the incoming water, as the pipes pass through the frigid earth.

If you rely on a rainwater tank, the water temperature drop can be even more dramatic.

Tank water has been known to freeze so it would not be unusual for the water to be in the low single digits.

The fact that the incoming water is well below the 25°C assumption makes all of the manufacturers’ recovery time and flow rate data meaningless.

Recovery times will be longer with hot water tanks.

Tankless units will have lower flow rates with colder water.

The Problem – Less Hot Water Requires More Heater Capacity

The obvious problem is that the colder incoming water could leave you without enough hot water.

The selection of type and size of your hot water heater becomes a crucial decision.

How Hot is Hot Enough?

Remember, the average shower temperature is 38-42°C.

Given that the typical hot water temperature at the tap is 50°C, people end up mixing in cold water to achieve the desired temperature range.

Effect on Tank Hot Water Tank Sizes

With hot water tank sizes, there are two issues at work, both of which will reduce the amount of hot water available in cold weather.

First, as you withdraw hot water from your tank, it is replaced by cold incoming water.

The colder the incoming water, the more heat it absorbs from the hot water already in the tank.

The net result is less available hot water.

The second issue is the lower temperature of the cold water from the cold water tap.

It means that you are also mixing in more hot water to attain the sought after temperature range.

The combination of these two facts means that your hot water will not last as long as it did during the summer months.

Worse yet, it may run out.

Effect on Tank Recovery Time

Hot water tank recovery times are also based on the 25°C assumption.

Having much colder water coming in means that the amount of time and energy required to achieve the minimum required 60°C thermostat setting will be much greater.

It obviously takes longer to go from 2°C to 50°C than it does from 25°C to 50°C.

Heat Pumps Pose Special Problems

Heat pump hot water systems are usually a poor choice for cold climates.

The fact that they extract heat from the ambient air makes their effectiveness marginal in cold weather.

Effects on Heater Capacity of Tankless Continuous Flow Heaters

Tankless continuous flow hot water systems are rated in litres per minute flow rates.

Once again, all of the calculations are based on the 25°C incoming water assumption.

Tankless heater flow rates are based on achieving a 50°C temperature regardless of the relative to the temperature of the incoming water.

For example, 20ºC incoming water would not affect the pre-set temperature but the flow rate would be reduced.

So, a factory pre-set 50°C unit can reach the maximum temperature at the maximum flow rate if the incoming water is 25ºC.

As the incoming water temperature drops, the flow rate is reduced so the temperature is still achieved.

This problem is easily addressed by just selecting a model with a higher flow rate.