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What Size Hot Water Heater Do I Need? What Size Hot Water Tank Do I Need - Water Saving Shower Head

You can save a substantial amount of money by buying the correct size hot water heater.that is larger than you really need. 

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

"What size hot water heater do I need?" or "What size hot water tank do I need?" is one of the most common questions we hear. And a water saving shower head makes a huge difference.

Learn more...

What Size Hot Water Heater Do I Need? - Hot Water System Size Guide

What Size Tankless Instant Hot Water Heaters Do I Need:

If you have one water saving shower head, you will need a ≥7L per minute tankless unit. With 2 showerheads you need ≥14L per minute unit. With 3 showerheads you need ≥21L per minute unit. With 4 showerheads 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 Heater Tank Do I Need:

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 Guide

The hot water system size guide is best determined by counting the number of simultaneously used showerheads and whether or not you have a water saving shower head, as your hot water system size guide.

What Size Hot Water System Do I Need?
Hot Water System Size Guide Chart
Showerheads 1 2 3 4
Flow Rate with 3-Star Water Saving Shower Head (L/min) 9 18 27 36
Flow Rate with 3-Star Water Saving Shower Head
with cold water mixed in. (L/min)
7 14 21 28
Flow Rate with non-Water Saving Shower Head (L/min) 18-25 36-50 54-75 72-100

The hot water system size guide is best determined by counting the number of simultaneously used showerheads and whether or not you have a water saving shower head, as your hot water system size guide.

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

This hot water system size guide 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 system 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.

Shower heads matterHot Water System Size Guide Factors

A key indicator of a hot water system size guide 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 hot water system sizing.

• 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 showerheads?

• 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?

Above are the various hot water system size guide factors.

Please also see:

How to Switch to LPG Hot Water for Your Home

Star Ratings for Gas Hot Water Heater Systems

How Long Do 45kg Gas Bottles Last

Calculating What Size Hot Water System Do I Need? What Size Hot Water Tank Do I Need

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. 

Showers and water saving shower headss are the key. 

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

Choose a water heater with 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.

Assuming an average 8 minute hot water shower, consumption is 56 litres (15 gallons) of hot water to take a shower. This is based on a water saving shower head using an average of 7 litres (1.8 gallons) of hot water per minute.

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.

Hot Water System Size Guide Helps You Save Money

When it’s time to get a new hot water system, one of the key questions relates to the sizing or 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 by Number of People or Bathrooms?

Saving moneyThe 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 Tank Size 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 with a flow rate of at least 16L/min. To accurately calculate the hot water tank size 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 headwill 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.

*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 Systems for Family of 5

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 safely use 8L/min to calculate this.

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
Flow Rate Calculator
Showerheads 1 2 3 4
Flow Rate with 3-Star Water Saving Shower Head (L/min) 9 18 27 36
Flow Rate with 3-Star Water Saving Shower Head
with cold water mixed in. (L/min)
7 14 21 28
Flow Rate with non-Water Saving Shower Head (L/min) 18-25 36-50 54-75 72-100

Buying Too Large Wastes Money

The result is that many people end up buying a larger system 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

Shower heads should be 3 star ratedShowers use the most hot water in a home. People generally shower for 5 to 10 minutes. 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. 

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

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

Storage Hot Water TanksRinnai HOTFLO 170 gas hot water system

In the above example, if all four people take their showers in the morning, you would only need a storage hot water system with 224 litre first hour 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 be quite large 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 tank. 

The bigger tank would also cost more to purchase.

Tankless Continuous Flow Hot Water Systems

Tankless continuous flow hot water systemFor 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 size of 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.

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

For those who live in cold weather areas...

Choosing Hot Water Heaters for Cold Climates

There is less hot water ouput 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

cold weatherGround 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

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?

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 Delivery

Electric Storage Hot Water Systems

There are two issues at work here, 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

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 60°C than it does from 25°C to 60°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 Tankless Continuous Flow Heaters

Gas Continuous Flow Hot Water Systems

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 25°C temperature increase relative to the temperature of the incoming water. 

For example, 20ºC incoming water would increase to 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.

Compare Gas Hot Water Heater Systems Prices

Now you can compare prices for all the major brands of gas hot water systems including Rinnai, Bosch, Kelvinator, Rheem, Dux and AquaMAX.

Click on your category of interest to compare gas hot water systems prices:

Tankless Continuous Flow Gas Hot Water Systems Prices

Gas Storage Tank Hot Water Systems Prices

Gas Boosted Solar Hot Water Systems Prices 

 

 

Gas Hot Water Special Offers

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Bosch 17e, 21e & 26e Gas Hot Water Systems

Rinnai Infinity 16 & 20 Gas Hot Water Systems Sale

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The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.