Elgas LPG Gas Blog

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 Pump Water Heater Cold Climate Pose Special Problems

Heat Pump Water Heater Cold ClimateHeat pump water heaters in cold climates pose special problems.  Heat pumps are usually a poor choice because they extract heat from the ambient air, making their effectiveness marginal in a cold climate.

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.

Factors to Consider

Simultaneous use is one of the fundamental determining factors. 

The number of people in your home combined with how and when they use hot water is the key.

  • 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 Showerheads (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?

Simultaneous Showers Are the Key - Water Saving Shower Head

Showers use the most hot waterShowers use the most hot water in a home. People generally shower for 5 to 10 minutes.

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 whilst reducing the size and cost of the hot water system required.

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, you will consume about 56 litres of hot water per shower or a family of four would consume 224 litres.

In the above example, with a water saving shower head, if all four people take their showers in the morning, you would 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, with the colder incoming water factored in.

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

Using a Tankless Continuous Flow Hot Water System

Bosch 26EFor a tankless continuous flow unit, it would depend on how many showers are being used simultaneously.

Using the example above, if you take showers at different times, you only need a unit with a minimum flow rate of 7L/min at 50°C or 9L/min at 42°C.

If you have 2 people showering at exactly the same time, you would need a minimum 14L/min at 50°C or 18L/min at 42°C. 

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.


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

In general, hot water heater sizing will need to be larger in a home where the incoming water temperature is substantially below 25ºC.

If simultaneous showers are a requirement, then a sufficiently sized tank or larger tankless unit are your best picks. 

If you only use one shower at a time, then tankless is definitely the way to go, as you won’t have to worry about recovery times or running out of hot water.

Taking the time to consider your actual usage, before buying, can save you from a costly mistake and living with cold showers.


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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.