Adjusting Your Heat Pump Hot Water System For Cold Weather

In the colder months of the year, your heat pump hot water system may need adjusting.

Heat pump hot water systems are highly efficient because they don’t generate heat in the same way as traditional electric hot water systems. Instead, they draw heat from the surrounding air and transfer it into the water.

Give The Heat Pump More Time to Run

We can think of the heat in the air as the heat pump’s fuel. When the air is warmer, there is more available heat, and the heat pump can produce hot water easily. As the temperature drops, less heat is available, and the system has to work harder and longer. This is why the settings that work well during warmer months may not be ideal during the colder ones.

One of the first things to consider is the operating timer.

A common setting for the heat pump to operate is between 11 am and 3 pm. We set up heat pumps to run during the middle of the day to leverage more heat in the air, as well as cheaper and greener electricity.

During warmer months, this may provide plenty of time for the unit to heat the water.

In winter, however, the water may take longer to heat. A four-hour window may not always be long enough for the heat pump to complete its heating cycle, particularly after heavy hot water use.

A simple first step is to lengthen the operating window. A few weeks back, I noticed my heat pump was still working at around 4 pm, just before it was supposed to turn off. I checked it and noticed the temperature hadn’t quite hit the target of 60 degrees. I lengthened the timer to come on 1 hour earlier, 10 am instead of 11 am.

This doesn’t necessarily mean the heat pump will run continuously for six hours. It simply gives the system a larger window to hit the target temperature (usually 60 degrees). Whether it works for one hour or six hours will mostly depend on how much hot water I used the day before.

Cold Winter Nights

Heat pumps generally don’t perform as well when outdoor temperatures are very low. In places such as Melbourne, Canberra and Hobart, overnight temperatures can fall into the low single digits during the cold season.

At these temperatures, some heat pump hot water systems can run for a very long time without producing hot water as effectively as they would during warmer parts of the day.

This can create a problem if the system is restricted to a short daytime timer. If we use a short timer and too much water is used outside of the timer hours, the unit will run out of hot water. It will only kick in when the timer is set to restart the next day. 

The most logical solution here is to extend the timer again, let’s say from 8 am to 8 pm. 

Again, this doesn’t mean the unit will necessarily run for 12 hours. It means it has access to the warmer parts of the day and has plenty of time to replenish the hot water supply.

But there’s still a risk that too much water is used outside these hours, leaving someone with a cold shower in the morning.

The Backup Heating Element

Some heat pump hot water systems also have a traditional electric heating element for backup or to raise the temperature of the stored water.

If you heat the stored water to a higher temperature, you effectively have more usable hot water stored in the tank. This hotter water in the tank can then be mixed with colder water from the tap. This increases the volume of usable hot water in your tank because you’re using less hot water to wash your hands.

This can be particularly useful when you want to make sure there is enough hot water available outside the heat pump’s operating hours.

For example, the system could use the heat pump in the middle of the day and, where appropriate, use the electric element to further heat the tank during periods of excess solar generation or other low-cost/free electricity.

This can be a sensible way of storing energy as hot water.

The goal is to have plenty of hot water available when the heat pump isn’t operating, such as early in the morning or later at night, all without unnecessarily relying on expensive electricity.

Of course, the exact settings and whether to use the backup element will depend on the specific system and your electricity tariff.

Think About The Seasons

Many homeowners want to set their hot water system once and forget about it. That’s understandable.

However, for those of us who are more focused on efficiency, there is a good argument for having two basic settings:

Heat pump settings for the colder months vs. the warmer ones.

When spring arrives, and the weather starts to warm up, it’s worth resetting the timer rather than leaving the winter setting in place all year round.

The Simple Winter Adjustment

If you have a heat pump hot water system, the most important thing to remember as winter arrives is this: cold weather usually means your heat pump takes longer to heat your water.

Before you assume there is something wrong with the system, please check the operating timer.

If you currently have a short window, such as 11 am – 3 pm, consider extending it to 10 am – 4 pm. If you’re in a particularly cold climate or the system struggles to recover after overnight use, a much longer window, such as 8 am – 8 pm, may be appropriate.

Then, when the weather warms up again, review the settings and return to a shorter operating window. 

For more, read our detailed guide to hot water heat pumps.