Can a Heat Pump Give You a Hot Bath on a Cold Day?

It's a frosty winter evening. The wind howls outside, and all you want is to sink into a steaming hot bath. But can your heat pump deliver when the temperature plummets? It's a common concern, yet recent groundbreaking research from 2025 provides a definitive answer. The University of Salford's Energy House 2.0 study demonstrates that modern heat pumps are more than up to the task. Even in typical UK winter conditions of 0 to -5°C, these systems reliably provide hot water with impressive efficiency

How Heat Pumps Work for Hot Water in Cold Weather

Think of your refrigerator working in reverse. While a fridge extracts heat from inside to keep things cold, your heat pump cleverly extracts warmth from outside air, even when it's freezing.

Here's how it works: The refrigerant in your air source heat pump absorbs heat from the outdoor air via the evaporator. Most modern heat pumps use a natural refrigerant called R290 (Propane) due to its low Global Warming Potential (GWP).

The boiling point of R290 is –42.1°C, which is important because once it changes from a liquid to a gas, it can be compressed, raising its temperature.

The hot gas then passes through a heat exchanger, where the heat is used to provide heating and hot water in your home. The refrigerant then passes through an expansion valve, cooling down and preparing to repeat the cycle.

Because R290 has such a low boiling point, even at outdoor temperatures as low as –5°C, there’s still plenty of thermal energy available for the process to work efficiently.

Although Air source heat pumps work effectively and efficiently at temperatures well below freezing, the efficiency will gradually decrease the colder it gets.  Ground source heat pumps on the other hand,  maintain more consistent performance because the ground temperature stays relatively stable year-round at about 10-12°C just a few feet underground. Both types deliver reliable hot water for your bath, regardless of the weather is doing outside.

Your Hot Water Storage System Makes the Difference

Your cylinder size helps determine whether you'll enjoy that perfect bath or face disappointing lukewarm water. Getting this right is crucial for both comfort and efficiency.

Standard sizing guidelines recommend 35-45 litres of hot water storage per person per day. Here's a practical breakdown:

Your typical bath requires a minimum of 100 litres of water at 40°C. When you store water at the recommended 60°C and mix it with cold water, you'll need approximately 60 litres of hot water from your cylinder for each bath. This means a 180-litre cylinder can easily provide hot water for multiple baths.

Recovery time matters too. After draining hot water for a bath, your heat pump needs to reheat the cylinder. Good heat pump system design will account for this and help reduce waste between heating cycles

The evidence is clear: your heat pump absolutely can deliver that perfect hot bath, even on the coldest UK winter day. With proper sizing and smart operation, you'll enjoy reliable comfort while maintaining impressive energy efficiency.