Hot Water and Heat Pumps: What To Consider Before Buying

This guide covers everything you need to know about hot water systems for heat pump installations, including storage options, efficiency considerations, and the pros and cons of different technologies.

How Heat Pumps Heat Water

Why Heat Pumps Don’t Provide Instant Hot Water Like Combi Boilers

Most UK homes currently rely on combi gas boilers, which instantly heat water on demand. This means you can take long showers or fill multiple baths without worrying about running out of hot water. However, heat pumps generally use a different approach, prioritizing hot water storage over instant heating.

Why the Difference?

• Heat Pumps Are Lower Temperature Systems: They typically operate at lower flow temperatures than gas boilers, which limits their ability to provide instantaneous hot water.

• Power Considerations: While it’s possible to use a heat pump for instantaneous hot water, the cost and complexity of such systems make them impractical for most homes.

Hot Water Tanks: The New Normal?

For the vast majority of heat pump installations, a hot water cylinder is required. This is a significant change for homeowners moving from combi boilers, which typically do not require a hot water tank.

Advantages of Hot Water Tanks:

• Energy Storage: Store heat when energy is cheaper (e.g., during off-peak hours or when solar panels are producing excess power).

• Consistent Water Temperature: Maintains a steady supply of hot water for multiple users.

• Improved Efficiency: Allows heat pumps to run more efficiently, as they can operate continuously at lower power levels.

How to Avoid Running Out of Hot Water

Continuous Heating vs. Timed Heating

One of the biggest fears for homeowners switching to heat pumps is running out of hot water, especially if they have large households. However, modern heat pump systems prioritize hot water production, using sophisticated controllers to keep your tank topped up as needed.

How It Works:

• Continuous Monitoring: The heat pump’s controller continuously monitors the water temperature and diverts energy to the cylinder when needed.

• Smart Tariffs: Some homeowners use time-of-use tariffs to heat water when electricity is cheapest, reducing overall running costs.

• Solar Integration: If you have solar panels, you can divert excess solar power to your hot water tank, effectively using free energy.

Alternative Hot Water Solutions

For those concerned about running out of hot water, there are several alternative approaches:

1. Immersion Heaters: Use a traditional electric immersion heater to supplement your heat pump during periods of high demand. This is a simple, but less efficient, solution.

   
   

2. Hybrid Systems: Use a small heat pump specifically for hot water, separate from the main heating system.

   
   

3. Heat Batteries: More compact and efficient than traditional tanks, these phase change devices store large amounts of heat in a smaller space (more on this below).

   
   

Heat Batteries: A Compact Alternative

What Are Heat Batteries?

A heat battery is a compact, innovative alternative to traditional hot water cylinders. Companies like Sunamp have developed these systems, which use phase change materials to store large amounts of heat in a smaller, more space-efficient format.

Key Benefits:

• Space Savings: Typically 75% smaller than an equivalent hot water tank.

• Fast Recovery Times: Rapidly reheat after use, providing a more consistent supply of hot water.

• Energy Efficiency: Highly insulated, minimizing heat loss and maximizing efficiency.

Real-World Applications

Heat batteries are becoming popular in space-constrained homes and apartments where traditional hot water cylinders would be impractical. They can also work well with solar PV systems, storing excess solar energy for later use.

Legionella Safety and Hot Water

What About Legionnaire’s Disease?

One potential concern with hot water storage is the risk of Legionnaire’s Disease—a potentially fatal respiratory illness caused by Legionella bacteria, which thrives in warm water.

Mitigating the Risk:

• Sterilization Cycles: Most heat pump systems are configured to periodically raise the water temperature above 60°C to kill any potential bacteria.

  
  

• Smart Controllers: Use smart timers to ensure the tank reaches sterilization temperatures regularly, often overnight when electricity is cheaper.

Hot Water Cylinder Design: Modern Innovations

Space-Saving Alternatives

Traditional hot water cylinders are vertically oriented, taking up significant floor space. However, newer designs are emerging to address this:

• Horizontal Cylinders: Designed to fit in lofts or crawl spaces.

• Slimline Models: More compact for tight spaces.

• Thermal Batteries: As mentioned, these offer a high energy density in a small form factor, ideal for space-limited homes.

Final Thoughts: Hot Water and Heat Pumps Can Work Together

Switching to a heat pump doesn’t mean sacrificing comfort or convenience. With the right design and equipment, you can enjoy reliable, efficient hot water without the risk of running out.

Top Tips for a Successful Hot Water Setup:

• Plan for Peak Demand: Make sure your system is sized correctly for the number of bathrooms and potential peak usage.

• Consider Alternative Technologies: Heat batteries and hybrid systems can provide flexibility and efficiency.

• Think About Insulation: Modern cylinders are extremely well insulated, reducing standby heat loss.

As the heat pump market evolves, more innovative and space-efficient solutions will continue to emerge, making it easier than ever to upgrade your home’s heating and hot water systems.