Heat Pump Hot Water Priority – What It Means and How It Affects Heating
Hot water priority is a common heat pump setting that controls whether your system pauses space heating to focus on reheating hot water. When misunderstood, it can make homeowners think their heating or hot water isn’t working properly.
This guide explains what hot water priority is, how it works on heat pumps, when it’s useful, and when it can cause confusion or problems.
What is hot water priority?
Hot water priority means the heat pump will temporarily stop heating your home in order to reheat the hot water cylinder.
When hot water priority is enabled:
- The heat pump switches fully into hot water mode
- Space heating pauses
- The cylinder is reheated as quickly as the system allows
- Once complete, heating automatically resumes
This behaviour is normal and intentional.
Why heat pumps use hot water priority
Heat pumps have a limited maximum output. Trying to heat the home and hot water at the same time would make both slow and inefficient.
Hot water priority helps by:
- Ensuring reliable hot water availability
- Preventing long, inefficient reheating cycles
- Simplifying system control and design
- Protecting compressor operation
Most domestic heat pump systems in the UK are designed around this approach.
What you may notice when hot water priority is active
When the system switches to hot water mode, you may notice:
- Radiators or underfloor heating cooling temporarily
- The controller showing “Hot Water” or “DHW” mode
- Heating demand ignored for a short period
- Hot water reheating taking 30–90 minutes
In a well-insulated home, this pause in heating is usually barely noticeable.
How long does hot water priority last?
Hot water priority lasts until the cylinder reaches its target temperature. Typical reheating times are:
- 30–60 minutes for small to medium cylinders
- Up to 90 minutes for large cylinders or low-output systems
Once reheating is complete, the heat pump automatically switches back to space heating.
Common misunderstandings
“My heating keeps turning off”
This is often just hot water priority operating normally. If it happens once or twice per day, it’s expected behaviour.
“My heat pump ignores the thermostat”
During hot water priority, heating demand is temporarily overridden. This does not indicate a fault.
“The system is stuck on hot water”
If hot water priority never ends, there may be a cylinder sensor fault, incorrect target temperature, or flow issue.
When hot water priority causes problems
Hot water priority may contribute to issues if:
- The cylinder takes an unusually long time to heat
- The system switches to hot water very frequently
- Space heating struggles to recover afterwards
- Hot water is scheduled too often
These problems are usually related to settings, flow rate, or cylinder performance rather than the priority function itself.
Hot water priority vs running both at once
Some systems allow limited simultaneous heating and hot water, but this is less common on domestic installations.
Running both at once can:
- Increase reheating times
- Reduce hot water temperatures
- Lower overall efficiency
For most homes, hot water priority gives the best balance of comfort and efficiency.
What homeowners can safely check
- Hot water target temperature (typically 45–55°C)
- Hot water schedule frequency
- Whether the system returns to heating after reheating
- Any boost or one-time hot water settings
Avoid disabling hot water priority unless advised by a professional.
When to call an engineer
Contact a qualified heat pump engineer if:
- The system never returns to heating
- Hot water reheating takes excessively long
- Error codes related to hot water appear
- The cylinder never reaches target temperature
- The system has never behaved correctly
In most cases, issues can be resolved through settings, sensor checks, or flow adjustments.
Key takeaway
Hot water priority is a normal and intentional feature of heat pump systems. It ensures reliable hot water by briefly pausing space heating when needed.
If heating pauses occasionally but hot water is reliable and the system recovers quickly, hot water priority is doing exactly what it should.