Heat Pump Flow Problems – Causes, Symptoms & Fixes
Flow problems are one of the most common reasons heat pumps perform poorly or shut down. They can cause cold radiators, frequent faults, noisy operation, or a heat pump that runs but doesn’t actually heat the house.
This guide explains what flow is, how to recognise a flow problem, the most common causes, and what you can safely check before calling an engineer.
What does “flow” mean on a heat pump?
Flow refers to the movement of heating water around your system:
From the heat pump
Through pipes, valves, and pumps
Into radiators or underfloor heating
Then back to the heat pump (return)
A heat pump is designed to move a large volume of water at a low temperature. If that water can’t move freely, the system can’t transfer heat properly.
Common symptoms of flow problems
You may have a flow issue if you notice any of the following:
Radiators or underfloor heating are cold or only warm at the bottom
Heat pump keeps starting and stopping (short cycling)
Error codes relating to flow, circulation, or temperature difference
Heat pump is running but the house isn’t warming up
Loud rushing, gurgling, or whining noises from pipes
Very high flow temperature but little heat output
System works upstairs but not downstairs (or vice versa)
Typical heat pump flow error codes
Many manufacturers monitor flow electronically. Common fault descriptions include:
Low flow
Flow switch open
Circulation fault
High delta-T (large difference between flow and return temperatures)
Pump speed or pump fault
If you see an error code, use the manufacturer error code finder before continuing.
The most common causes of flow problems
1. Closed or partially closed valves
This is by far the most common issue.
Check for:
Isolation valves near the heat pump
Valves on the cylinder or buffer tank
Zone valves stuck closed
Recently fitted valves left shut after servicing
Even one partially closed valve can restrict flow enough to cause faults.
2. Air trapped in the system
Air prevents water from circulating properly.
Signs include:
Cold spots at the top of radiators
Gurgling or bubbling sounds
Inconsistent heating between rooms
Radiators, manifolds, and sometimes the heat pump itself may need bleeding.
3. Circulation pump not running correctly
Your system relies on a pump to move water.
Possible issues:
Pump stuck after summer shutdown
Pump set to too low a speed
Electrical or control fault
Pump running but seized internally
Some pumps can be freed manually, but many require an engineer.
4. Blocked filters or strainers
Most heat pump systems include magnetic filters or strainers.
If blocked, they restrict flow and can cause:
Flow faults
Overheating
Reduced efficiency
Filters should be cleaned regularly as part of servicing.
5. TRVs or zone controls restricting flow
Unlike boilers, heat pumps do not like lots of valves closing.
Problems occur when:
Many TRVs are shut
Rooms are zoned too aggressively
A single room thermostat controls the whole system
This can starve the heat pump of flow, especially during mild weather.
6. Incorrect system design or commissioning
Some flow problems aren’t user-fixable.
Examples include:
Pipework too small
No low-loss header or buffer where required
Incorrect pump sizing
Poor balancing
These issues often show up as ongoing faults since installation.
Quick checks you can safely do
Before calling an engineer, check the following:
Heat pump and controller are powered on
All visible valves are fully open
System pressure is within normal range
No obvious leaks or dripping pipes
Radiators have been bled
Filters haven’t been recently cleaned (if accessible)
If anything looks unfamiliar or requires tools, stop and call a professional.
Flow problems vs temperature problems
It’s easy to confuse these two.
Flow problem: Water isn’t moving properly
Temperature problem: Water is moving, but not hot enough
If radiators are hot near the heat pump but cold elsewhere, it’s usually flow.
If all radiators are lukewarm, it may be a temperature or control issue instead.
When to call an engineer
You should contact a qualified heat pump engineer if:
Error codes persist after basic checks
Pumps are noisy or not running
Filters are inaccessible
Flow faults keep returning
The system has never worked correctly
Ongoing flow issues can damage components and significantly reduce efficiency.
Related guides
Heat pump system pressure explained
Heat pump thermostat settings
Heat pump not working – step by step checker
Heat pump error codes by manufacturer
Key takeaway
A heat pump cannot work without good water flow.
Most flow problems are caused by closed valves, air, blocked filters, or control issues – not failed heat pumps.
Understanding flow helps you diagnose problems faster, avoid unnecessary call-outs, and get your system running efficiently again.