Filters, Strainers and Magnetic Filters
Filters, strainers, and magnetic filters all exist to do the same basic job: protect your heat pump by keeping debris out of critical components. When they block, they commonly cause flow faults, cold radiators, noisy pumps, and repeated breakdowns.
This guide explains the differences between filters, strainers, and magnetic filters, what each one catches, where they are typically located, and why they are so important on heat pump systems.
Why heat pumps need clean water
Heat pumps move a large volume of water continuously at relatively low temperatures. Any restriction to that flow has a much bigger impact than it would on a traditional boiler system.
Contaminated system water can damage:
- Circulation pumps
- Plate heat exchangers
- Flow sensors
- Control valves
Filters and strainers exist to stop this damage before it happens.
Magnetic filters
What is a magnetic filter?
A magnetic filter uses a powerful magnet to capture magnetite sludge – a black, metallic residue formed when steel radiators and pipework corrode internally.
What they protect
- Circulation pumps
- Heat exchangers
- Motorised valves
Where magnetic filters are usually fitted
- On the return pipe to the heat pump
- Near the indoor unit or hot water cylinder
- Sometimes inside the heat pump casing
What happens when a magnetic filter blocks
- Reduced system flow
- Flow or circulation error codes
- Cold or uneven radiators
- Increased pump noise
Magnetic filters are one of the most common causes of repeat flow faults on recently installed heat pump systems.
Strainers (mesh filters)
What is a strainer?
A strainer (often called a Y-strainer) uses a fine metal mesh to catch physical debris suspended in the heating water.
Common debris includes:
- Solder blobs
- Plastic swarf
- Dirt from installation
- Limescale particles
Unlike magnetic filters, strainers do not attract sludge. They rely purely on physical filtration.
Where strainers are usually located
- Inside the heat pump
- Near plate heat exchangers
- On flow or return pipework
Many heat pumps contain internal strainers that homeowners may not be aware of.
Standard filters vs magnetic filters
| Component | What it catches | Typical blockage |
|---|---|---|
| Magnetic filter | Magnetite sludge | Black, oily residue |
| Strainer / mesh filter | Physical debris | Grit, solder, flakes |
Many systems use both. Either one can cause serious flow problems if blocked.
Symptoms of blocked filters or strainers
- Radiators or underfloor heating going cold
- Heat pump running but not heating the house
- Flow or circulation error codes
- Frequent stopping and starting
- High flow temperature with poor heat output
- Whining or noisy pumps
These problems often develop gradually, rather than immediately after installation.
Can homeowners clean filters themselves?
Sometimes – but not always.
You should only attempt to clean a filter if:
- You know exactly where it is located
- Isolation valves are clearly fitted
- The manufacturer allows homeowner access
- You are confident managing system pressure
If you are unsure, do not attempt it. Incorrect cleaning can introduce air, cause leaks, or void warranties.
How often should filters and strainers be cleaned?
- First year: very likely to need cleaning
- Annually: as part of routine servicing
- After system work: radiators, pumps, or valves
Older radiator systems often require more frequent cleaning, particularly in the early years after a heat pump is installed.
When to call an engineer
Contact a qualified heat pump engineer if:
- You can’t locate the filters or strainers
- The heat pump casing needs opening
- Error codes persist
- System pressure becomes unstable
- The system has never been serviced
Cleaning filters and strainers is routine work for engineers and often quick to resolve when done correctly.
Key takeaway
Filters, strainers, and magnetic filters are small components with a huge impact on heat pump reliability. When blocked, they are a leading cause of flow faults and poor performance.
If your heat pump used to work well and has gradually declined, checking filtration should be one of the first diagnostic steps.