Heat Pump System Water Treatment: Inhibitor, Filters and Annual Water Tests

Most heat pump owners know the outdoor unit, the cylinder and the controller. Far fewer know that the water inside the heating circuit is one of the most important parts of the system, and one of the most neglected.

Poor system water rarely causes a dramatic breakdown. It tends to cause slow, confusing problems: weaker flow, noisy pumps, rooms that heat unevenly and an efficiency that drifts down year after year. This guide explains what system water treatment is, what UK standards expect and what to check.

The short answer

• Heat pump circuits should be chemically cleaned and flushed before commissioning, whether the system is new or an existing system being converted.

• A permanent in-line filter should capture both magnetic and non-magnetic debris.

• The water should be dosed with a suitable corrosion inhibitor, compatible with any glycol in the circuit.

• The inhibitor level, pH and cleanliness should be tested at every annual service.

• If nobody has told you any of this was done, ask.

Why water quality matters more on a heat pump

A boiler heats water hot and fast, so it can tolerate some dirt. A heat pump works differently:

• It circulates a large volume of water continuously, at relatively low temperatures.

• Its heat exchanger passages are narrow, so debris and scale build up on them quickly.

• It depends on steady flow. Restricted flow shows up as fault codes, short cycling and poor heat output.

Contaminated water can damage plate heat exchangers, pumps and filters, and the damage is gradual. A system that “used to work well and has slowly declined” is a classic sign.

If your heat pump keeps stopping and starting, read our guide to heat pump short cycling. Dirty water is one possible cause among several.

The three layers of protection

1. Clean and flush at commissioning

Water treatment starts before the heat pump is switched on. Old systems contain rust, sludge and installation debris. Even a brand-new system can contain flux, swarf and pipe scale. A chemical clean and flush removes this before it reaches the heat exchanger.

This applies to existing radiators and pipework being reused. Converting an old system without cleaning it first is one of the most common ways to inherit someone else’s sludge.

2. A dedicated filter

A magnetic filter on the return pipe captures iron particles (magnetite) that form when steel components corrode. It should be emptied during the annual service.

Magnetic isn’t enough on its own. Non-magnetic debris such as limescale, installation debris and degraded glycol passes straight through a magnet. That’s why heat pump systems typically use a filter combining a magnet with a fine mesh sieve.

Filters can also block. A blocked filter reduces flow and can increase pump noise, so it needs cleaning, not just fitting.

3. Inhibitor and biocide

Inhibitor slows corrosion and scale formation inside the system. It’s dosed at commissioning and should be tested regularly.

Two points are easy to miss:

• Underfloor heating and plastic pipework don’t remove the need. Even with no steel radiators, the system should be treated after cleaning, usually with a biocide.

• Glycol can break down without biocidal treatment. It can form a gel-like substance that a magnetic filter can’t catch.

Follow the product label for dosing. More inhibitor isn’t better, and it can cause its own problems.

What the standard expects (BS 7593)

BS 7593 is the UK code of practice for treating water in domestic heating systems. The current version, BS 7593:2019+A1:2024, includes heat pump systems in its scope.

One major manufacturer suggests re-dosing at roughly five-year intervals. Your inhibitor product and heat pump manufacturer may specify something different, so check the label and the installation manual rather than assuming.

Water treatment if you have glycol (or don’t)

Many UK installations use either glycol or anti-freeze valves for freeze protection. They affect water treatment differently:

• With glycol: the inhibitor must be compatible with it. Glycol concentration also needs testing, because both too little (freeze risk) and too much (reduced heat transfer, harder pumping) cause problems.

• With anti-freeze valves: the circuit runs on treated water. It still needs cleaning, filtration and inhibitor.

Our guide to anti-freeze valves vs glycol explains how to decide between them.

Signs your system water may be a problem

None of these proves poor water quality on its own, but together they’re worth investigating:

• Dark, dirty or rusty water when draining or bleeding a radiator

• Radiators cold at the bottom or in patches

• Flow-related fault codes or repeated low-flow warnings

• A noisier circulation pump than before

• Rising running costs with no change in weather or settings

• Pressure or flow problems that return after being fixed

• A heat pump that heats the house but needs a higher flow temperature than before

For general heat pump faults, Why Is My Heat Pump Not Working Properly? is a good starting point. If your system is working but running hotter than it should, read What Flow Temperature Should My Heat Pump Run At?

What the water side of an annual service should include

A genuine annual service should go beyond the outdoor unit. On the water side, expect:

• System pressure checked, and topped up if needed

• Expansion vessel checked

• Magnetic or in-line filter cleaned

• Strainers checked

• Inhibitor level and water condition tested

• Glycol concentration tested (if used)

• Flow and return temperatures recorded

If your last service didn’t mention any of this, it may have been a unit-only check. Our guides to how often to service a heat pump and what a heat pump service should cost and include cover what you should get for your money. For the wider picture, see how often an air source heat pump should be serviced.

Questions to ask your installer

Whether your heat pump is new or years old, these are fair questions:

1. Was the system chemically cleaned and flushed before the heat pump was connected?

2. What filter is fitted, and where?

3. Which inhibitor was used, and was it tested after dosing?

4. Is there glycol in the system? At what concentration?

5. When should the inhibitor and filter be checked next?

6. Where is this written down in my handover documents?

If you’re comparing installers, see How to Compare Heat Pump Quotes for what a good quote includes. If you want a second opinion before committing, read When Should You Get an Independent Heat Pump Review?

What not to do yourself

• Don’t add inhibitor without checking compatibility. Mixing products, or dosing a glycol system with the wrong one, can cause more harm than good.

• Don’t open a sealed system carelessly. It affects pressure and can introduce air.

• Don’t assume “it’s working” means “it’s fine”. Water problems build slowly.

• Don’t ignore your warranty. Check what your manufacturer requires.

Does poor water quality shorten a heat pump’s life?

It can. Debris and scale wear pumps and valves and foul heat exchangers, all of which affect longevity. Our guide on how long heat pumps last covers the factors you can control, and annual servicing is the biggest one.

FAQs

Does a heat pump need inhibitor?
Yes. The standard that governs domestic system water treatment includes heat pump systems, and inhibitor is part of it.

Is a magnetic filter enough?
Not on its own. Heat pump systems also need to catch non-magnetic debris, so look for a filter with a fine sieve as well as a magnet.

How often should system water be tested?
At every annual service, covering inhibitor level, pH, glycol concentration (if used) and general cleanliness.

Can poor water quality cause flow faults?
It can contribute, particularly through blocked filters and fouled heat exchangers, but flow faults have other common causes too, such as air, pressure and pump settings.

Do I need a powerflush?
Only if the system is genuinely sludged. Signs include dirty water and poor circulation. A professional assessment should come first.

Who is responsible for water treatment on a new installation?
The installer, as part of commissioning. Ask for it to be documented.

Final thoughts

Water treatment isn’t glamorous, but it protects the most expensive parts of your system. A clean circuit, a proper filter, the right inhibitor and a yearly test are cheap compared with a failed heat exchanger or years of unexplained inefficiency.

If you aren’t sure what was done on your system, start by asking, and get a second opinion if the answers are vague.