---
title: "What is Suspended Solids mg/L?"
description: "Suspended solids measure the amount of undissolved particles, such as rust, corrosion debris, sludge and dirt, circulating within the system water. High levels can indicate a dirty or corroding system and may contribute to blockages, reduced circulation, component wear and poor heating performance."
url: https://help.fernox.app/en/articles/150-what-is-suspended-solids-mgl
collection: "Chemical Parameters"
updated: 2026-10-02
language: en
alternates: [es, de, fr, pl, nl, cs, fr-be, nl-be, ro, sk, tr, sv]
---

# What is Suspended Solids mg/L?

Suspended solids measure the amount of undissolved particles, such as rust, corrosion debris, sludge and dirt, circulating within the system water. High levels can indicate a dirty or corroding system and may contribute to blockages, reduced circulation, component wear and poor heating performance.

### Suspended Solids — mg/L

**Technical definition**  
Suspended Solids is a measurement of the concentration of **insoluble particles suspended within the heating system water**, expressed in **milligrams per litre (mg/L)**.

These particles may include **corrosion products, magnetite, rust, scale fragments, installation debris, dirt, sludge and other particulate contamination** circulating within the system.

Unlike dissolved substances, suspended solids are not chemically dissolved in the water and can potentially be removed by filtration, separation or settlement. Their concentration provides a useful indication of the **physical cleanliness of the heating system water** and the extent to which particulate contamination may be circulating through system components.

**Simplified definition**  
Suspended solids are the small particles of dirt, rust, corrosion debris and other material floating around in the heating system water.

High levels can indicate that the system is dirty or that corrosion and sludge are developing. These particles can circulate through the boiler, radiators, pumps and valves and may cause **blockages, wear and reduced system performance**.

### Why are Suspended Solids important to test?

Suspended solids are important because excessive particulate contamination can affect the **efficiency, reliability and longevity** of the heating system.

An abnormal suspended solids result may indicate:

-   **Corrosion products** circulating within the system, including iron oxides such as magnetite.
    
-   Accumulation or mobilisation of **sludge and deposits** within radiators and pipework.
    
-   Installation debris remaining within a new or recently modified system.
    
-   Scale or other deposits breaking away from internal surfaces.
    
-   Inadequate flushing or cleaning of the system.
    
-   Poor filtration or ineffective magnetic separation.
    
-   Ongoing corrosion that is continuously generating new particulate material.
    
-   Contamination introduced during installation, maintenance or system modification.
    

High levels of suspended solids can contribute to a range of system problems, including:

-   **Blockages and restrictions** within narrow waterways, valves and heat exchangers.
    
-   Reduced circulation and uneven heating.
    
-   Accumulation of sludge in radiators, potentially creating cold spots.
    
-   Increased wear on pumps and other moving components.
    
-   Fouling of boiler heat exchangers.
    
-   Reduced efficiency of control valves and thermostatic radiator valves.
    
-   Increased risk of component failure.
    
-   Reduced effectiveness of heat transfer and overall system performance.
    

Fine magnetic corrosion products such as **magnetite** are particularly significant in modern heating systems. These particles can circulate with the system water and accumulate within pumps, valves, heat exchangers and other components with small internal waterways.

Suspended solids testing therefore provides an important indication of whether the system water is sufficiently clean and whether further investigation, **system cleaning, flushing or filtration** may be required.

### Important interpretation note

Suspended solids should **not normally be assessed in isolation**. The quantity of particles present indicates the level of particulate contamination but does not necessarily identify their composition or the underlying cause.

For example, elevated suspended solids could result from active corrosion, historic sludge being disturbed, installation debris or deposits released during cleaning.

The result should therefore be considered alongside parameters such as **iron, copper, aluminium, turbidity, inhibitor concentration and pH** to help determine whether the contamination is associated with corrosion or another system condition.

It is also important to consider whether the sample accurately represents the circulating system water, as suspended material may settle when water remains stationary and can therefore vary depending on **where and when the sample is taken**.