Sludge in Radiant Systems

Black sludge and other debris is often the initial sign of trouble for a hydronic system. Left untreated, corrosion will drastically shorten the life of any hydronic heating system. But how is corrosion treated? To answer this question it is important to understand what corrosion is and how it is caused. Different types of corrosion will require different types of treatment.

The most common cause is basic oxidation, or rust. Oxidation happens when oxygen, which is entrained in the water, reacts with ferrous (iron based) components. The rate in oxidation is doubled with every 18 degree rise in water temperature. Being a closed-loop radiant system, the internal water generally operates in the 100°-140°F range. Even a small amount of oxygen can cause significant corrosion. A simple field test can be used to visually verify corrosion. Partially fill a clear, clean glass with system fluid and then hold a magnet up to the side of the glass. Watch as the floating black particles migrate towards the magnate. This happens because the black particles are rust, composed mostly of iron, which is naturally attracted to the magnet.

Another fairly common cause for corrosion is an improperly balanced pH level. An ideal pH level is 7 on the pH scale, which is neither acidic nor basic. A low pH level will turn the system fluid acidic, causing the fluid to “eat away” at the ferrous components. Conversely, a high pH level is basic. Basic levels can be equally aggressive depending on the metals used. For the most part, basic pH levels do not adversely affect hydronic components themselves. Instead, a high, or alkaline, pH level tends to cause unwanted scale deposits. All the dissolved materials naturally present in the water supplied to the system will more likely precipitate out at higher pH levels, causing potential obstructions or reduced system performance over time. An example of a high pH system generally involves glycol. Most glycol based systems tend to start out more towards the basic pH range due to the inhibitors used.

The downside to glycol is the pH level is not constant and will drop as the glycol ages. Aging is a result of the glycol solution absorbing oxygen (which makes it an effective tool against oxygen permeation). A good practice to follow is to perform a yearly check of the glycol and re-fill with a new solution or add inhibitors when needed.

If the corrosion takes on the form of orange sludge, or has a pungent order, the system might be experiencing microbial growth. With water temperatures below 160 degrees, oxygen, and a food source such as glycol or sulfur rich water, microbial life can begin growing in the system. The resulting sludge is created when these microbes die. This is a particular problem with areas using artesian wells, which tend to be a good source of sulfur.

The last main cause for corrosion is electrolysis due to dissimilar metals. Because of all the trace metals present in city water, the possibility of these different metals reacting with each other and various system components is fairly significant. Metallics suspended in water create an excellent electrical conductor, increasing the chance of corrosion in systems with high ferrous content.

The best solution to any of these conditions is to ensure the system has been properly treated. To better evaluate the situation and determine which problems are causing the corrosion a fluid test should be performed. Companies which conduct these tests will be able to determine the nature and root-cause of the corrosion as well as offer a means of correction. Most “cures” include adding a specially formulated additive to the system. Although these additives will cure the current corrosion issues they won't eliminate the need for regular system maintenance and fluid checks. Inspect the hydronic system before the beginning of each heating season and take the necessary steps to ensure a corrosion-free season!

Christopher Campfield
Watts Radiant System Designer