When is it necessary to replace old steel unions with new ones
Steel fittings are a critical point in every heating system, where rigid pipe meets the radiator or boiler, yet leaks often occur here due to corrosion or mechanical damage. Replacing old parts with new ones is not only a matter of aesthetics, but primarily of safety and long-term functionality of the entire system.
Signs of necessary replacement
Corrosion on the surface of the fitting, indicated by rust spots or peeling paint, signals the beginning of material degradation, which can lead to cracking within a few years. If you notice even small drops of water directly from the body of the fitting or around the threaded connection, it is an immediate sign that the sealing is no longer functioning and the material has lost its integrity.
In practice, we have seen cases where neglected corrosion on a small fitting spread to the entire body of the radiator, requiring its complete replacement instead of simply replacing the parts.
Importance of correct size and type
Incorrect selection of the fitting size can cause insufficient tightening or, conversely, cracking of the material when trying to compensate for the mismatch, so it is essential to accurately measure the outer diameter of the pipe before purchasing. For standard home heating, the standard sizes are 15 mm and 18 mm, while larger diameters use 22 mm versions, which require different force parameters during installation.
- For 15 mm pipes, we recommend the model Connecting fitting - with internal thread - C - 15mm-Rp1/2".
- For 15 mm pipes with higher flow, use the variant Connecting fitting - with internal thread - C - 15mm-Rp3/4".
- For 18 mm pipes, the ideal choice is Connecting fitting - with internal thread - C - 18mm-Rp1/2".
A basic mistake during replacement is underestimating the difference between the outer diameter of the steel pipe and the inner diameter of the new fitting, which leads to instability of the entire connection.
Installation technology and pressure
When installing new parts, it is crucial to adhere to the maximum operating pressure of the system, which should not exceed the value specified by the manufacturer, to prevent destruction of the threads. When tightening the fitting, it is necessary to use a torque wrench or follow the manual, as excessive tightening can damage the threaded surfaces and create micro-cracks.
We also recommend reading the guide How to properly tighten threaded fittings to avoid damage, where the correct technique for applying tape and the step-by-step tightening process is described in detail.
Impact of temperature and water chemical composition
Thermal expansion and changes in the chemical composition of water can cause stress in the connections, which is manifested by loosening of the threads or cracking of old parts, especially if they were installed years ago without thermal insulation. If the system operates at temperatures above 80 °C, it is advisable to replace the original fittings with those designed for higher temperature ranges and resistance to thermal shocks.
In practice, it happens that old fittings, originally intended only for cold water, begin to behave unstably after the first heating season due to different thermal expansion of the materials.
When to choose press fittings instead of threaded fittings
If you are planning a comprehensive system renovation, consider switching to press fittings, which eliminate the risk of leaks caused by improper tightening of threads and provide a longer lifespan without the need for regular maintenance. Press fittings are an ideal solution for modern systems where fast installation and high reliability are required, without dependence on the quality of manual tightening.
For more information on this alternative, you can read the article Press fittings vs. threaded fittings: when to use which solution, which will help you decide on the optimal solution for your specific project.
Corrosion prevention and extending lifespan
New fittings are always a better investment than repairing old ones, as new parts have a corrosion-resistant layer that has already been compromised in the old ones. To prevent future corrosion, it is important to regularly check the condition of the fittings and, if necessary, apply an appropriate preservative or replace the sealing materials.
We recommend reading the section How to prevent corrosion on the connecting fitting in heating, which provides specific tips for maintaining the integrity of connections throughout the year.
Purpose and function of expansion gap
New fittings often include an internal expansion gap that compensates for the different thermal expansion between the steel pipe and the cast iron body of the radiator, thus preventing the formation of stress cracks during sudden temperature changes.
Frequently asked questions
Can I just tighten the fitting if it is leaking?
If water is dripping, it means the seal is damaged or the thread is worn, so tightening will only provide temporary relief, and the problem will soon return.
How can I determine if I have a 15 mm or 18 mm pipe?
Simply measure the outer diameter of the pipe with a caliper; a 15 mm pipe has an outer diameter of approximately 15 mm, while an 18 mm pipe has a diameter of 18 mm.
Is it possible to use an old fitting on a new system?
No, you should always replace an old fitting, as its material resistance and sealing properties are already compromised and a leak is likely.
Replacing an old steel fitting with a new high-quality part is one of the most effective investments in the long-term safety of your heating system, preventing costly repairs due to subsequent damage.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.