Low Temperature Systems & Non-Barrier Radiant Tubing
Reprinted from Weil-McLain’s Technical Services Bulletin #SB-0604
Will the use of non-barrier tubing in a radiant floor system affect the boiler?
The answer to this question is answered with two distinct possibilities that affect boilers in different ways:
Low supply and return water temperatures, used in radiant systems, can cause flue gas to condense and corrode the flue side of cast iron boilers.
Potential for excess oxygen, which creates oxygen corrosion affecting the water side of both cast iron and cast aluminum boilers.
Low Supply and Return Water Temperatures
When fuel is burned, water vapor, carbon dioxide and heat are produced. If the flue side surface temperature drops below the dew point (in the range of 110°F to 130°F) because of low water temperatures, water vapor will condense on the surface. Start-up condensation will cause the surface to flake away, significantly shortening the life of the boiler (see Figure 1). Cast aluminum boilers do not have this problem and are made for use with low water temperatures.
Low water temperature systems have been in use with cast iron boilers for many years. Diagrams provided in Weil-McLain installation instructions for cast iron boilers, clearly show how to handle these applications to prevent any damage to the boiler.
Potential for Excess Oxygen
Just like water dissolves salt, water can also dissolve gases such as oxygen. Like dissolved salt, dissolved oxygen is invisible. It is contained in the water and cannot be removed by even the best air separators. Even after you have eliminated all the air bubbles you can hear or see from the system fill water, dissolved oxygen is still present.
The only oxygen that gets into a typical copper pipe system enters either with the fill water or through minor leaks. When the water is heated, the oxygen becomes less soluble, comes out of the water, and is eliminated through the air vents. Therefore, very little oxygen is present.
If oxygen continues to enter the system, it can attack iron, steel and even brass in the system.
A constant supply of oxygen will damage system components, cause sludge build-up and shorten the life of the boiler due to excessive corrosion. This is true of cast iron and steel boilers. Oxygen dissolved in water has less influence on aluminum corrosion than on iron and steel corrosion.
Unlike metal pipe, non-barrier tubing can “breathe” (called oxygen permeation), much like oxygen passes through our lungs into the blood stream. This process can allow a constant supply of new oxygen to enter the system. You may not see or hear air bubbles, but dissolved oxygen is there. The amount of oxygen that passes through the tubing walls into the water depends on the tubing material and construction. A good analogy is the improvement is U.S. food packaging technology. Today, potato chips stay fresher much longer than 20 years ago because the plastic packaging includes barrier layers to prevent air from getting inside. Similar technology has been applied by some radiant floor tubing manufacturers to protect the system from oxygen permeation.
What are Weil-McLain’s recommendations?
There is no U.S. guideline for oxygen permeation in radiant tubing. The recommended maximum oxygen permeation rate for radiant tubing is based upon a European standard of .1 mg/l/day (one-tenth milligram per liter per day) or less. This is equivalent to the amount of oxygen that would enter the system if it were drained and refilled three times a year. Weil-McLain believes this standard to be viable and uses this as our recommendation of oxygen permeation.
How can you be sure you won’t have a problem with non-barrier tubing?
All Weil-McLain boilers should be isolated from high oxygen permeable systems. A simple solution is to use a corrosion-resistant metallic heat exchanger, sized for the system load, between the tubing and the boiler