Best Practices for Hydronic Systems Part 6: Keeping Air Out of Your Hydronic System

Air in your hydronic system can result in noisy operation, reduced heat transfer, and equipment damage and corrosion. Fortunately, by observing a few simple rules, you can keep system air to a minimum.

Keep Your System Pressurized!

Hydronic systems should be pressurized so that there is at least 4PSIG at the top of the system piping. System pressurization should be done when the system is cold and the pump is off. This is because the amount of air that can be dissolved in water increases with pressure and decreases with temperature. When pressuring the system, the pressure reducing valve will need to be adjusted for the proper cold fill pressure. Remember that 1 PSI Water Pressure = 2.31 Ft Elevation. So if you know the elevation between the top of the system and the pressure reducing valve location you can calculate what the PRV setting should be. To review this simple calculation, please see our blog Air Management and Pressurization – Part 3 How to Calculate System Fill Pressure. As long as you have at least 4 PSIG at the top of the system, you should have enough water pressure to vent air out of the manual air vents.

Install Air Separator in Optimum Location for Air Removal

All hydronic systems need an air separator to assist in getting air out of the system. The best location for the air separator is usually at the point in the system where the water is the hottest and the pressure is lowest. Again, this is because the solubility of air in water increases with pressure and decreases with temperature. Of course, the lowest pressure and highest temperature are not necessarily going to occur at the exact same spot. Generally speaking, the point of lowest pressure will suffice, but if that is not possible, go with the point of the highest temperature (with the possible exception of high-rise buildings because the elevated pressures might prohibit air coming out of solution).

Install Balance and Control Valves on Return Side of Coils

Air in coils is particularly problematic, as it not only can cause noise but it also interferes with heat transfer. By installing the balance valve on the return side of the coil we are able to keep the water pressure higher in the coil. This extra pressure keeps the air in the solution while it is circulating through coil—which in this case is a good thing because it keeps free air from causing poor heat transfer or noise from the coils. Alternatively, installing the balance valve on the entering side of the coil reduces the pressure inside the coil, which creates more opportunity for air to come out of the solution and cause problems.