The following is an excerpt from one of my books "10 Successful Repair
Strategies". With the weather turning cooler (at least in my little corner
of the world) this seemed like it might be the appropriate time to present
it.
Air Problems in Hydronic Systems
by Gary R. Lloyd CMS
Let's imagine a very long pipe - the diameter doen't matter - that's standing on end with the bottom capped. Now fill the pipe with water to the 28 ft. mark.
The weight of the water will cause a pressure at the bottom of the pipe of 12 PSI.
Water pressure regulating valves (PRVs) usually are factory adjusted for 12 PSI. This pressure is adequate in a hydronic system only if the system is less than 28 ft. in height. If any part of the system is higher than 28 ft., the pressure is not adequate to fill that part of the system.
Importantly, if an air vent is installed at the system's highest point - and that point is higher than 28 ft. - it will let air in instead of out. And that air will find its way into other parts of the system and cause air locks.
Another symptom of air entering the system is when the water level drops in the expansion tank. Given a constant pressure and temperature, the water level cannot drop unless air enters the tank.
Where does the air come from? The vacuum at the top of the system. This isn't the only source of air problems, but it's by far the most common.
At this point, the difference between fill pressure and expansion pressure should be explained. The PRV causes fill pressure. Expansion pressure is caused by adding heat after fill pressure has been achieved.
Let's add a few things to the pipe. Add a water source, a PRV (set at 12 PSI), and a pressure gauge at the bottom of the pipe.
Turn the water on. The pressure will rise to 12 PSI and the water level will rise to 28 ft. Now heat the pipe. The water will expand and rise in the pipe, and the pressure will increase. This is expansion pressure.
Now drain off some of the water until the PRV starts to feed. When the PRV stops feeding supply water, the pressure again will be 12 PSI (fill pressure) and the water level again will be at 28 ft.
Now let the pipe cool off. As the water contracts, the PRV will again feed water to maintain 12 PSI (the fill pressure).
You cannot accurately adjust the fill pressure until the expansion pressure has been removed from the system. The expansion pressure can be removed by draining off system water or cooling the system, until the PRV starts feeding water.
Most manufacturers recommend a cold-fill procedure to adjust the system pressure. This works fine for engineering or start-up purposes, but cooling the system isn't always possible. Draining to remove expansion pressure without cooling the system is a much more practical method for the serviceman.
In order to calculate the proper fill pressure, the height of the system must be known. Measure carefully. For those who enjoy mathematics, multiply the system height by 0.43 and add 4 PSI to determine the proper fill pressure.
I just divide by 2. It works.
The following is a practical, service-oriented procedure for adjusting fill pressure in hydronic systems:
Measure the height of the system and divide by two (or multiply by 0.43 and add 4 PSI) to determine the proper fill pressure.
Turn off the boiler and circulating pump(s).
Drain the boiler until the PRV starts to feed water.
Adjust the PRV to stabilize at the proper fill pressure.
Vent air from the system.
Adjust the water level (one third to one half) in the expansion tank.
Turn on the circulating pump(s).
If the pump discharges toward the boiler, check the pump inlet pressure. If it's less than 2 PSI, increase the fill pressure until it reads 2 PSI.
Turn on the boiler.