One of the most overlooked and misunderstood components of a hydronic heating system is the expansion tank. When a heating system is designed the heat loss and boiler sizing are usually carefully calculated. The circulator is sized based on the length and heat carrying capacity of the piping. When it comes to the expansion tank, most guys just install the same size tank they have always used. The phrase, “We’ve always done it that way.” comes to mind. Oddly enough, most of the time it works. It’s when it doesn’t work that the problems arise. The most common symptom of an undersized expansion tank is the relief valve blowing off. To avoid this, the installer should understand the function of the tank and the importance of proper sizing.
Let’s start with the basics. We all know that things expand when they are heated and contract when they are cooled. Water is no different and this becomes a major factor in a closed loop hydronic heating system. Upon installation of a new system, you fill the pipes with 40 to 50 degree water and purge the air. Then when you fire the boiler and heat the water to 180 degrees, the water expands. As the water gets hotter the volume increases. Without compensating for this expansion of the water, the pressure in the system would go up and at 30 pounds, the relief valve would open. An expansion tank is designed to accommodate this increase in water volume in a closed loop system. Water is not compressible but air is and the expansion tank uses a cushion of compressed air to accept this increased volume of water as it is heated.
Old style expansion tanks mounted between the floor joists in the cellar ceiling were made out of steel and had tappings for water, air inlet, and a water level gauge. When the system was filled, the water entered the tank and compressed the air at the top. As the water heated, the level in the tank rose and the air compressed even more. Eventually, the air cushion would be absorbed into water and the tank would become waterlogged. This of course caused the pressure to rise which set off the relief valve and always led to a service call.
In the early 60’s the modern diaphragm tank was developed. It uses a rubber diaphragm to permanently separate the system water from the compressed air in the tank, thus eliminating the waterlogging problem. These tanks became popular because they were small and compact, inexpensive and needed much less plumbing. This type of tank, however, still can cause potential problems if not sized properly.
Let’s take a look at the five things we need to know to properly size a diaphragm expansion tank.
Fill Pressure ---In a closed loop system, static height of the piping is a factor only when filling the system. One pound per square inch (PSI) of water pressure is required for every 2.31 feet of static height. In the average 2-story house with the boiler in the basement, we must raise that water about 16 feet which would require about seven PSI. In order to ensure positive pressure and aid in air removal, we should add another five PSI to this. Pressure reducing valves (PRV) are factory set at 12 PSI and expansion tanks are pre-charged to the same 12 PSI. Most of the time the 12 PSI pre-charge is enough but if the system height is greater than 16 feet the pressure reducing valve must be adjusted higher and the air charge in the tank must be increased accordingly. A tire gauge and a bicycle pump are all the tools you need to accomplish this. Always remember the tank must be isolated from the system when checking pressure. When checking the tank pressure, the system pressure must first be reduced to zero and the tank isolated by means of the isolation valve. If you don’t follow this procedure, you are not reading the tank pressure; you are reading the system pressure.
Relief Pressure---We need to know this in order to size the tank. Most residential relief valves are set at 30 PSI.
Fill Temperature---When you fill the system with tap water, the temperature is usually around 50 degrees.
Average Water Temperature--- This is the normal high limit temperature of the boiler. The more we heat water, the more it will expand.
Total Water Content--- This is critical to tank sizing. Old gravity systems that have been converted to forced circulation will have much more water than a new baseboard system. Three quarter inch copper pipe contains .025 gallons of water per linear foot while inch and a half steel pipe holds .106. Measure the total footage and multiply by the appropriate amount. Then add the boiler water content and allow a couple of extra gallons for the fudge factor.
There are many factors in determining expansion tank size and any unusual variations in any one of these areas could dramatically affect the tank size. Manufacturers have plenty of experience in this regard and have developed guideline charts based on average systems. Looking at the chart, you can see that systems with higher water content like cast iron radiators and baseboards will require a larger tank than a copper baseboard system.
