Expansion Tank Sizing

Mon, June 06, 2016

Losing your marbles? Are air problems getting you down? The details surrounding expansion tank sizing and location, air separation sizing and location, and fill pressure result in more problems and seem to be shrouded in more mystery than all others I have run into combined. Here are the guidelines you need to do them properly: Expansion Tank Sizing: Expansion tanks are placed in hydronic heating systems to absorb the increase in water volume as the water in the system is heated. The tank must be big enough to allow the water to expand without increasing the system pressure beyond the relief valve setting. Actually, it is wise to allow for ten percent below the relief valve setting, because many valves will begin to relieve at ten percent below their rated pressure. So, a 30 PSI valve may actually begin to lift at 27 PSI. You need five pieces of information to size the tank: 1.) System Volume, 2.) Fill Pressure, 3.) Relief Valve setting, 4.) Fill Temperature, and 5.) Design Temperature.

1. System volume refers to the amount of water in the system. The expansion volume is directly related to the system volume. In an existing system, volume can be established by draining it and measuring what comes out. The volume of a new system is calculated by an Engineer using the lengths of pipe, their lengths, and the volume of the boiler, and of the tank volume. Below, for your reference, is a list of the linear volume of typical pipe sizes:  = .1 gal,  = .2 gal, 1 = .3 gal, 1-1/4 = .4 gal, 1-1/2 = .5 gal, 2 = .6 gal. Consult the boiler manufacturer for the volume of the boiler in the system.
2. Fill pressure into a hydronic heating and/or cooling system should be set so the cold pressure at the top of the system should be a minimum of 4 PSI. Most of the time, make-up is at the bottom, so your PRV should be set for the height of the system, converted to PSI plus four. The conversion feet to PSI is to divide the height of the system in feet by 2.31, then add four. That is the cold fill setting for the PRV. If your building is 30 feet tall, divide that by 2.31, and then add four. 13 + 4 = 71 PSI. If the PRV is left at the factory-set 12 PSI you will have problems! The primary reason to have a minimum of 4 PSI at the top of the system is so the air can be properly vented.
3. Relief valve capacity is determined by the boiler manufacturer. A relief valve is typically provided with the boiler, and is sized with the proper pressure and BTU ratings. Many boilers are available with more than one option for relief valves. A replacement valve would be sized for the same pressure setting, and the same or greater BTU rating.
4. Fill temperature in the Chicagoland area is often assumed to be 40F because that is the coldest the water is coming from the city.
5. Design temperature is the maximum temperature of the water in the system. For the purposes of tank sizing, that temperature should be the same as the high-limit setting of the boiler.

The expansion tank should be located, or joined to the system close to the suction side of the pump. This is because the tank is 'the point of no pressure change' in the system. That is often misinterpreted as the point at which there is no pressure change at all, but that is incorrect. There should be no pressure change relative to whether the pump is on or not. The pressure should change as a result of the increase in temperature in the system. The properly located tank will assure proper pump operation.