The expansion tank talk

By Bob “Hot Rod” Rohr


This article will get you going on proper expansion tank selection, installation, and maintenance. Expansion tanks are such interesting and important components in hydronic systems.

In the hydronic world there are two types of expansion systems you are likely to encounter. The first is considered an air management type expansion tank system. These are generally found in older homes and buildings. They are not so commonly installed these days. This type of tank is often called a compression tank, or plain steel tank.

The tell-tale sign that you have a compression tank is a horizontal steel tank supported above the boiler, often squeezed into a joist bay, or just below it. The proper installation involves the tank, and a fitting into the tank, and in some cases an air purger installed into the boiler itself.

With this system, any air that is captured and eliminated from the boiler or piping is directed up into the top of the expansion tank, sometimes called compression tank. The air space needs to be maintained in the tank for the expansion of the water as it is heated. In these horizontal tanks there is a diaphragm or separation between the fluid and the air bubble. You cannot install an auto air purger or separator on this system without directing the expelled air up into the tank.

This is where the term air management comes in. When these tanks are improperly installed, with a separate air purger for example, the air space in the tank can be vented out, fill water enters and the tank becomes waterlogged. This may cause the relief valve on the boiler to seep or discharge as you do not have expansion “room” in the tank any more.

Often these tanks have a sight glass on one end to monitor the water level in the tank. These tanks are commonly 12- or 15-gallon capacity. So, when waterlogged, they can be very heavy and weigh well over 100 lbs. Be sure they are adequately supported.

The second system is known as an air elimination system. This system will have a tank that physically separates the water and air. Diaphragm tanks are most common, and you will know this tank by the crimp indent around the tank where the butyl rubber diaphragm is locked into place. Other versions have a bag or bladder inside that contains the water in lieu of the diaphragm. With this type of expansion vessel, you will want to have a properly installed quality air separator to remove all air from the boiler and piping. A central air purger at the boiler and possibly some high point auto vents should be used. These tanks will be about one-third the size of a plain steel compression tank since the air bubble is now captive in the tank.



Here are some tips for installing the air elimination diaphragm or bladder tanks.


  • Always check the pre-charge pressure before the tank is installed into a system. It is common to pre-charge the tank to the fill pressure you intend to set the system at. This is called static fill pressure.


  • A number of manufacturers offer expansion tank isolation/ service valves. These valves allow the tank to be serviced or removed for replacement. Some versions have a hose connection drain port valve allowing you to drain a damaged, waterlogged tank for quick and easy replacement. These are well worth the money. All tanks will fail at some point – make it easy for you, or the next technician, to service or replace a tank. Air pressure should be checked every three to five years, so these iso-valves also facilitate that service step.


  • Be sure to properly support the tank. There are various brackets and arms available to solidly mount the tank.


  • Remotely mount the tank, never over the top of the boiler or electronics as they may leak at some point.


  • Do not locate the tank directly under a separator, air or dirt, as this forces debris down onto the diaphragm, possibly shortening the tank’s life expectancy.


  • If you work on radiant systems that have non barrier tube, there are lined and stainless-steel tanks available now. Spend the money to use a radiant specific tank.


  • Use the online sizers available from most every tank manufacturer to ensure you have adequate capacity in the tank.


  • Multiple tanks can be connected together; just be sure the pre-charge pressure in the tank is set correctly on all the tanks.


  • When installing, where you locate the tank connection into the system establishes the point of no pressure change (PONPC). It is best to locate the circulator pump downstream of the expansion tank, referred to as “Pumping Away.” Be sure to read Dan Holohan’s book by the same name for more on the topic.


  • If you use glycol in your systems, always use the multiplier to increase the tank size.


  • There is no harm in over-sizing an expansion tank if you fall on the line between too small and over-sized.


  • With solar thermal and chilled water expansion tank installation we often over-charge the tanks. So set the tank pressure a psi or two below the fill pressure to allow a small amount of fluid to enter the tank. This ensures you maintain pressure in the tank and system as water cools and contracts. On a solar thermal system, you can experience a very wide temperature swing. Temperature will be at the ambient air temperature, and can reach well over 300°F under stagnation conditions, so be sure to use the solar expansion tank formula to assure you have adequate capacity (see Appendix 2 in Idronics 3 January 2008, Note that only the collector and outdoor piping will experience the wide temperature swing. The formula splits out the capacity of exposed piping. If not, the tank size gets incredibly large if you assumed the entire fluid content would swing 300° or more.


  • With chilled water systems we need to assure the circulator always has positive pressure on the inlet side to avoid potential cavitation. The tank over-charge assures you always have this pressure as water contracts when temperature is lowered. This is important if you embrace A2WHP, which can be used to heat and chill the system fluid.

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