Why is an expansion tank above the boiler?

**Why is the expansion tank always found above the hydronic heating system? **

An added benefit or function of an expansion tank (in addition to providing a place for expanding heated water) is that the boiling point of the water in the hydronic heating system can be raised by elevating the tank.

When you elevate or raise an expansion tank you increase the head, which is a term used to describe the difference in elevation between two points in a body of fluid. When you increase the head, you increase the pressure.

This results in the ability to heat water at a significantly higher temperature without generating steam. The more heat that is supplied to the heat-emitting units, the better.

That is why you will always see an expansion tank over the boiler.
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Your explanation is shaky. There are two types of expansion tanks used on hot water boiler systems. The steel expansion tank and the bladder type expansion tank. These expansion tanks are used to provide a cushion for the expansion of water when it is heated. Without this cushion of air, the pressure in the boiler would rise and exceed the setting on the pressure relief valve, resulting in the PRV opening and allowing water to discharge and ultimetely relieve system pressure. Often times, when systems are on, one will hear a whooshing sound, and see discharge of water from the system’s PRV. While it could theoretically mean a defective PRV, most times it is a defective expansion tank.

Height and hydrostatic head have little to do with why these tanks are installed. Often, the smaller bladder style tanks are installed adjacent to the heating system, sometimes nearly at-level with the boiler tank, itself. The main reason that the older tanks were installed between the joists, far above the actual system, was a matter of convenience.

Technically speaking, hydrostatic head is a result of a combination of volume and column height. It is defined as follows:

“…the pressure at a given point in a liquid measured in terms of the vertical height of a column of the liquid needed to produce the same pressure…”

Expansion tanks, themselves, primarily serve as a receptacle for the expanding water. Fluid in these tanks are a byproduct of the fact that heated water expands. When the sealed units fail, and the air bladder no longer provides a sealed cushion, they become water-logged and require replacement.

Expansion tanks should not be installed UNDERNEATH a boiler. :slight_smile:

Yes, Ben…

Never said below. Said adjacent to.

My comments go to the notion of hydrostatic head, the height of the tank somehow being better, and what the primary design of the tank is intended for.

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