Hydronic Expansion Tanks, Then and Now

Many homes and commercial properties use hydronic (hot water) heating systems. The energy source can be oil, natural gas or even wood. In a modern hydronic system, the flame in the boiler heats water, and a circulating pump pushes the water through piping to radiators and convectors. The name “boiler” is misleading, as these systems don’t boil water. They can create excellent comfort levels as they radiate heat to our bodies. They also allow easy control of heat in different indoor zones, adjustable for various temperature and heat-loss needs.

Hydronic boilers (Illustration H074C) heat cold water to about 160 degrees F. When heated, the water expands about 5%. Where does that extra water go?

Way back before the 1930s
Home heating systems before 1930 had no pumps (Illustration H071C). In this so-called gravity system, a boiler beneath the first floor heated water, which expanded and naturally flowed upward in large pipes. A tank in the attic held the heated water. As water cooled, gravity pulled it down through return pipes to radiators within the home, where the water gave off heat, and finally to the boiler. This “open” system also provided the attic tank with a drain line to dump excess water outdoors. Gravity systems were installed before pressurized water mains became available, so the heating contractor had to carry buckets of water up to the attic to fill the system (Photo 1).

The Roaring 20s Harnessed Electricity
Once electrical power became available in the 1920s, heating systems started using circulating pumps to move water through the piping. These pipes could be smaller than in the previous systems and the radiators also could be smaller. The system depended less on a perfect pipe layout and was less expensive to install. It also was more efficient, as it had no openings to the outdoors. A compression tank was located in the ceiling near the boiler (Photo 2).

The compression tank operated with about two-thirds water and one-third air. As the heated water expanded, it pushed into the tank and compressed the air. This raised the system pressure a few pounds. Remember: Water cannot be compressed. If water is heated in a closed system, the excess water must be stored or released. When a compression tank is filled with water and the air cushion can’t be compressed any further, the excess pressure will pop the pressure relief valve on the boiler and dump the water until the pressure is below the maximum of about 30 psi.

Modern Systems Keep Air and Water Separate
Modern systems more efficiently solve the problem of excessive pressure and dumping. Excess heated water is stored in an expansion tank (Photo 3). These small tanks use an air-filled rubber bladder that separates the air from the water. The bladder is compressed when excess water enters the tank, so the air cushion is not lost to the water.