Tags: Expansion Tanks

Troubleshooting Expansion Tanks


An undersized expansion tank or one that is completely filled up with water will cause the boiler pressure to increase when the water heats. Because the expansion tank is too small or too filled with water to absorb the excess pressure, the relief valve will begin to drip. The dripping relief valve is only symptomatic of the real problem, and replacing the valve will in no way solve it.

There is not much you can do about an undersized expansion tank except replace it. As a rule-of-thumb, expansion tanks should be sized at 1 gal. for every 23 ft2 of radiation, or 1 gal. for every 3500 Btu of radiation installed on the job. In Table 10-3, the allowance is slightly higher.

If the problem is a completely filled tank, it should be partially drained so that there is enough space to permit future expansion under pressure. The first step in draining an expansion tank is to open the drain valve. The water will gush out at first in a heavy flow and then tend to gurgle out because a vacuum is building up inside the tank. Inserting a tube into the drain valve opening will admit air and break the vacuum, and the water will return to its normal rate of flow. After a sufficient amount of water has been removed, the drain valve can be closed.

table 10 3 Troubleshooting Expansion Tanks

Diaphragm Expansion Tanks


The air in a diaphragm-type expansion tank is separated from the water by a flexible rubber membrane (see Figure 10-44). These tanks are smaller than the closed steel tanks and come from the manufacturer precharged with compressed air. When the tank arrives at the site, the diaphragm is fully expanded against its inside surfaces. When the tank is installed and connected to the system piping, water enters the other side of the tank chamber and presses down on the diaphragm.

As a rule, diaphragm tank manufacturers will precharge their tanks to 12 psi, which is sufficient to match the water-fill pressure requirements of the typical house or small commercial building.

diaphragm expansion tank Diaphragm Expansion Tanks

Closed Steel Expansion Tanks


The closed steel expansion tank has no moving parts (see Figure 10-43). It is normally two-thirds filled with water and one-third with air. As heated water expands and its excess volume enters the tank, it compresses the air at the top of the tank. The compression of the air in the tank results in an increase of system pressure, which is indicated on the boiler pressure gauge.

When the system water cools down, its volume contracts, and the air in the tank expands back to its original volume, causing system pressure to fall. To sum it all up, the rise and fall of system pressure is created by the expansion and contraction of the air in the expansion tank.

One problem encountered with a closed steel expansion tank directly connected into the system is that the system water can absorb the air and send it to the radiators and convectors by gravity circulation. Installing a gravity-flow check valve on the expansion tank will prevent gravity circulation.

closed steel expansion tank Closed Steel Expansion Tanks

Expansion Tanks


Expansion tanks (also sometimes called compression tanks) are installed in hydronic (hot-water) space heating systems to limit increases in pressure to the allowable working pressure of the equipment and to maintain minimum operating pressures.

When the temperatures rise during the operation of the system, the water volume also increases and builds up pressure. The pressure in the system is relieved to a certain extent by the storage of the excess water volume in the expansion tank. When temperatures drop, there is a corresponding drop in water volume and the water returns to the system.

Maximum pressure at the boiler is maintained by an ASME pressure-relief valve. Minimum pressure in the system is generally maintained by either an automatic or manual water-fill valve.

Closed steel expansion tanks and diaphragm tanks are used to contain the expanding volume of heated water in residential and light-commercial hydronic heating systems. Some typical installations using ITT Bell & Gossett expansion tanks are illustrated in Figures 10-40 through 10-42.

airtrol Expansion Tanks

conventional expansion tank Expansion Tanks

top outlet boiler Expansion Tanks