Gas and Oil Home Heating Furnaces - expansion tank?? in hot water heat system

I have a very old hot water heat system in my home. There is a 'tank' above the boiler (in the floor joists) that have sprung a leak on a seam. I disconnected it so I could continue to heat the house. How important is it?? There is only one (1) pipe leading into (or out of) the tank. It is a branch off the boiler out pipe. There is a drain valve on the bottom.

Would really appreciate info about this.
Dave

That would be your expansion tank by the sounds of your explanation. Get it repaired A.S.A.P. By not using it, you could or will find yourself in an over-pressure situation. Get a boiler guy in there to install a new diaphram tank.

When h20 is heated, it expands, filling more space, this compression tank allows for the expanding h20 & is a good place to accumulate air that has entered he closed system. without the tank, each time cooler h2o is heated, the increasing pressure would cause the pressure relief to open, venting the excessive pressure & wasting resources, also it acts as a shock absorber to help reduce water hammer in the system, it must be fuctional for proper operation. Take care

The "drain tube" you mentioned actually has a dip tube that goes part way up into the tank, this allows partial draining of the tank to ensure that the top portion of the tank has air, as tank should not be completely filled with water, a full (flooded) compression tank will cause that pressure relief to open as mentioned b4, because there is nowhere for the expanded water to go. See
ya