If you do not want to suffer the possible consequences of this service procedure, ask a qualified professional to do this for you.
A hot water heating system is a sealed system. Water expands about 4% by volume when heated from about 50 to 240F, so an expansion tank accepts this ‘extra’ water. If there is no acceptable place for the water to go, the water will go out of the safety relief valve.
In the years around 1910 to 1930, the combination of an automatic feed and relief valve was used to add and release water as the wood or coal stoves cycled only 3 times per day and the amount of water lost was negligeable. Todays boilers cycle 3 to 6 times per hour. This brings in too much corrosive oxygen.
Expansion tanks permit the system to be a truly sealed system. The automatic feed, per manufacturer instructions, should remain shut until service is required to prevent fresh water makeup due to leaks. If the automatic feed water valve had been closed, the need for repair would have been more obvious and immediate because you would have bubble noises in the system and no heat from the upper radiation.
The total internal volume of the expansion tank must be about 4 times the expanded water volume made when heating the water from 50 to 240F. You can find how much water will be produced by letting your system cool down to room temperature (with the automatic water feed valve open) then close the feed valve and run the burners until the boiler shuts down on the high limit control. Just turn the thermostat all the way up.
Watch the boiler temperature to make sure the boiler temperature does not rise above this point. Shut the power switch down if it does rise too high. Cool the system and replace the Aquastat, then start the process again. If you do not want to suffer the possible consequences of this service procedure, ask a qualified professional to do this for you.
Make sure you have a bucket under the relief valve pipe to catch the extra water. As you have the mathematics to calculate the internal volume of an expansion tank, you will size it properly. It is OK if the new expansion tank is a little larger than required. The manufacturer has tables that indicate the acceptance volume of their tanks. If you would like more professional software, Amtrol has a web site with calculation tools.
The Aquastat is the electric control used to sense the water temperature within the boiler. The minimum function of the aquastat is to limit the highest temperature of the boiler. Residential boilers are required to remain below 250F. the Aquastat will shut off the power to the burner at 240F. The word Aquastat is the property of Honeywell. Other companies use a more generic name like water temperature limit control.
A tankless coil is a long coil(s) bundle that fits within the boiler to permit cold domestic water to enter the coil and leave (heated) to pass on to the taps. this requires a more complex limit control to set the boiler at a minimum and maximum domestic water temperature as well as limit the boiler maximum temperature.
There should be no free air in the system. When water enters the system from the street or well water there is about 4% of the water that is air by volume. By heating the water, the bubbles become larger and rise to gather at high spots in the system. This water should be bled from the system periodically by opening bleed valves when bubble noises become noticeable in the pipes. Automatic vents should not be left open, as oxygen can enter through openings by the law of partial pressures. Water inside a hydronic (hot water) heating system will retain about 2% entrained air that can not be removed. The air will become nitrogen as oxygen rusts ferrous metals within the system. After this, the water within the system is valuable as deoxygenated treated water. Be sure that the fresh water added is of good quality without excessive hardness or corrosive properties.
To see the proper piping, pictures of hot water systems and tankless coil instruction, visit http://www.hydronic.net