Older homes were often heated by a steam boiler and cast iron radiators. Those old radiators not only threw a lot of heat, they held it for a long time. With steam the radiator had a valve that would release a small amount of steam into the air in effect providing some humidification. These were great systems in their day and the truth is they are still quite good. But often it is thought that changing the "old" steam system to a forced hot water system is more modern and therefore better.
Changing the system is not necessarily better or more efficient. In fact it can be less efficient and detrimental to the new boiler.
The big problem with steam is controlling the temperature. You must boil water to get steam. Steam moves freely through the large pipes. There is no modulating control to regulate the temperature. Because of this draw back the system is often converted to forced hot water.
A pump is installed in the system piping along with a temperature controlling device. The boiler no longer has to make steam to provide heat, just hot water. The pump moves the hot water through the pipes when the controller tells it the proper temperature has been reached.
The problem with a steam to forced hot water conversion is the piping. Steam pipes are very large, 3-4 inches in diameter. This volume is necessary in order for the steam to move through the pipes and the condensed water to move back to the boiler. A forced water system requires much smaller piping, usually three quarters to one inch in diameter.
When a system is converted the old pipes are almost always left in place. This creates a huge problem for the new boiler. The volume of water in the system is much greater than practical because of the old steam pipe size.
Consider this, a one inch diameter pipe one foot in length has a volume of about five cubic inches, a pipe of the same length three inches in diameter has a volume of about 85 cubic inches. That is 17 times greater! That means the boiler has to heat and move 17 times more water.
Another problem is the pipe configuration. Steam pipes are not installed in a continuous loop. When forcing water through pipes, loops from the main pipe do not allow for good flow to the radiator. What effectively occurs is water trying to enter the loop from both ends.
A boiler designed and sized to heat a 1500 square foot home by pumping hot water through the pipes will be massively over burden. It will be unable to heat the home quickly or efficiently and ultimately completely breakdown prematurely.
The first photo is of a steel tank boiler found in a home that had been converted from steam to forced hot water. If you look carefully to the left and behind the boiler you will see the "old" cast iron boiler. This steel boiler is doomed to an early grave. Cast iron is the only type of boiler that should be used in this type of conversion application. Steel can not handle the extreme stress from the large volume of cold water entering the tank from the pipes on circulation. However an undersized cast iron tank boiler will suffer the same fate, it may just last a little longer than its steel counterpart. 
This boiler was test run during the inspection. It ran constantly for one hour and no heat was received on the second floor after that time. The second photo shows the pipe reduction and the size difference between the pipes.
When these conversions are made it is imperative that the boiler be correctly sized to compensate for the large volume of water in the system. If this limitation is not considered, the unit will be extremely inefficient and will fail well before its intended design life.
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