The building currently has natural gas and uses radiator heating, there is no central cooling system.

A good way to lengthen the life span is to use 40 or 60 watt LED bulbsOld radiators will leak, especially with steam.

There are two 6' and one 2' radiators tied together and from there the line goes up to one room in the 2nd floor unit and  then return.

Electric oil filled radiators . 

However, after the installation, one specific steam radiator is incredibly slow to heat up. 

The attic really doesn't need any significant work, I can tell people have lived in it before - It's got bedrooms walled in already, there are radiators up there, windows, etc.  

All the heat is done by old radiators with a 100 year old radiant heat furnace in the basement coated in asbestos that can't be removed because it's too large.  

I bought a really old 3 unit (each unit about 1300 sqft) and had radiator heat and window units.

Boiler system is adding  baseboard radiators with pex line would be my first guess  More than likely it had enough residual heat from above to keep it above freezing and so pipes wouldn't freeze before.

Does the house have forced air or hot water heating, and if so, where are the vents/radiators?