We purchased a 3 unit home that was built in the early 1900's.  Two units are section 8 from the Feds and one is free market.  The families have been there forever... At least 12 - 20 years.  The previous owners had a horrid prop. management company, a Real Property Management affiliate, and there was basically no work done to maintain the home.  The home is not to our standards and is borderline unsafe, so we've been slowly fixing the items we're allowed to fix.  The furnaces are 40+ years old and the boilers are from the 80s. Since we are paying the heat, we need to find a way to  lower the heating bills.  The monthly bills are 200+ a month in the winter per 2 bedroom apt.  We were thinking of installing an ecobee so we can monitor the usage, but I was just told that the entire building has radiators.  How do we convert the radiators to something that is more efficient while the tenants are still living in the units?