In 2015, i had radiators installed on the 2nd floor (bottom pic).

We get a call at 10pm on a weds night from a tenant saying that the radiator in their master bathroom has exploded open and is gushing water.

I should've asked my contractor to get on the roof and check it as well as the appliances, the boiler, the radiators, the windows.

Even if they only clean when you are coming thru, that is better than years of piling up.5) Gives you information to determine if you want to renew the lease.6) If you have regular inspections, then if a tenant is withholding rent, it gives you the opportunity to check on your property.Older homes with radiators and no fans in the bathroom are a nightmare.

If it's a boiler / radiator system, or an oil furnace, then I'm not familiar with what those "should" cost.

Let me tell you, it practically radiated charm through the screen, and I just had to reach out to share my enthusiasm about it.Now, you might be scratching your head, wondering, “How on earth did they find me?”

In addition, several of the radiators which are located in Unit B are controlled by the regulators in Unit A and only heat Unit A after Unit B is completely heated up.

In the middle of the summer if you go out an hour or two after dark in the city it is one temp and you can feel the heat radiating off of the block walls and asphalt streets still.

I would check with a local HVAC (Hydronic) wholesaler or installer to see if you can still obtain them {be prepared for them to cost almost the same {probably more when you factor in labour} as replacing the entire unit.As for using PEX, I'm not certain if the heat-rated PEX (used for in-floor hydronic heating which runs around 140F) can be used for a boiler and radiators (which typically run around 180F).  

My 5 unit in Bucktown has a gas bill of $95/month on the budget plan which covers hot water for all 5 units and the building laundry as well as for radiator heat in basement studio apartment and the common areas.