So, installing them outside or in a corrosive environment will shorten their lifespan.Regarding the relief valves...When water is heated, it expands.

Should I raise the rents and pay the heat bill myself?

Also make sure if it was newly renovated they didn't lipstick the property with carpet,paint, replace door knobs etc.You want big items like heating,a/c, roof, plumbing, electrical,etc. to have a lot of life left on them.If you buy based on inflated rents and then have flat to depressed rent in coming years, higher turnover and make ready costs, and high capital costs for large ticket items you could lose a bunch of money.Make sure the property insurance policy is not a master one insuring multiple properties as the owners policy could be low in cost compared to what you will pay.

For example, I am looking at https://www.redfin.com/IL/Chicago/3313-W-Cermak-Rd...its a tough call, its nice and clean and doesnt need repairs.... but its next to a vacant lot, heating is 2 gas space heaters in each unit(6 bedrooms, and no heat in any of em!)

At that time we updated the electrical, heating, and in Dec 2016 put on new roof.

Oil is a pain in the butt and the tenants will complain of high heating bills. 

Since then they've improved some of the mechanicals including 2 water heats, and installed an oil burning forced air furnace for one apartment.

When I asked for the heating to be repaired I knew I had the law on my side...

In addition, the renters pay something called "nebenkosten"(additional costs), this additional cost covers all insurances, property maintenance, lawn maintenance etc. they need to pay the electricity and heating charges separately: so total rent would be: Net rent(Kaltmiete)+Nebenkosten(additional costs)+ electricity/heating(paid by the renter directly depending on use)I.e 1450(net rent) + 200(addition rent or nebenkosten, which covers all maintenance)= 1650(Gross rent)So the figures I gave you does not include insurances etc: So the monthly payment I need to make the seller excludes maintenance etc, and hence 800€ is the amount remaining after everything is paid of.

Cloud MN same renter since oct 2011. 2) 2800sq ft modified 2 story in Big Lake, MN with 6 heated garage stalls on 2 acres.