I weigh my monthly rent against the following items: Taxes Sewer and Water Trash Heat/Utilities HOA Cap Ex and Ops (this is the fund for replacing the items @Glenn McCrorey mentions + snow removal + misc extras.

Especially if the HVAC is central, with single pane windows the owner is paying to 'heat the whole neighborhood!'

Fixed operating costs (taxes, insurance, minimal heat) and debt service do persist regardless of the revenue stream; this is why one creates a "vacancy & credit allowance" when budgeting or analysing a property .... it is also why lenders like to see debt coverage ratios (DCR) of 120+%.You are basically asking at what point of missed revenue (vacancy or bad debt) will by boat cease to float.

I also like looking at trulia's heat maps.

I know there are state programs that offer financial incentives (rebates) to upgrade heating/cooling systems.

Bill Gulley   At the risk of sticking my nose in and getting involved in a heated battle for which I'm quite sure someone will try to say I'm wrong...  

I pay the gas/heat/water, which gas averages around 500 during the winter months.With me living in one unit, it will bring in 1600 a month.

They also do electric, heat, and gas sub-metering devices.  

)       $6,500   Labor (My contractor is the best)       $500   Kitchen cabinet Doors ( Replaced the doors only)       $2,700   Front/Back concrete pad       $1,300   Appliances (Stainless steel range, MW and range)       $3,470   Heat and Air ( New 13 seer Heat and Air system installed!

I know I found a heating and air company via Angie's List.