Now lets take of look at the heating system for the home you own or plan on buying. Here in the Houston Texas area we hardly even think about our heating systems until it's cold. I will let you know what type of system you have is it electric or gas. When testing the unit i'm going to evaluate the flame pattern this can give me evidence if the is a possible crack in the heat ex-changer, does the unit cycle correctly, looking for incorrect installation, if unit is gas are there gas leaks present I use my nose and a gas detector, correct gas line installation the gas line my be missing the sediment trap or the line may be installed where the flex line goes through the cabinet. If the gas line is going through the cabinet I will recommend that an HVAC technician install correctly. Over time gas flex lines going through the heater cabinet can leak, because the cabinet vibrates and wares a hole in the flex line. The correct way is to have hard pipe go through the heater cabinet. Many times I find that the unit is missing covers.There may be exposed electrical wires at the unit. Rust can be observed in the cabinet or water in cabinet from a/c evaporating coils. I may hear irregular noise coming from the unit. Improper venting, dirty filters once again like for the a/c systems making sure the filter is not clogged with dirt is very important to your heating unit. It will restrict the air flow lowing the performance of the unit reducing the energy efficiency and shorting the life of the unit. If the unit is gas you need a carbon monoxide detector present in the home carbon monoxide is odorless that's why they call it the silent killer. There may be some physical damage at unit etc. Annual maintenance is recommend this can alert you to any potential hazards like back drafting or gas leaks.