Also took into consideration the roof is less than 5 and the heat pump (air/heat unit) is under 10.

Heating fuel tanks that can be above found or buried underground.

Once you add-in the cost of utilities (especially heat), furnishings (couches, tables and chairs, beds, sheets, towels, table cloths, pots and pans and plates and silverware, napkins, toilet paper, TV's and radios, washer and dryer and refrigerator - yes, it's common for a long-term SFR not to include a washer, dryer, or refrigerator - etc, etc, etc), plowing/lawn/trash (or HOA fees), cleaning, extra repairs and maintenance (I promise that a long-term house renter takes better care and causes less wear and tear than a revolving door of vacation renters), turnover management and screening, etc, etc, you are not making any more with the vacation rental than with the long-term SFR where the tenants pay all the utilities (possibly with the exception of water/sewer), and take care of the lawn and snow, and provide their own furnishings soup to nuts.

In December my tenants wrote saying the Co2 alarm had been going on since November when they turned on the heat (10 mos after we bought the place and 1st winter).

and it seems that the variable cost of heating in the winter is chewing up the potential profits of most multi family properties on the market, so originally my plan was to invest and hold onto small two or three unit multi family properties, but now it seems like small single family homes, I'm thinking ranch style, are maybe better because it's not expected for me to pay any portion of heat, and if the property is on the smaller side it would be easy for the tenant to heat. 

I suppose my main concern is that installing coils won't fix the problem and that this issue will pose a problem for selling the house later on.

All units are occupied and in great condition Separate utilities , Tenants pay heat , cooking gas and electric Owner does current managementHere is the very simplified income/expense report from the owner:Now property number two is a 12 unit building.  

As much as it was exhausting, I never hated going to the house (it was not heated and I was working through a bad/cold winter).

The condo I have is a 1BR / 1 bath with a unit next door in far worse condition (unit I acquired did a complete rehab - new kitchen, baths, plumbing, heating, electrical and windows) is under agreement for asking price of $129k.

With little to no appreciation and limited rent growth due to the area you are now break even or under with the value of the property.My house I live in I just replaced the whole heating and air for 17,000.