Replacement for apartment sized space heater

[Jacob A.]

Hi everyone, I have a question about apartment sized space heaters. In my local area we have a ton of triple deckers and I would say about 85% of them still have these bulky very non energy efficient natural gas space heaters that are used to heat the entire apartment. I try to shy away from houses that have them because they're dangerous and I think it's only a matter of time before landlords are forced to convert them. If going through a total rehab, then it's no big deal and a no brainer to just replace them with a new heating system, but how about a house that is in good condition but has a space heater on the 2nd or third floor? What are the replacement options for that without having to rip open walls on the floors below them, etc. 

To heat a large 3br apartment, electric heat would be really expensive, and those ductless wall systems I hear require a lot of upkeep/maintenance which I can't depend on tenants to do and would prefer not to have to be there once a month to maintain myself. 

Anyone know of other options for replacements of these heaters?