Heating/AC, appliances, electrical, windows, paint, etc. 

Guests, just like those at your personal residence, require all sorts of amenities we don't often think of beyond the heating, cooling, water, waste, electricity and internet that hosts are responsible for.

However, due to the leak, the water heater kept trying to heat water constantly and the electric bill doubled.

Also when evaluating old dilapidated houses they have often sat with no heat through cold seasons and these rigid metal pipes don't hold up to the forces of freezing water very well.

They are energy efficient and help reduce the monthly heating bills.

I am a HVAC technician in new york, and recently came across the NYSERDA clean heat program incentivizing the installation of electric heat pumps.

Upon taking over I immediately raised the rents on the MtM tenants to $800 knowing I was also planning to switch the heat to a system where the tenants would eventually be paying for their own heat.

They are energy efficient and help reduce the monthly heating bills.

I was quoted 12-14k for new central heat and air which included all the duct work.

The other issue to think about is mechanicals i.e. heating and cooling access and control, water heater access, and washer and dryer access.