I recently decommissioned this old steam boiler and dropped 2 hot air furnaces in the attic for the 2 2nd floor apts, and put 2 small hot water boilers in basement for the 2 first fl apts.

Plus it's their fault for letting the electric service to run out, and the entire summer rainy season water has been penetrating inside and no air conditioning.

Moisture and lack of air flow = mold.A regular soaking of the ground near and under the foundation/slab could easily create enough moisture for drywall to grow mold.

In my area, a dry basement is a huge plus, as well as a decent sized fenced yard and central air.

I guess they thought the house needed airing out.

I guess sunlight and fresh air are over-rated.

Take a look at Rinnai's page:http://www.rinnai.us/hydronic-air-handlers/If what you are looking to do is just use a tankless water heater to supply hot water for both plumbing and your hydronic heat it may not work.If you go the electric route the "whole house" models require either 200 or 300 amp electrical service.Who pays for utilities?

I don't think i'd be willing to close though with so much "up in the air".

mold and air quality?

I can also relate, as I started real estate investing as an active duty Air Force officer.