2 Naturiopath Doctors leased a home from me for 18 months upon moving out we discovered black air filters, also they had large structures used to grow indoors a few times complaints about marajuana smell was noted in their files.

Contact a contractor and see what they will charge per unit to install Natural Gas (hot water or hot air) and then maybe just do the 1st and possibly the 2nd floor with NG.

Knob & tube is the first generation of wiring and still prevalent in Providence, RI, where I live, work and invest.  

@Michinori KanekoI live in port saint Lucie fl, the property is a 4/3, 3000 sq feet under air and built in 2005.

We also had my personal Heat and Air team come take a look at the system to make sure it was fully functioning and without any leaks.

We also had my personal Heat and Air team come take a look at the system to make sure it was fully functioning and without any leaks.

Took out the electric heating and put in a forced hot air/cooling system.

If it were me in the situation I would look either at the most doors I could get for the buck in a stable market, like a $750,000 16-unit multi-family in my market that is currently c-class (window unit air conditioners, dated stylings, and 10+ year old appliances, etc), extremely well located, and, based on all that, probably a good target for adding value over time and increasing the appraisal value substantially. 

These chemicals and pesticides leach into our soils and water along with drifting over in the air for miles. 

Purchase an Air Purifier for their son's room and main living area.  2.