I honestly believe that he at-least THINKS he smells mold and that he isn't just trying to get out of the lease.What would you do? 

The 4-unit bldg has 4 newer heating systems (natural gas).  

Last winter in the northeast was terrible, and we bought them a $300 room heater to try to help with the heating costs.

So if higher quality homes, I guess using a realtor is the alternative)I will stick to the fundamental in investment, for instance, investing in stocks i prefer to evaluate the fundamental of companies instead of chasing the most popular or heated stocks for long term success.

Tenants pay heat and I'll get about $425/month rent so $1275$800$340 main.

My tenants will pay their own Electricity and Heating utilities, the house has separated gas and electricity meters.  

@Dan Perrott for most insurance companies any wood burning heating device is prohibited.  

It is winterized so the furnace(Heat) is not on.  

(I usually play this from my cell phone.)g) If it's winter, I'll turn up the heat. 

Are the heating systems separate per unit or is there one boiler?