I too lowered rents with my very first property during the first dot com bust.

My second choice would be sealed-combustion natural gas furnaces sized appropriately. 

I would drop it to about 18 inches, that way you can get better air flow for flame combustion and you do not restrict the heat it gives off.  

Also, as EV's Tesla drivers don't currently pay anything into the gas tax, so it's further subsidized by internal combustion cars at a current rate of $34 billion per year.

It's a train of combustion.

A BPI audit will also inspect the safety of all combustion appliances in the home.

This service will clean the fireplace of soot, sap buildup, bird’s nests and other combustible debris.

The 'common vented' set up you have is a combustion/CO safety problem & one of the most dangerous installation allowed by code.For the price you quoted we would replace everything including an on-demand HW tank & eliminate the chimney completely. 

Waiting for the next real estate crash to buy real estate is like waiting for the next dot com bust to by internet stocks.

That's adding a LOT of combustible to the house.