Analogy: To keep an internal combustion engine running, the engine has to keep exploding little bits of gasoline. 

It is my belief that anything can happen in the short term to affect prices whether it is the Great Recession, Dot Com bust or Corvid. 

Also remind her that the house has to be safe and that means not putting combustible material by the heaters. 

Gas hot water tank is properly vented and has outside air for combustion.

Although Bill Gulley raises one problem to consider with back drafting (where exhaust gases are sucked back in due to the need for more of an air supply to perform proper combustion), that problem is better solved by using an "air make up system" such as a Skuttle to supply additional outside air without having to pull it through the chimney. 

Debris near electrical panel, furnace, or other combustible areas7.

Success to me is not being forced to sell, to  others it may be buying inventory at the bottom and keeping it until recovery.I had a property in San Mateo that we tried to sell during the dot com bust.

They have a higher combustion effeciency and you don't have to have 30-50 gallons of water constantly heated on standby.In consideration of a rental, I would not pay between $1,500 to $2,000 for tankless instead of $500-600 tank per unit for replacement.

If your electric rates are low enough, you could even get by with strip heat for backup and not need fossil fuel at all.If you do put in a fossil fueled furnace, go with a sealed combustion 90% model.

(many cities and areas are restricting the use of "open flame" and "combustible materials" on job sites.)