Converting to electric (including a new upgraded 200A feeder and larger panel) is less expensive when compared to the in kind boiler replacement by about $1000.Several factors being weighed besides price is:- Resale Value of the house - Overall cost of use for the tenants - Safety for the tenants (Gas/water/steam/combustion/leakage issues VS. electric baseboard overheating)- Future maintenance costs- Reliability and expected lifespan of electric baseboard (I don't have much experience with them for a PRIMARY heat source)-Future considerations; If converted to electric baseboard the Range/Dryer/Hot water heater can all be converted later to electric thus being able to eliminate the gas bill its fixed charge/service fee.

Others require them to be at grade and separate from combustion (furnace, water heater).

It's like speculating about the combustible engine and its future uses in 1909.  

It would be unlikely that you could get an attic space hot enough from lack of ventilation to melt outdoor siding, because that would be a ridiculous inner surface temperature that might ignite anything combustible in the attic. 

EnviroTabs works with any type of internal combustion engine.

If natural gas combusts complete, you get water and CO2.

Direct contact with piles of clothes,,,too close to curtains or other combustables,,,garbage and the house can get hotter than she thought.

I opened the outside cover of the heater, and looked through the round window to see if the ignitor was glowing - couldn't see anything that seemed to be glowing.I found this manual that explained operation of the "glow" ignitor, and how to troubleshoot the ignitor under the "BEFORE YOU START" section:http://www.forwardthinking.honeywell.com/related_links/combustion/glowfly/install/69_2334_glowfly_installation.pdfI figured I'd better be certain that the ignitor's glow wasn't being hidden by anything, so I switched off the electricity to the heater and I removed the screws to the burner box taking the cover off too, and then flipped the electric switch back on.

Adding another boiler might require you to introduce more air to support combustion, or else run the risk of backdrafting of fresh air through the exhaust system (the chimney).

I could see that the exhaust has leaks, but you only get monoxide as a result of combustion, and it must first light before you get anything to burn ...I posted something in this other thread that might give more info on this:http://www.biggerpockets.com/forums/52/topics/58638-gas-appliances-in-rental-