Mike, This may not be an issue for you, but keep in mind that coal and coal dust is acidic.

Now we finally got it rented back out and the property has bugs so we have to spray for bugs and the AC is old so we needed to do an acid wash on it.

I would recommend the gel in addition to using boric acid (both tablets and powder).There were a few roached in my primary when I first bought it. 

The heat exchangers fail most frequently on the propane boilers due to the high acidity, low temperatures the propane condensing units run at, my friend who owns a 3rd Generation heating/plumbing company doubts those units will ever come close to outlasting a traditional 3 pass boiler.Agree the pros of having a propane boiler/furnace-fuel is cleaner, services tend to be cheaper, tanks can be bought or fuel company can supply tank, work great if you also have a tankless hot water heater with same fuel source.Efficiency of Maine does offer rebates as well for the high efficiency units, they don't mention this but, if you insulate all baseboard piping that is exposed in the basement, take photos and have a high efficiency oil boiler they will qualify as the same rebate as the propane unit.

The fix there is going to be a steam cleaning followed by steel brush with some TSP or oxalic acid (Bar Keeper's Friend) and digging out the top layer of the grout with an oscillating tool.There was no need to replace the tile around the fireplace to deal with that stain.

I use RTAs from Kitchen Cabinet Kings.Flush hot water heater between tenantsUse drain enzyme to clean out drains between tenants to clean old plumbing lines.Use a powdered boric acid based bug killer before tenants move in to keep bugs in check.Use stainless steel and ceramic for sinks.  

I've tried the boric acid,borax, foggers,sprays, even professionals have come twice but I fear they will come back.  

Many sewer services have cameras that can show you the problem and what type of system you have.Assuming you are on City sewer, after you have the lines cleared, treat the plumbing with an acid treatment--I use CLEAN SHOT--4x per year. 

The acid down the field lines was smoking and gurgling , I put down abour 20 gallons and let it sit . 

And am familiar with the income and acid papers I need to provide with her information.