We thought it was the perfect combination of attic fan kicking on, boiler running, and dryer running with the door to the furnace room closed, all at the same time which was causing a deficiency in combustion air, leading to unburnt gas.

Keep them away from combustibles and concentrate on where you have pipes,  bathroom, kitchen utility room etc.

Also, if you are staining a wood floor, be careful with your oily rags, they actually can spontaneously combust and start a fire in your recently rehabbed kitchen...

Electrically heated appliances and equipment shall be installed with the required clearances to combustible materials.And, most manufacturers will require 5' on a top discharge unit.Most installers hate enclosing them even with this clearance -- makes the units work harder.Keep them in the wide open. 

Make sure you get the "IC" (insulation contact) rated housings if you will be using them anywhere they may come in contact with insulation or other combustibles. 

This person will spontaneously combust or get mowed down by an unmanned Zamboni.

Then, use a hammer and chisel to break it free.As stated above, you can also melt it, it will be VERY smokey, because it should have white oakum ( highly combustible);  so, establish proper ventilation, a fire watch, and get a good respirator.

Keep in mind if he has direct vent gas furnaces, they still produce condensation from combustion.

Sniffed around the hot water heater -fine, but he does flag a cut in the plug gasket where the ignition assembly enters the combustion chamber.

It's very unlikely that welding sparks would be any risk to the house itself, as long as there isn't combustible material around.