When you "enclose" these appliances, they have some spacing requirements for air flow around the sides (cooling / safety), usually in the installation manual or on the units themselves in some cases.

Filter had been changed a few weeks earlier and was clean enough but supposedly restricted air flow can cause the problem...guy said to wait 4 hours and turn system back on if it happened again.

The Crape Myrtle requires train pruning when young and routine pruning to maintain shape and promote airflow through the crown as the tree matures.

First, some thought needs to go into placement and air flow to make sure the home is heated thoroughly.

Cut them trees away from the house - you need air flow to keep this stuff from staying damp. 

This sealed off the dust, and gives a grand storage/workshop area,   HOWEVER, it also sealed off the airflow, with the result that any dampness wicking through from the outside soil makes any nearby surface mold heaven.

The system has to heat the whole building and even if you to 'isolate' the non-occupied units, the heat will still migrate there via air flow or simple conduction.

I have a flu that does not seal well on a fireplace.  I have a gas log that I am pretty sure does not need a vent.I can feel a draft up the chimney.  What would be an inexpensive way to seal it?  I still would like to...

Good Ole Clorox seems to work, be sure to wear a breathing mask when applying as well as get some good air flow and circulation.I know Home Depot sells something for like 30 bucks called "Mold Control" it has you mix two containers into an spray bottle.

Smart vent is probably your best bet (Or something like it) because it is difficult to get adequate air flow with round holes.Smart vent is tricky to install properly.