Rust shows there’s a leak, but where?

Q: If there is a rust residue that is left from something leaking, what type of pipe would cause this rust residue to form? I live in a condo where my utility room has an A/C condenser near a furnace, and apparently something has dripped down the front of the furnace and on the floor behind the vinyl molding. It is damp behind the sheetrock of the molding, and both places show this rust-colored powder or residue. Any ideas? I have a few calls placed to A/C and plumbing/heating contractors. — Lisa G.

A: Good call — to the contractors, I mean. It’s hard to say exactly what is causing that residue. However, if you can’t locate the source of the leak, or if water is dripping through the ceiling or from behind a wall, it could be a leaky pipe.

However, I don’t think this is the cause, because the only type of pipe that rusts or drips rusty water is older, galvanized steel or iron pipe. This material is not commonly used in plumbing today; rather, copper pipe or flexible tubing are used to deliver hot and cold water to faucets, while PVC pipes funnel used water out of the house.

Your utility room contains both the furnace unit and the air-conditioning condenser. The A/C condenser itself is a source of extra humidity in the small space, since water collects there. Condensation on the outside of the unit, due to a significant difference in temperature between the rapidly cooled water and the (probably) warm room, could be occurring. If the utility room is especially hot, for whatever reason, the water that has condensed outside the A/C unit could become vapor again, rise to the ceiling and condense once more, dripping down onto both the condenser and the furnace.

Another possibility depends on the type of furnace you own. Some forced-air heating systems have a humidifier attached that adds moisture to the heated air before it is sent through the ductwork. It’s a part that must be maintained regularly to work properly. The evaporator pad should be inspected monthly and replaced at the end of the heating season. The humidifier reservoir should be checked for leaks and the drain line inspected for blockage or leaks.

That’s all speculation on my part, however. The only fact is that there is unwanted moisture causing damage to the outer surface of the furnace and to the wall. The source of this moisture must be found quickly and eliminated. Therefore, calling in both a plumber and a heating/air conditioning contractor to look at the situation is a wise move.

HOME TIP: As winter sets in, remember to clean or replace your furnace filter each month that the furnace is in use.