Q. My family lives in a 1968 Rancher. We’ve been here for over 10 years and have a problem that we’ve not been able to resolve. We are hopeful that you might have an idea or two that we’ve not yet tried.

The furnace intake vent is located on the ceiling just as you leave the large living room and head down the hallway towards the bedrooms and baths. The intake vent is constantly covered in mildew. We have scrubbed the intake cover, washed the entire area down with a bleach solution, painted many coats of Kilz and still the problem continues. We really would appreciate any ideas to help us finally get rid of this unsightly and unhealthy problem.

A. You sure do have a problem and mildew’s only the symptom so let’s march down a list of possibilities to see if we can get lucky and stumble on the root cause. Mildew spores are everywhere. We know we can’t escape them so keeping them out of the house is not an option-- there here. In order to have mildew grow and become established to the point that you can see it growing requires a couple of conditions to simultaneously exist.

Mildew needs temperatures within a certain range to survive and they just happen to be the same temperatures that we humans require too so that factor is unavoidable. Mildew needs a food source and the dust on the leading edges and surfaces of the hallw ay return air grille where you see it forming provides the food source.

The most important thing that mildew needs to thrive is moisture, so somehow your house is providing interior humidity levels high enough to allow the growth to get started. Worse yet, in your house you are telling me that it’s been going on for a long time so what we need to be looking for is a constant moisture source that is contributing the quantities needed for mildew. Tracking down that moisture source is where things can get sticky. The first thing to do is to rule out the usual suspects. Make sure your clothes dryer is in fact ducted and is venting to the exterior. Peer over the back of the dryer and see if the hose is split. Repair it if it is and just don’t tape it, replace it. Ditto for all bath vent fans.

After you done that start looking to the heating and air-conditioning system. Does the system have a humidifier attached to it? If so, trace the little quarter inch copper water line that feeds it back to its source and turn it off and leave it off. No w here is where things start to get tricky. If the ducts leading from the furnace of your rancher are located in the slab then you need to examine the ducts in the slab for any water intrusion. Water can get into the slab duct from outside-- storm wate r-- or from a pipe leak under the slab itself. Any house that was built around here in 1968 that has copper water lines running under or in the concrete slab is at risk for a pipe leak or a burst. It tends to happen more often on the hot water side in m y experience and finding the leak takes a plumber with a stethoscope.

If your rancher is located over a basement it is imperative that the basement be dry. Moist air can migrate from a wet basement or crawl space and create problems from the bottom to the top of a modern house.

Back to the HVAC equipment, make sure if it’s in the basement that there isn’t anything leaking onto the system or its ducts from any source, anywhere. I think from your description that your furnace and air-conditioning equipment are in a closet-like a rea or in a laundry room so have the section where the air conditioning coils are located checked to see if the condensate pan is draining properly. Your heating system is gas have your service provider double check the heat exchanger to be certain that moist flue gasses aren’t getting into the interior air-stream.

One of the above most likely is the culprit. Good luck finding it and don’t give up. Mildew spores are not healthy as you’ve noted and people in your house may be suffering now but can’t put their finger on the cause.