There is substantial visible mould growth inside hard surface (e.g., sheet metal) ducts or on other components of your heating and cooling system. There are several important points to understand concerning mould detection in heating and cooling systems:

• Many sections of your heating and cooling system may not be accessible for a visible inspection, so ask the service provider to show you any mould they say exists.
• You should be aware that although a substance may look like mould, a positive determination of whether it is mould or not can be made only by an expert and may require laboratory analysis for final confirmation. For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mould or simply a substance that resembles it.
• If you have insulated air ducts and the insulation gets wet or mouldy it cannot be effectively cleaned and should be removed and replaced.
• If the conditions causing the mould growth in the first place are not corrected, mould growth will recur.
• Ducts are infested with vermin, e.g. (rodents or insects).
• Ducts are clogged with excessive amounts of dust and debris and/or particles are actually released into the home from your supply registers.