Mold in HVAC ducts

Mold in a supply plenum made with fiberboard.

Lets discuss some of the causes of mold inside of HVAC ducts. Tice Engineering was asked to investigate the cause of mold that was found in a residential HVAC system. We have done several similar investigations and this is what we found. The home owner (alias Mr. Smith) indicated that a newer HVAC system that they had installed did not seem to cool the house as well as the previous one. The house was cool but still felt uncomfortably damp. The owner had ordered an inspection from an industrial hygienist to look for mold in the house as there were suspicious stains in parts of the house. The IH opened the duct work and found significant mold in the supply plenum (photo above) which is the distribution manifold downstream of the fan/coil unit (FCU).

The Smith's horizontal fan/coil unit (FCU) mounted in the attic.

The Smith FCU was a horizontal unit installed in the attic of the house. This type of installation is more sensitive than a vertical unit because in a vertical installation gravity helps collect condensate water to a drip pan. Also in this type of installation the condensate water drain is downstream of the evaporator.

Mold habitat: For mold to survive, it must have moisture and food. Most older duct systems have a significant amount of dust inside of them. This is what provides the food for the mold. What follows below is a discussion of how moisture gets into the ducts in sufficient amounts for mold to survive.

Duct interior Insulation: The rigid ducts in the system were made of fiberboard duct board. Fiberboard is a clever invention that allows the installer to custom build ducts without heavy equipment. The fiberboard can be easily bent to form the shape of the duct and it doesn't have to be insulated in a separate step. The insulation is built in. The potential problem with fiberboard or ducts with fibrous insulation inside is that the surface makes a fine habitat for mold to grow on if it stays wet for extended periods. The mold lives on the moisture and accumulated dust that gather on the fibrous surface. Duct interior insulation that has a smooth film surface doesn't provide a good habitat for mold like a fibrous surface does.

Fan Speed: I checked the fan speed by measuring the vacuum level in the return section while it was cooling. In this case, the fan speed suited the application. This is important on these units because if the velocity of the air is too high it can pull liquid water off of the evaporator coil and into the supply ducts. The velocity must be low enough to insure that the water drips onto the drip pan where it is collected. The fan speed must also be low enough to insure that air will not be pulled through the water trap in the condensate line. The best way to determine proper fan speed is by measuring the vacuum level in the unit. This is because the duct system design affects the vacuum level.

The Smith's shallow trap on the condensate drain line.

Condensate drain: The condensate drain is a particular area of concern on a horizontal FCU. The line must have a trap and the trap must be deep enough to keep unconditioned air from being drawn into the unit. The water trap serves to keep warm moist air from entering the air handler. If the trap is not deep enough or there is no water in the trap, warm air can be pulled into the FCU downstream of the evaporator coil. When that happens the air coming in will not go through the coil and will not be cooled and dehumidified. This air mixes with the conditioned air and increases it's temperature and humidity level. Also, when this moist air contacts the cooler duct surface it can condense on that surface and provide water for any mold that may be present there.

Another potential problem is clogged drain lines. Over time these lines can become clogged by accumulated dust and inhibit proper drainage of the condensate.

Another horizontal FCU installation that has no trap on the condensate drain line.

Another potential problem of condensate lines is improper plumbing. The photo above shows an installation with no trap on the condensate drain. In this case there is nothing to keep warm air from entering the unit through the drain. On the Smith's system I found that the condensate drain went down two floors to where it exited the house. The problem was that there was no vent on the drain pipe downstream of the trap. These drains are basically the same as a drain on a sink or other plumbing fixture. When a drain has a long vertical section there must be always be a vent pipe downstream of the trap that is open to the ambient air. The vent pipe ensures that larger slugs of moving water in the line will not cause a vacuum in the drain pipe and pull the water out of the trap. This is a fairly common occurrence on plumbing drains that have been installed without a vent or in which the vent becomes clogged.

This installation has a good deep trap but should have a vent downstream of the trap if the pipes have a long vertical section to lower floors. This trap also includes a built in brush to manually clean the trap occasionally and a switch to stop the unit if the trap clogs.

Conclusions: There are several areas of concern on HVAC cooling systems that can lead to mold problems in duct work. Mr. Smith's installation had items 1, 2, 4 & 6 on the following list. The main one was item 6 which explains why the house felt moist even though it has a new air conditioning system. It was drawing warm moist air into the supply ducts through the dry condensate pipe.

1 - Ducts with fibrous insulation inside including fiberboard duct work. Provide a favorable environment for mold.

2 - Horizontal fan/coil units are more likely to carry over water into the supply ducts if the following conditions exist:

3 - Blower speed set to high for the application. This can cause condensed water to be pulled off of the evaporator coil.

4 - In-adequate depth of the condensate trap. This can allow unconditioned air to be pulled through the water in the trap.

5 - No trap installed on the condensate line. This allows unconditioned air to enter the supply ducts.

6 - No vent on a condensate drain line with long vertical sections. This causes the trap to become dry and allows moist warm air to enter the supply ducts.

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