• HVAC system design is an area of expertise to many mechanical engineers. They design HVAC systems so that the equipment will perform to optimal efficiency. However, modern products are certainly not the products of yesterday, and even the best mechanical engineers designed HVAC systems with some quirky aspects. One of those is ductwork in slabs.
  
  

Condensation Issues

• Ductwork in concrete will have condensation issues due to the amount of temperature difference between the cold concrete slab and warm ductwork. This condensation forms into a puddle inside the base of the ductwork, sitting stagnant. This is a potentially harmful situation in terms of indoor air quality, as the stagnant water will collect dust, dirt, and allergen particles that will stick to the surface of the water. This affects the air that in distributed through the ductwork and can increase allergy symptoms.
  
  

Rodent Living Space

• One of the issues with having a tunnel underneath a concrete slab is that it becomes an easily accessible area for rodents to live undetected. This could cause huge problems with air quality as you would be breathing in the excrement of the rodents. Even when they are discovered and terminated, the ductwork would have to be thoroughly cleaned in order for the system to be decontaminated.
  
  

Rusting/Crumbling Ductwork

• Rusting ductwork can occur even if no obvious moisture is present. Since the metal encapsulated in the slab can condensate, the pipe can literally rust through from the exterior of the pipe. This is less of a health issue and more of a system integrity issue as weakened duct pipes can collapse under the slab and cease supplying areas with heat. Some old ductwork is also made of clay which cracks and crumbles as well.
  
  

Remodeling

• Any kind of remodeling project that requires ductwork modification becomes extraordinarily expensive due to trying to tie into slab ductwork systems. Metal is not used in under slab ductwork anymore and has been replaced with a PVC piping system. Tying the two together would be a hard conversion to make.
  
  

Radon

• Radon is a heavy gas that has been linked with causing cancer. Higher levels of Radon exist in basements because of the weight of of the gas. However, the gas can be distributed throughout the house through ductwork in the slab where the heavy gas is likely to sit. There are several Radon testing kits available for purchase if you want to test the amount of radon in your home.