Appropriate venting for gas hot water heaters and gas furnaces is a critical element to safe operation of these appliances. Some high efficiency furnaces now use plastic PVC venting because they discharge spent gasses which are not very hot, and they are vented differently that what is pictured here. But in the majority of cases the exhaust from these units is similar to what is pictured here and venting the appliance causes the steel piping to become very hot.
It is not safe for hot metal surfaces to come into immediate contact with combustible materials like wood or paper because of the potential for a condition known as Pyrolysis to occur. Pyrolysis is a thermochemical decomposition of organic material which occurs at elevated temperatures even in the absence of oxygen. Contact with the materials with heat from the venting over long periods of time can eventually cause the wood or paper to change internally, become dry and brittle and even catch fire in some cases. It is very important to make sure that any venting which travels through combustible materials have proper spacing between the materials and heated piping.
Standard practice is to have the vents from furnaces and hot water heaters attach from the appliance to a solid single wall pipe, and then connect to a solid double wall pipe. The double wall pipe then vents through to the exterior. The double wall pipe is required to have at least one inch clearance from combustible material and be secured firmly. If single wall piping is used, it should have a minimum of six inches of clearance between combustibles, as standard practice.
The photo on the left was taken in a home located in Aurora Colorado during a home inspection by our company, Magpie Property Inspections LLC. It is an example of an incorrect vent pipe installation from a hot water heater. Single wall venting is installed and is vented through plywood decking in an attic with less than one inch clearance in places. It is not well secured. This can lead to a serious fire risk as explained. The photo on the right was taken during a home inspection in Castle Rock Colorado and illustrates the correct installation of furnace and water heater venting.
If you notice any issue with your furnace and/or water heating venting, similar to the installation in the photo on the left, we recommend having a qualified HVAC technician evaluate the venting and advise on any corrections which are needed.