This is probably a surprise to most people, it was to me the first few times I saw it. I am referring to the number of times in new houses, where the heat ducts have not been hooked together or, if they have, they have come loose. Sometimes I think it is the result of subcontractors forgetting to complete the job, other times the ducts were not properly secured or someone working under the home managed to contact them in such a way that they came apart. I have also found ducts that were crushed, so no heat would pass through them. In those cases, it is apparent that the problem was caused by impact damage by workers. The photo below was from a brand spanking new home. Especially in new houses, I turn on the furnace and test each duct with a laser thermometer to make sure it is part of the system. This is another reason to encourage a home inspection at a new home. Many builders just do not finish the job!