Having recently moved from a brand new townhome into an older single family home in Millbrae (CA), we are slowly repairing issues caught and missed by our property inspector during closing. The most recent fixer upper has been the heating vents under the house in the crawlspace and in the attic. Thankfully, both heaters (one upstairs and one in the garage) appear to be properly functioning. The major problem is the connected heating ducts all had some issues. Thankfully, the heating duct problem did not come by surprise because it was caught by our housing inspector.
The heater in our garage is large and for good reason. The bottom floor covers are larger distance, so I was told a larger heater is required to push hot air to every vent on the bottom floor. So even though the heater was large enough to handle the distance, all the heating ducts under the house were old and required replacing. The ducts were all solid metal (not flexible) piping that had rusted, was missing insulation, and not strapped to the top of the crawlspace. Some parts of the vents were completely disconnected. In fact, our HVAC guy told us there is a decently large pool of water in the crawlspace about the equivalent of half the house. Since the vents were on the muddy ground and made of metal, it explains the rust. Without second guessing, we decided to replace all the heating ducts under the house.
The heater on our top floor, though still working, was a smaller than the garage heater. Possibly too small for its application. Although the total length of the heating ducts in the attic is shorter than in the crawlspace, there are more heat vents upstairs. Therefore, even though the ducts were not disconnected and rusted, the heat vents lacked the pressure to push hot air to every vent. An additional problem was the attic heating pipe was an 8 inch pipe, and not a more standard 6 inch pipe, so the heater has to work harder to fill more volume. Lastly, a bit of asbestos insulation was found in the attic.
Since we've just moved in and our home inspector told us the heater was "heating the crawlspace", we decided to completely replace all heater vents. It was replaced to flexible piping throughout the house. In the crawlspace, the piping was secured to the ceiling of the crawlspace. In the attic, the asbestos was remove and all piping was reduced in size to 6 inches. All in all, we were charged approximately $1000 which included removal and haul away of old vents, new vents and installation, and removal of asbestos.
The installation took a full day with a 2 man crew. Ever since the change, the air blows much stronger throughout the house and stays on much longer. Even though I doubt the $1000 will be offset by a lower gas bill, it is worth it for the health reasons (in case small animals like rats were living in the heating vents and removal of any dust).