Over the past 4 years we have cleaned allot of duct work and seen allot of strange things. In the process “down-flow” furnaces have become known to Royalty Services Group as almost always having some kind of problem or issue associated with them. A down-flow furnace is one that blows or pushes the conditioned air down instead of up. In Utah, up-flow furnaces are much more common since most of the furnaces are located in the basement and service the main floor of the house.These down-flow furnaces are usually found in the basement and the duct work is usually laid on top of a layer of sand with the concrete slab poured over them. Since they are encased in concrete, they provide some unique challenges.

Some very common problems we find with this type of duct work is rusting out, collapsing, and large voids.

air ducts in concrete

Rusting Out

When the metal duct work sits in the concrete for 10-30 years, water starts to penetrate it though cracks and crevices. This leads to the metal starting to rust and decay. Eventually the metal is gone and the sand or dirt below the duct work starts getting sucked in and blow out of the vents and into the rooms of the basement.

Collapsing

In the past, duct work made out of a cardboard or a paper type material was used. It looks very similar to the the cylinder tubes you can purchase from the big box stores used to pour concrete for a fence post. Anyway, after many years water seeps in and permeates this material causing it to expand and warp. After this cycle has occurred many times, the walls and ceiling of this duct work starts to collapse and fall in on itself. This adds restriction to the air space and causes a lack of air pressure in some areas of the basement. We have worked on a job where this was happening in the basement of a church in Bountiful, Utah. We were pulling larges chunks of this duct work out of the vents to try and improve the airflow.

Large Voids

In both of the previous situations you can be left with large voids in the duct work due to the deterioration of the material used. Once these large voids are present, there is nothing stopping the material on the outside of the duct (sand, dirt, gravel, bugs, etc….) from entering the duct work and getting blown out of the vents and into the finished space. This turns out to be a particularly bad problem because of the lack of cost effective solutions. The sand and dirt will just keep coming and coming. The ducts can be cleaned, but there is not way to effectively remove all of the dirt and sand below the duct work since there is an unlimited supply. The only effective solution is to jack hammer up the floors and replace the old duct work with a pvc type duct that will not rust or deteriorate as easily. There is one other solution and that is to run new duct work through the ceiling and walls if there is room for it.

I understand that in some cases installing a down-flow furnace is the best solution but I think that too many engineers and contractors don’t understand the long-term problems associated with this style of ductwork encased in concrete. Just this past week we were called out to help a client in Salt Lake City, Utah who was complaining of a bad odor coming from the ductwork and wanted the air ducts cleaned. Here HVAC system consisted 2 furnaces installed in the basement. One was an up-flow furnace that services the main floor while the other was a down-flow furnace that services the basement with ductwork in the concrete.

After the initial inspection it became obvious that the very bad odor was coming from the down-flow ductwork in the concrete and it seems to be strongest at the furnace itself. This was a challenge since the furnace covers that box that distribute air to all of the ductwork. After callling in an HVAC tech and having the furnace removed we ran our video inspection camera down each duct line and found water in each of them. The odor definitely seemed to be coming from the water. We suspected a possible sewage pipe leak/break that may have allowed this stinky water to get into the ductwork. Next a plumber was called in to check the lines and it was verified that one of the drain line under the concrete slab had ruptured and was leaking sewer water and fumes into the ductwork in the concrete. This is going to be very expensive to have the foundation dug up, pipes repaired, air ducts cleanedand everything put back together.

This is just one more example of the crazy problems with find with down-flow furnaces and ductwork run in the concrete.