Underground A/C Inside Arrangements

Leaving, Entering the Building

The two holes through the concrete basement wall need to be pretty large, at least 13" square, so that the bundle of the 9 tubes can pass through, OR bigger still so the 15" sewer main could pass through. If more tubes are used, twelve in a 3 x 4 or 4 x 3 bundle, or sixteen in a 4 x 4 bundle, an even larger pass through is necessary.

Once they are all in place, and the backfill mostly in place, new concrete can be poured to fill in the spaces between the tubes where it goes through the concrete wall. Once inside the basement, the nine tubes can go into a "plenum chamber" (air box) where they all join together to become one large standard rectangular duct. In our example, each 4" tube has a cross sectional area of around 13 square inches, so nine of them have a total area of around 117 square inches. This would then make sense to become an 8" by 16" rectangular duct (or larger) for connection to the house air ducts.

If the building has no basement, the tubes should extend slightly under the house and then each elbow upward. When the concrete slab floor is poured, the two bundles of tubes would therefore stick up through the floor.

The main page drawing shows the Exit bundle and the Enter bundle being substantially apart as they join the building. In that case, an interior main (large, rectangular) duct would be necessary inside the building or basement. An alternative that is more convenient for HVAC connections is to continue one (or both) of the bundles underground along the building wall, such that both bundles enter the building near each other, and near the building's furnace or air handler.

Note: the underground tubes do not really have to be bundled together, and they would actually perform slightly better if they were all separate, but that would involve far more trenching. Bundling the tubes, EXCEPT in their main field where they should be six feet apart, only slightly affects performance, not enough to worry about.

Standard Furnace - A/C

In a standard forced-air system, there are Cold Air Return ducts which allow room air to be drawn back to the furnace, where the furnace blower then pushes the air through the furnace and then through an air conditioning coil. From there, the air proceeds through Warm Air Ducts back to the various rooms of the building.

That arrangement allows three possibilities:

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This System

Most of the system is exactly the same. The Cold Air Return ducting is all absolutely standard, as is the furnace and the Warm Air Ducts.

Near the outlet of the furnace air, the large Main Distribution Duct splits in two, each of the full size. A Motorized Damper is installed at that location to direct the air coming out of the furnace into either of these paths. The one path is the standard one, which directly feeds into the normal Warm Air Duct system. When the furnace is called on to provide heat, or the SUMMER FAN switch is turned on, the air follows a path that is exactly like it would otherwise be, no difference except for passing through an (open) damper.

The second air path sends the furnace's air through a large duct to send the air into all the underground tubes, connecting to one of the bundles with a plenum (air box). That air would then be forced through all the tubes and would arrive back in the building through the second bundle of tubes. Another plenum (air box) and large duct sends that air to re-join the original Main Distribution Duct just after where the Motorized Damper was installed. The result is that, whenever the Motorized Damper is actuated, the air coming out of the furnace gets diverted through the underground tubes, but then comes right back into the same original duct.

The wall thermostat wire that normally turned on both the furnace blower and the A/C compressor, now turns on the furnace blower and the Motorized Damper. Most modern furnaces do not require any wiring change for the wall thermostat, as they generally have a terminal board inside that the A/C compressor relay would normally connect to. The single item of wiring is to instead connect the (24 volt) Motorized Damper to that terminal.

Then, when the standard wall thermostat is set to request cooling, the furnace blower turns on and so does the Motorized Damper, which then pushes all the house air through the underground tubes to be cooled and de-humidified. Once that air is back in the Main Distribution Duct, it cannot be distinguished from air that would otherwise have been cooled by a conventional A/C coil (where the Motorized Damper now is.

For cautious or skeptical people, a conventional A/C system and coil can still be included, exactly as it normally would be. In that case, the Motorized Damper and duct split would normally be installed downstream of the conventional A/C coil. But the wall thermostat could only turn one or the other on, so a second wall thermostat might be necessary.

Bypass Version

In nearly all installations, it is most economical to use the method described above. However, there can be some unusual applications where an entirely independent system is desired.

In this case, the Cold Air Return trunkline is split in two, both of full size. One air path continues into the furnace cold air intake, and all that is completely standard.

The second air path would provide air to an entirely separate (new) blower, which would have the single function of pushing air through the underground tubes. That blower must be the same size and capacity and horsepower as the blower in the furnace. As above, the underground cooled air coming back into the building then feeds into the Main Distribution Duct to feed that cooled air to all the rooms.

In this case, an individual HVAC installer can determine whether a Motorized Damper is necessary or appropriate. If not, then the wall thermostat COOL wire needs to activate a 24 volt relay which simply turns on this second blower, which then provides cooling.


No major modification of any of the house ducting is involved, and even its design calculations and sizes are still the same. The only additional HVAC work necessary is to add the two branch Main Ducts and install the Motorized Damper. Any type or model of forced-air furnace may be used as well. The differential cost regarding these factors should be minimal.

There appears to only be one way that this could be messed up! If, ANYWHERE, the cross-sectional area of the air path is SMALLER than the cross-sectional area of the correct-sized Main Distribution Duct, then the airflow would be reduced, performance reduced, and the blower overloaded. So, if even a three-foot-long piece of 8" diameter (small) ducting is used to carry all the air, everything else becomes meaningless! We use the analogy of firemen using their 3" firehose, but needing to borrow your 1/2" garden hose as an extension. All their 3" firehoses would become pretty meaningless, because the amount of water that could pass through would be entirely limited by your garden hose. Even though it would be a wonderful gesture to offer your garden hose to firemen, DON'T! Ditto, regarding any air path that has too small an area.

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