So...the subslab duct is supply, which would provide dilution to the subslab zone through the existing air leaks. The non-operational system would provide pressure relief venting when the furnace is operating and potentially allow outside air to enter the gravel if stack affect is strong enough. The pipe sizes will only support 80 cfm, based upon my knowledge of the common fan curves used for radon. I suspect the fan is a Fantech HP220, which given the ability with duct size, will move more than 220 cfm . The savings numbers to me should also recognize the difference in ventilation caused by pressure changes either by the operating radon system or the HVAC system. I love that you can measure house tightness and pressure changes and develop a pretty good sense for the change in total flow.
Seriously, underground ducts? I've never seen it done where it didn't end up with rotted ducts, condensation standing in the ducts and growing healthy mold, attracting rodents who like the conditioned and safe nesting areas, and becoming a conduit for radon. They are a mess from an IAQ perspective and they perform very badly from an energy perspective. As Nancy Reagan was fond of saying, "Just say No!"
look in to its like sewer liner
It,s put in buy the group who do sewer pipe restoration. 6" is the very min. It,s about 1/4" thick I saw it done on 36" under ground PVC pipe with 12" branch runs out of thick steel and boots out of 24 gage sheet steel. I have seen the group put in large flue liner with the same tools-pumps. using fire proof liner.
most under ground duct are heat only, and sized that way
Uninsulated duct will require 150 btuh per linear foot of duct for heating.
1" insulated duct needs 90 btuh per linear foot for heating, 40% less energy
2" insulated duct will need 75 btuh per linear foot of duct pipe for heating, 50% less energy
Eric Kjelshus Energy
Not sure how much if any air is leaking. If it is leaking where does it ultimately go? I agree, doubt it's worth messing with. If the A-coil is over 5 years old and not been cleaned, cleaning would yield a much quicker payback time.
I disagree with cleaning the A-coil being more effective than fixing or moving away from underground ducts. Huge difference.
In ground ducting - if rotted out will leak air into the ground - even through the clay. The clay may be impermeable to water but the air would move through it. Trees grow in clay, the roots need to breath. Flood the clay field with trees on it they will die if the roots don't get air. (yes there are trees that grow in swamps - but try growing a pine tree or spruce tree there....)
If the in ground ducts are leaky, the house may also have a dust problem, moisture and mold problems... not caused by the A-coil, but a result of warm heated air in contact with the dirt and remnants of rotten duct work.
Not sure what Kevin has done, but duct testing should be done - and then the hard decision is to try to fix or abandon and replace with something above the ground.
Mini splits would be my choice... but that may be difficult to explain to customer if they just replaced the furnace. If so - build them a recommended replacement and if (when) the furnace fails - do it right at that time...
If the ductwork is damaged and/or has water in it, that needs to be addressed right away. If the ductwork is dry and not collapsed then go for the A-coil cleaning. Water in ductwork will show up as sauna like conditions in the winter if the leak is bad enough..