TJ I took my 'block wall IR' training from unitedinfrared.com. If you want to go this route; as for Peter and tell him I said to give you the BPI courtesy discount.
Any way; the theory is that the sun needs to heat the wall, (solar loasding), sufficiently to allow the cooler cores to show the Delta T needed. Similar to flat roof scanning where we can locate moisture under a roof membrane and prior to interior ceiling damage.
You will need to take scans at different intervals and camera settings through out the day. This way you can compare images and capture the best anomalie. The color setting usuallly works best in IRON Bow or similar. You will also have to adjust the camera range at 20 degrees from high to low.
That is why interior IR doesn't work as well. The masonry wall will be more uniform through out unless there is a major moisture leak or perhaps heating duct break in the wall. I would suggest going out side and scanning some block walls at different times to get a feel for it. The sun hits different wallls at different levels so more than 1 wall cn be tested.
How are you progressing on your 'wall scan"?
One comment on the continuous running furnace fan; did the customer put the fan on 'run' instead of auto? Is the furnice an older moel that has a blower control thermostat in the hot air plenum that controls the fan and is defective?
It will cost energy to continuously run the blower and could cause excessive wear and tear and failure.