I had a home inspection recently to sell my home, and the NACHI certified inspector did a Delta-T on my furnace (it was 95F outside) using a laser temp reader, and the differential was 25 deg. He reported It needs to be repaired because it is out of range. It was my understanding that home inspectors are not qualified to perform a Delta-T on a furnace as it is more complex than just pointing a temp reader at the intake and vents. Now I have to get an HVAC contractor to “repair” my furnace. Can you tell me if this was out of line for the inspector to do?
That would not be the correct way to measure delta t. The proper testing equipment is needed for wet and dry bulb temperature. The location of the testing probe holes is essential.
Jason, an inspector is typically allowed to perform any noninvasive test, within a reason, according to his knowledge, skillset, and comfort zone. The SOP (standard of practice) only sets the bare minimum for a home inspector to follow and perform. Many home inspectors will go beyond it to provide a more thorough report. Please understand, you would want them to do the same if you were the buyer and not the seller. If the inspector measured the delta T correctly, then there may indeed be something wrong, but simple as a dirty filter or closed registers… From there the unit could be oversized, overcharged, have improper duct work, restriction in the ductwork, etc…
In your state, per your contract, are you required to fix EVERYTHING that the inspector finds? Talk to your attorney about this and then you can decide if you want to proceed or sell the house to someone else 
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It depends, but don’t depend on it… … …
Repair the unit & sell the home.
Although home inspectors are not required by Florida Statute
(61-30.804, Standards of Practice, HVAC Systems) to check the
temperature differential many inspectors, including us, do
Jason. Maybe the CPI was an HVAC technician.
Delta T is the most common use of the word delta in the HVAC industry, meaning temperature difference. If the ambient temperature before a cooling coil is 75F and the temperature after the cooling coil is 55F, subtract 55F from a 75F to find a delta t of 20F.
Other deficiencies likely led the inspector to ‘Recommend’, not require the owner to have a system maintenanced.
Nobody has to do anything.
Recommend: commend or entrust someone or something to (someone).
Jason. When was the last time you had a licensed HVAC contractor service the furnace? Invoices would help.
*Furnaces require servicing annually.
Hope that helps.
I’d be curious If he told you what the range was. Then if an hvac technician measured the temp correctly and it was in range I’d send the bill to that inspector.