For as long as I've owned my '89 F250, I've been annoyed by the fact that it seems to run warmer than it ought to in the cabin. I had written this off to the diesel engine running warm, but there was always this nagging suspicion in the back of my mind that this was not the actual story.
It turns out that the design of these trucks is different than I'm used to in vehicle heaters. There is no such thing as truly "off"; the coolant is always circulating through the heater core in the cabin, even with the heat turned off. Add in a geriatric blend door that doesn't seal properly, and you've got a recipe for a diesel powered mobile sauna. Hardly the thing you want when heading out to go camping in the late spring.
Ford used to make big rigs, back before they sold to Freightliner (part of Daimler-whatever-they're-called-this-year) in the late 90s and subsecuently discontinued the product line a decade later in favor of the Freightliner and Western Star lines. Apparently this problem affects them too; lots of big rigs use pickup truck bits liberally throughout their cabs.
You'll find a bunch of write-ups in various forums about this, usually couched in terms of "how to make your air conditioning blow cold". Making my A/C blow at all would require a conversion from R12 to R134a and some TLC from an expert, so same problem as a truck with no A/C at all.
There are plenty of home-grown approaches out there, from a choke-cable-actuated cutoff valve to a neat, vacuum-valve-actuated-from-the-dashboard implementation.
As the BFT makes its way down the road to being a true Frankentruck, though, I care less and less about such things, and though I wasn't really on board for slapping something together out of random bits from Lowes I was OK with a seasonal shutoff valve that required popping the hood.
Despite the always-on design of the OBS Ford pickup truck and some folks wanting to make the problem a lot more difficult than it is, it is entirely permissible to block off that loop with a simple valve. it turns out that this problem is endemic enough to the Big Rigs that the folks who make big rig HVAC parts have a valve especially for this application - Airsource MEI part number 2455. It cost a bit more than Ryder wants from the local FleetPride, but at least I was able to pick up hose clamps at the same time.
Note that some folks think this is a bad idea alleging that it will cause the heater core to rust out quickly. They're an easy swap in this truck. They cost $50. I just got a new one put in less than a year ago, so I guess we'll find out!
Installation took less than 90 seconds on a cold engine and only made a little mess of spilled antifreeze. What a joy to drive home from Northern Maryland and be a little bit chilly rather than being steam roasted.