HEATER PLUMBING
Fleet Mechanic
Joined: Sep 2010
Posts: 1,833
Likes: 467
From: Mason City, Iowa
Looks correct. Normally the hot water or heat temp control valve is between the head and the bottom water inlet of the heater core. That way during the summer your heater core isnt radiating heat from the coolant from the cylinder head. But some have said and its been my experience too it doesn't seem like much of a problem.
Hotshot
Joined: Oct 2009
Posts: 16,173
Likes: 4,781
From: Burbank, WA
Hooler is correct. The hot water flows from the head to the bottom of the heater core, where the flow pushes upward through the core taking any trapped air with it, theoretically. The upper heater core tube routes to the water pump, where it pulls the water through the heater system and recirculates the cold water back into the block, mixing with the water drawn from the bottom of the radiator. Ideally you'd want a shut off valve in the hot, lower hose to prevent that hot water circulation in the summertime, when you don't want any more heat in the cab than necessary. Even if you put the shut off in the upper, cold hose, you can still get some convection heating from the heat travelling through the water to the core. It may not be as hot, but it can still warm the cab when you don't need/want it.
Fleet Mechanic
Joined: Sep 2010
Posts: 1,833
Likes: 467
From: Mason City, Iowa
Hooler is correct. The hot water flows from the head to the bottom of the heater core, where the flow pushes upward through the core taking any trapped air with it, theoretically. The upper heater core tube routes to the water pump, where it pulls the water through the heater system and recirculates the cold water back into the block, mixing with the water drawn from the bottom of the radiator. Ideally you'd want a shut off valve in the hot, lower hose to prevent that hot water circulation in the summertime, when you don't want any more heat in the cab than necessary. Even if you put the shut off in the upper, cold hose, you can still get some convection heating from the heat travelling through the water to the core. It may not be as hot, but it can still warm the cab when you don't need/want it.
I appreciate you describing how the heater core system works as I think its very important to the overall cooling system function.
Just my expereince but my heater never worked very well in my truck. It didn't bother me much as I usually dont have it on the road in the winter, but I still drive around a lot in the cool, even cold part of late fall. I always thought these heaters were a joke because even though I was running at 180 degrees there was little heat coming out of it. I was wondering if the core was clogged. Once I figured out that my hoses were connected wrong I changed them around, put the valve in line with the bottom inlet port, burped the cooling system with engine running from a cold start, the radiator cap off, the heater valve open, and what a difference! Now it will get almost hot in that cab, with temps in the 20's. And my engine temp is stable now at around 185*. Before it was sometimes bouncing very high probably due to air pockets.
Last edited by hooler1; Jan 28, 2026 at 07:56 PM.
Fleet Owner
Joined: Dec 2000
Posts: 25,378
Likes: 5,387
From: Central PA
I ought my truck in June of 1977. A week later I drove from Lancaster County to pick up my college girlfriend who lived outside philly near Norristown. We drove to Wildwood on the Jersey Shore. I couldn't figure out why my heater box was so hot. I did not know there was a shutoff valve on top of the engine to keep hot water from running through the heater. So to get away from the heat from the heater she had to sit next to me!
Fleet Mechanic
Joined: Sep 2010
Posts: 1,833
Likes: 467
From: Mason City, Iowa
I ought my truck in June of 1977. A week later I drove from Lancaster County to pick up my college girlfriend who lived outside philly near Norristown. We drove to Wildwood on the Jersey Shore. I couldn't figure out why my heater box was so hot. I did not know there was a shutoff valve on top of the engine to keep hot water from running through the heater. So to get away from the heat from the heater she had to sit next to me!
Tuned
Joined: Aug 2024
Posts: 326
Likes: 158
From: Texarkana
I ought my truck in June of 1977. A week later I drove from Lancaster County to pick up my college girlfriend who lived outside philly near Norristown. We drove to Wildwood on the Jersey Shore. I couldn't figure out why my heater box was so hot. I did not know there was a shutoff valve on top of the engine to keep hot water from running through the heater. So to get away from the heat from the heater she had to sit next to me!
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