1972 Hot water shut off valve

[Edward McComas]

I think I recall that 1971 and 1973 Corvettes with A/C had a hot water shut off valve, but 1972 Corvettes did not. Is this correct? If so, is it possible to put one on a 1972 Corvette. Is the factory vacuum hose still there, or would I have to run a vacuum hose?

I did not see a hot water shut off valve in the AIM for 1972, but I seem to recall one for both 1971 and 1973. Any ideas on why Chevrolet dropped this for one year,
assuming my info is correct?

[Reba Whittington]

Re: 1972 Hot water shut off valve

Don't know about the 1973, but the '72 did not have one. I have added one to mine because of the marginal AC with a big block.

[Jim T.]

Re: 1972 Hot water shut off valve

1970 had one. My 70's valve is still functioning. Easy access by removing the side grille if you are going to install one.

[Harmon C.]

Re: 1972 Hot water shut off valve

Ed Not used on 73's

[Jack H.]

Can add....

but you'll have to 'create' the appropriate vac control line for it, Ed. If memory serves (I can check out and report back), the line that ran to the hot water shut-off valve was 'T-d' off another and your car won't have the line in 'er. I have to check this to make sure I'm NOT confusing the hot water shut-off valve's vac control line for the exterior air make-up door (recirculate vs. external make-up air source to the plenum....

As to why GM discontinued the 'feature' for a period of time, we'd really have to have primary 'testimony' from one who worked on the Corvette design team back then, but I can offer my hunch.

The way the system worked (documented in the AIM/CSM under C60 section with vac truth table for AC control/servo lines), the inline heater hot water shut-off valve was vac controlled. Designers had to reason out how this 'switch' should work--normally open or normally closed.

If the switch/valve was normally closed and the cockpit controls told it to open only when heat was needed (system temperature control off MAX cool setting), then in the event of some control system failure the result would be NO HEAT. The silhouette of this logic 'could be' lack of heater/defroster function and on an ice cold day, this would constitute a threat to life/limb/property--can't see through the windshield!!!!!!!!!

If the switch/valve was chosen to be a normally open device, then cockpit controls would have to instruct it to shut. In the event of a control system failure, the result would be a hotter than desired air box (hot water flowing through the heater core and radiating/convecting into the system), so Mr./Mrs. 'high roller' who'd bought a fancy new Corvette might sweat a tad crossing the desert in the dead of summer. It's obvious to me which way to default the system!!!!!

Now, the consequences of an A/C equipped Corvette with hot water shut-off valve aren't intuitively obvious to the novice owner/operator and are NOT really addressed in the owner's manual. Consider the case where you start the car, leave the A/C off on a warm summer day and venture onto the highway.

The whole system was cool (ambient). When the car started up with cockpit temp controls both in their OFF position, vac flowed to the hot water shut-off valve and closed it. The heater core in the A/C system essentially stayed at ambient, and any air leakage through the system (internal vents were NOT known for absolutely/positively sealing 100% shut) dribbled out at essential ambient temperature. BTW, this effect (leakage) is MUCH worse in a convertible with top down than it is in a coupe with T-tops installed due to low pressure created in the cockpit due to air flow turbulance over the car.

NOW, the engine warms up and you decide to stop (get gas, run an errand, have lunch, Etc.). You shut down the engine and after some period of time the vac storage can leaks down, the hot water shut-off valve loses its control, and reverts to its normally open position.

By this time engine coolant is REALLY hot from lack of water pump motion and the relatively cool heater core has a circulation path for hot coolant water to move by convection flow. It comes, essentially to thermal equilibrium with the block as a result.

You finish your errand, jump back in the car, fire the engine and drive away. Suddenly you notice that there's HOT air leaking out of the interior air box onto your feet, Etc. It's happening because the heater core is thermally 'soaked' and there's SOME residual air leaking through the air box.

You conclude these Corvettes are HOT and start looking for alternatives to fix the problem (better thermal barrier under the carpet, Etc.). BUT, if you understood the system as you do now, you'd simply drive away, reach down and run the A/C for 5-minutes or so to 'suck' the pent up heat out of the heater core!!!!!!!!!

[Mike McKown]

And the short answer is

If you are trying to get heat out of there, go down to the NAPA store and buy one of their heater hose shut off valves and stick it in one of the two hoses. That'll keep the hot water out of your core and you should have to turn the valve only twice a year. Cheap, quick and simple.

If you're trying to do something else, this does not apply.

Mike

[Jim T.]

Re: Can add....

From what I remember reading about my 70's heater flow shut off valve, the only time that the valve stops the flow of circulation of coolant through the heater core is when I select MAX for air conditioning and this selection also recirculates the air from inside the car. Move it down one position and it reverts back to a open system and uses outside air through the evaporator for cooling.