Vacuum operated heater hose shut-off valve

[James B.]

My '72 BB wasn't equipped with one. I'm curious why didn't GM install one for this application, even though GM had installed them on some other Corvette applications.

[Larry M.]

Re: Vacuum operated heater hose shut-off valve

These were originally installed on the mid-year cars with factory air conditioning to help improve interior cooling. I'm sure that as air conditioning became more common, GM probably made these standard (??) Or maybe they were only for the CAC cars (??)

GM/Chevrolet was not about to spend extra $$ on a car unless they had to do it to satisfy customers.

Larry

[Dick W.]

Re: Vacuum operated heater hose shut-off valve

I have installed the valves on several '72 cars, just to reduce the hot air in the car, and improve the A/C performance. Just hook up the vacuum lines per the '70 instructions in the Service Manual

[Jim T.]

Re: Vacuum operated heater hose shut-off valve

My 70 came with one, last time I checked its vacuum operation it still worked and it is original.

[Jack H.]

Re: Vacuum operated heater hose shut-off valve

I suspect it was a cost reduction issue as A/C equipped Corvettes had the hot water shut-off valve though '71 and it was re-introduced in '76. In addition to the cost saving (valve itself + interconnect), the contribution was rather modest.

Valves have to default (open or closed) and this valve's default state was to be open. It closed when it received vac AND the A/C system was being driven in MAX mode. Therefore, hot water flowed to the heater core whenever the system was not in MAX A/C mode and that includes hot days when the engine turned off (refuel, get lunch, run into the store).

So, when the engine stopped and the valve opened, the heater core would 're-charge' with hot water making the air plenum re-heat and outlet air to be rather warm on engine re-start/initial drive away.

Even with Corvettes equipped with hot water shut-off valves, their contribution is rather minor and you have to teach yourself how to properly 'drive' the system... After a brief stop on a hot day, restart the engine and switch to MAX A/C mode as you drive off. That will close the hot water shut-off valve and sacrifice intitial cooling to 'suck' heat out of the heater core....

Some find it actually simpler to install a manual hot water shut off valve in the line to the heater giving them two modes of operation (summer and winter). But, this is not factory 'stock'....

[Jim D.]

Re: Vacuum operated heater hose shut-off valve

I have a question that relates to this thread. I have a 66 with vintage air conditioning, the vacuum shut off gets its vacuum from a tee going to the distributor, will that effect performance ?

[Jack H.]

Re: Vacuum operated heater hose shut-off valve

Sounds to me like the car's been 'jury rigged' to permanently invoke the hot water shut off valve IF the vac line that's T'd into provides full time vs. ported vac...

[Joe R.]

Re: Vacuum operated heater hose shut-off valve

If I recall correctly, in the C2 air conditioning system the air from the blower motor passes first through the A/C evaporator and then immediately through the heater core, with no intermediate doors or flaps to redirect the air.

So, when running the A/C, it is essential to stop the flow of coolant through the heater core.

[John H.]

Re: Vacuum operated heater hose shut-off valve

I have a question that relates to this thread. I have a 66 with vintage air conditioning, the vacuum shut off gets its vacuum from a tee going to the distributor, will that effect performance ?

Not unless the vacuum-operated coolant shutoff valve has a vacuum leak. I've built several project cars with Vintage Air A/C, and used the vacuum fitting on the intake manifold, just behind the carburetor, as the vacuum source for the Vintage Air system.

[Roger S.]

Re: Vacuum operated heater hose shut-off valve

I just installed a second vacuum switch and a bypass valve in my '73. May not be 'correct' but it looks original, at least to me.





Roger

[Jim D.]

Re: Vacuum operated heater hose shut-off valve

Not unless the vacuum-operated coolant shutoff valve has a vacuum leak. I've built several project cars with Vintage Air A/C, and used the vacuum fitting on the intake manifold, just behind the carburetor, as the vacuum source for the Vintage Air system.

Thanks John, I'll pick up a fitting and hook it up as you suggest behind carb.
JIM