A/C Hot water Bypass Valve Question

[Mark H.]

The vacuum hot water valve located behind the RH fender vent is supposed to block hot water from entering the heater core when the A/C is selected.

My question is, does vacuum applied to the valve BLOCK the water in the heater hose from getting to the heater core, or does it open the valve to allow it?

Thanx,

Mark

[Larry M.]

Re: A/C Hot water Bypass Valve Question

The vacuum hot water valve located behind the RH fender vent is supposed to block hot water from entering the heater core when the A/C is selected.

My question is, does vacuum applied to the valve BLOCK the water in the heater hose from getting to the heater core, or does it open the valve to allow it?

Thanx,

Mark

Mark:

The correct answer depends on the year of the car. For the C2 cars (1963-1967) applying vacuum to this valve opens it which supplys hot water to the heater. So with no vacuum, the valve should be closed. The vacuum control was in the HOT-COLD pull cable, and not in the AC on/off cable.

This changed somewhere during the C3 period. I do not know the exact year. After this time, applying vacuum would close the valve, whereas no vacuum opens the valve and supplys hot water to the heater. This was done to provide heat in the case that the vacuum supply or vacuum switch or vacuum tubing etc. failed.

Not certain about the 1971 model year. You would need to check the schematics in the Chevrolet Service Manual, or simply hook-up a vacuum source (Mighty-Vac) directly to the valve and check it yourself with the engine running.

If I had to guess, I would say your 1971 acts like a C2 car, and that it was around 1976 that things changed. But I have been known to guess wrong before.

Larry

[Mark H.]

Re: A/C Hot water Bypass Valve Question

Thanx Larry,

Just found the answer in the middle of one little paragraph in the '71 Chassic Service Manual.

The HWV is open UNLESS MAX A/C is selected, then vacuum closes the valve and does NOT allow hot water to the heater core.

Mark