Alan

Without vacuum the heater valve is always open to flow (if its normally operational). The flow of the coolant (or not) makes no difference in any way to the valve status - only the vacuum controlled by the solenoid.

Even with a perfectly sealed system - if under WOT (no vacuum being generated) you change the status of vacuum solenoids - then the volume of the actuator and tube feeding it from the solenoid become added to the total volume of the vacuum system after the check valve. Since all these area are bled off to no vacuum by the solenoid valve off - then the vacuum on other actuators will drop proportionally at that time - possibly too much to keep tham actuated - until the vacuum source returns. So if you swap the vent posistions around during WOT you may get some hot water in the heater core...

Its not a very likely scenario.

More likely is that there are (or will be) leaks sufficiently big that you will get vacuum drop outs with extended WOT - like climbing hills.

Alan

Aloysius

Domo Sensei! I am troubleshooting my HVAC and this was some good info: system actually connected to manifold vacuum and heater valve open all the time unless there's vacuum. I've started troubleshooting the leaks but haven't closed all the loops just yet <too busy - sigh>. My WOT sessions are very few, so yes, very unlikely scenario.