Two Heater Valves
 

In a post this summer about heat sneaking past the HVAP temp control flaps Greg Brown suggested using two heater valves (Inbound and outbound) to block off hot coolant to the heater core to prevent it from getting warm and thus impacting cooling efficacy because the foam sealing the HVAC cooling flaps has deteriorated. I am considering this modification since I have my intake off and coolant drained.

My understanding is the second heater valve spliced into the return line and is driven off the same vacuum line. Does anyone have any pictures of this setup? I'd like to see where the second value was spliced into the return line and how the vacuum lines were routed.

OK to add a second valve in the loop and connect it to the same vacuum source, but... It will still open on engine stop as the existing valve does. With just the existing valve closed, it's possible that heat from the engine will migrate back from the water bridge and vents to the reservoir and the heater core. If you tie the valves closed in summer as many do, you'll keep the heater completely isolated with engine off, and will have less heat to pull out of the airbox on engine restart.

A decade or so ago I looked hard at adding a second valve to the loop, but one with vacuum-to-open vs the vacuum-to-close used in the factory (and most other) valve. I identified a valve that would do the job, something native to a small Ford like a Focus, Escort or Capri IIRC. Plan was to hook it up to manifold vacuum, so the vacuum would go away instantly on engine stop and the valve would go closed. My figuring was that I wouldn't need to go through the valve tying effort every year while we lived in an almost-always-warm SoCal climate. Then we moved to central Oregon, where summer morning temps are generally around 50º and having morning heat is a blessing. Heat of the day here is nothing like SoCal heat of the day, so this modification dropped down into the not-that-important category.

So.... Do you need a second valve? It might help with HVAC performance especially following a hot-soak. How to hook it up? Depends on what valve you install. Having two of the factory-style vacuum-to-close valves won't buy you anything to help with hot-soak recovery. So decide how important that is, and make your decision from that perspective.