Close heater control valve for summer to maximize AC
 

1983 300D turbo - Texas summer heat, high humidity.

I'm still thinking about how to maximize by AC performance. I've noticed when I make several stops, the AC takes a some time to put out really cool air when I restart.

Here's my thinking: The heater control valve is closed when 12V is applied and cuts off hot water flow to the heater core. No voltage the heater gets hot water. In max AC mode, 12V is constant to the heater control valve. If you turn the engine off, the voltage is lost and the valve opens. That must allow some convection circulation of hot water into the heater core. Restart in a few minutes and the valve closes, but the heater core is already full of heated water and decreases the AC temperature until the water in the core is cooled. Make sense?

I'm considering installing a manual valve in the hose and turning off the valve for summer so that no water can circulate to the heater control valve. Make sense?

The only downside I can think of is the water that's in the heater core will be stagnant for the summer. I don't see why that would be a problem. Thoughts?

I guess I could just leave it idling like the commercial diesels do.

You would have to do both ends otherwise it will still convection heat.

I wonder if the heating is primarily convection or if the temperature differential actually creates some circulation? One valve would stop circulation.

Any reason not to try it?

Wouldn't it be simpler to run a 12v, switched wire to the valve?

Yes, I considered that but it would have to be hot regardless of ignition position and I wonder:
1. Would that damage the heater control valve mechanism - they are no longer made (just the core unit is available new) and I only have one spare.
2. What would be the battery drain if I (or the wife) forget to switch it off at the end of the day.

Actually, that may be the solution. All I need to do is put a timer in the circuit to kill the power after 30 minutes or so, right? Anyone know of such a timer?

switching the electric also only shuts off one line. I think the manual shutoff is the better solution. besides these leak a little so it should not allow the heater core to stagnate. timer relays are expensive and wont stand up to under hood auto use, and they usually only delay up to a minute

Thanks, I was also concerned that the new 12V circuit feeding back to the CCU might cause some damage.

I think I can find the right size ball valve in SS.

Actually it seems like more trouble than it's worth. There is already a check valve built into the monovalve, so you shouldn't be getting any convective flow from that side of the engine. I really doubt you are getting any significant convection from the head to the heater core with the engine shut off. It seems like a lot of trouble to increase the AC response time by a few seconds, if it actually makes any difference.

Might be easier to use something like this instead of fitting globe valves -

http://commons.wikimedia.org/wiki/Image:Hose_clip_jg.jpg

The one on the right, I mean.

Or use padded vice grips as an experiment to see if it makes any difference.

Sixto
87 300D

Craig is right. The check valve in the monovalve was deliberately designed into the system to prevent hot coolant from getting into the heater core when the engine is stopped. If the check valve leaks, that's another question.

Even with a perfectly working system, the ducting still will absorb a good deal of heat from the engine, the ambient air, and the sun. The more the car sits, the more heat will be absorbed. When you return and start the car, the a/c system has to remove all of that heat before you get cool air in the car. There's not much you can do about it, either, except park in the shade, cover the car, move to Alaska, etc.

I actually replaced my entire monovalve when I started to get a blast of heat after restarting the car in the summer because I assumed the check valve was leaking or sticking. It solved the problem, but I don't know if the check valve was really the problem or I would have gotten the same result with just a rebuild kit.

I've replaced the core piece but not the entire valve. When I restart the hot engine, I get an initial blast that feels like full hot from the heater. This slowly tapers to cold but the wait is a good 5 to 10 minutes, not seconds.

Maybe I have a bad check valve. Is this part of the valve assembly or another item?

I believe it is part of the monovalve but unfortunately not the replaceable part for which you can buy a "kit." AFAIK, you have to buy the entire monovalve.

That doesn't sound like residual hot coolant in the heater core, that sounds like the monovalve is staying open for several minutes after restart for some reason. Try an experiment on a hot day; put on full heat then switch to AC and see how long it takes to get cold, I bet it only takes about one minute (not 5 to 10), assuming the monovalve is actually going closed.

One other thing to check, I was getting occasional hot air from the AC at one point and found that the electrical connector on the monovalve wasn't making steady contact. Try tightening those contacts and/or putting some conductive "grease" on the contacts.