There are several forces at work here.

  First, consider the size of the heater core. However large it is, it can exchange just so much heat - that is, transfer just so much energy to the air passing through it. If the flow of hot water is restricted, the amount of heat available will drop. Usually we use the control handle for this, but the core can become crudded up, and the effect is the same as closing the valve.

Second, remember that total heat energy available? If you push MORE air thru, the air cannot get as warm, no matter what else is going on, because the energy is now spread that much thinner. The amount of air that passes thru the core at highway speeds is amazing! It is a wonder the air warms up at all!

  I have partially solved this problem by almost closing the air off, and only letting a small amount thru. At highway speeds, it is still enough, and it is much warmer.

  My guess is that most of the problem is just a partially blocked heater core. My thermostat works just fine, but I will still freeze if I don't restrict the airflow at speed.
  ----- Original Message -----
From: Andy Highland
Date: Tuesday, November 28, 2006 11:55 pm
Subject: Front heater: low heat at highway speed
To: vanagon@GERRY.VANAGON.COM

> I've been using my van (1990 Westfalia) in sub 40 F temperatures for
> the first time. When I'm at freeway speeds, the heater is blowing
> cold air. When I close the floor vents, (3rd lever pushed to the
> right) the air gets a little warmer, but not by much. Basically, the
> heat coming out is totally inadequate, but only when going fast in
> cold weather. In more moderate temps, or when I'm on city streets it
> seems OK. So is my front heater broken, or is this how other people's
> vans work? I find it hard to believe that people up north could live
> with such a pathetic heater.
>
> Thanks,
> andy
>