Think about it. The engine is way in the back, the heater is way in
the front.  The transfer tubes from rear to front are exposed to the
elements and share their captive heat with the radiator. (which
transfers heat to the external air mass.) So, the heater core is
disadvantaged from the beginning.

The heater has a fan blowing cold air across the heater core
(assuming you are set to outside air and not recirculating inside
cabin air which would slightly lessen the situation).

At idle the water pump is providing minimum flow hence minimum hot
water available for heat transfer in the heater core. Now add a
"tired" pump with worn or eroded vanes and you get even less flow and
less heat to transfer.  If the fluid is barely moving through the
core the latter will quickly cool and blow cold.  Pick up the revs
and you do two things. Add more fuel to the fire (gasoline) and pump
hot fluid faster and in greater volume. Voila, Heat.

  It's very common. My 3.3 L chrysler V6 van blows cool at idle
(172,000 kms) as does my Mazda 2.2 turbo (282,000 kms) when it's
below -20 C. ( Which it is today)

On 6-Jan-10, at 11:31 AM, mcneely4@cox.net wrote:

> Someone else suggested the no heat when stopped situation is normal
> during cold weather because the coolant temp drops.  In my
> experience, and that of a good many others, Vanagon usually runs
> cooler when moving than when idling, due to increased air flow
> through the radiator.  But, if the water pump is moving fluid
> poorly, perhaps the radiator and coolant pipes would have stagnant
> coolant.
>
>
> On Wed, Jan 6, 2010 at 12:13 PM, Don Spence wrote:
>
>> Tired water pump.
>>
>>
>> On 6-Jan-10, at 8:38 AM, Automatic digest processor wrote:
>>
>>> No heat at stoplights.