cold water in heater core/s does not go to the radiator.
it goes back to the engine.

both heater circuits have nothing to do with the thermostat or the radiator.
In all cars I have ever seen the heater circuit comes directly off the
hottest part of the engine ..
usually the cylinder head.
This is why ...
when it's really cold, don't start driving with the heater one ..i.e.
heater valve open..
as it will cool the engine, making it that much longer before the t-stat
can do it's job.
Subaru even published a bulletin on this ..
in really cold temps ( or a long initial downhill run in cold temps ) if
the heater is on from the get-go ..
in a strong case it'll never get into closed loop mixture control.

thermostats deteriorate in performance with age.
I'd say 5 years is long enough for a good german t-stat in a waterboxer
engine.

scott

On 9/7/2015 6:16 AM, Dan N wrote:
> I think it's normal behavior. When you open the front heater, cold coolant
> in the heater core flows to the radiator, warm coolant from the radiator
> flows to the heater core - an exchange - and you see  small dip on the
> coolant gauge. This will happen briefly until the warm coolant in the
> heater core reach the same temp. as the coolant in the radiator.
>