im not sure if this problem is boost related or coincidence,but i'll blame boost...
my heater always worked really good untill i turboed it.initially i used factory vacuum supplies,but then when the heater didnt work and after 6 months of my g/f complaining i got a vacuum manifold and tapped a fitting into the intake so all turbo stuff has a seperate vacuum source,but the heater is still dodgy,it gets warm...ish then just blows cold air at me.
has anyone else had heater issues after boosting and how did you fix it?

hi
i had a similar issue with my ed when i boosed it..
the heater would work but a few mins after i gave a few boot fulls.lol the heater would blow cold air. and would stay cool untill i started cruising again.
what i did is i used a blue/black one way check valve from the ef bbm solinoid.
and fitted it between the manifold and the vac line into the car.
so it will draw vacuum off boost and when the manifold was under pressure the valve being one way will block pressure going to the heater controls.
and the heater controls would hold vac for a while as it has a small vac chamber so aslong as you dont have any major vac leaks in the heater controls this work quite well..it has for me..

you could just get another gf? could just be easier

i considered just doing the ol' one way valve trick,but thaught it might be more than that,mine doesnt go back to warm air,well the furthest i regularly drive is like 40kms and it doesnt get warm again in that time...
new g/f hey?know anyone??jk

You could just remove the vac hose from the heater tap altogether. The default setting for the taps (sans vacumn) is on, so you should have no problems getting heat then.

In my experience, whilst the always open tap might delay engine temp getting up to speed by a very small margin, the constant flow of coolant through there is better than the slowly rusting heater core over summer. As long as your blend door is operating properly, you shouldn't have too many dramas!!!

It worked for me and I had no problems afterwards in my old EB XR8 when I did the CC conversion and had vac problems to the heater tap anyway.