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[Migv6 le Frog Fan]

Any help ?
https://www.youtube.com/watch?v=fq9leYUzAIY

The actuator arm in the video isnt easy to see when your in the car. Also a much more obvious one connected to the same servo which operates a flap nearer to the driver.
The one in the video had come adrift on my car and I had the same symptoms as you. It took quite a while to find it.
In fact I only found it once I discovered that video on youtube.

This is the one directly under the heater matrix, correct? I'll have to check the arm. How would I know to test this? Just turn heat to LO and HI and see if it moves?

Not the one directly under the heater matrix. Thats the one thats easy to see. The other one runs down the back of the matrix and is difficult to see. Shine a torch up there and you might see it dangling in fresh air.

[Doctor Gollum]

HBV failure generally manifests as a mahoosive external coolant leak... A gallon a minute type mahoosive if the setting allows.

Ergo, this issue will probably end up being a perished vacuum hose or failing vacuum switch. But you do need to test the HBV just to rule it out.

[cam.in.head]

does it  need vacuum for heat or bypass

[Doctor Gollum]

does it  need vacuum for heat or bypass

Without an HBV in front of me to test I honestly can't remember, but partial heat suggests a vacuum issue either way.

[RallyArde]

If thinking that there is any chance for external leak wrether in HBV or somewhere else, one option for finding it, would be to presurize the system

https://youtu.be/G9O--zV-ZQw

https://youtu.be/Ox2WtinRkBE

Couple vids from my case about year ago. Helped me to diagnosize faulty HBV easier

[Doctor Gollum]

If thinking that there is any chance for external leak wrether in HBV or somewhere else, one option for finding it, would be to presurize the system

https://youtu.be/G9O--zV-ZQw

https://youtu.be/Ox2WtinRkBE

Couple vids from my case about year ago. Helped me to diagnosize faulty HBV easier

Engine running and slowly change the temperature from Lo to Hi and back to Lo and you'll soon have coolant pouring out of it if it's leaking.

Generally they leak at one extreme or another. Keep your climate control set at 21 and you might have a pesky leak that you can't quite locate.

[underi]

A reservoir
Manifold connection at the one way valve by the brake booster
A vacuum switch controlled by the Climate Panel
At least three pieces of vacuum hose
The HBV

Had a look, the vacuum line at the reservoir and at the HBV is fine. I’d imagine if it was a vacuum issue it would be the switch. Is it located directly behind the climate control panel? I can see the two vacuum lines make a turn and go somewhere behind the heater matrix.

[cam.in.head]

easiest first is just to put your finger over the vac line to the valve . at max hot or max cold one way should create a vacuum . if vacuum is present suspect valve.       if not suspect control switch or vacuum connections somewhere.
your on the right lines now thou as something is keeping the flow from entering the matrix .  cold pipe !

[underi]

Finally had the motivation and time to check thing again. The HBV definitely works. I ended up running a straight line from the HVAC vacuum reservoir to the HBV to test it, after connecting the matrix was barely warm and after disconnecting the vacuum it got super hot, the pipes next to the matrix would burn my fingers after a second or two. So the matrix is definitely 100% warm. It was a cheap aftermarket part though so fitment wasn't perfect and it was leaking quite a bit. I used some expanding foam to seal it and ended up realizing that the leaking warm air was the only thing warming up the car lol. When fiddling with the temperature settings I can definitely hear the servo moving and on cold side it's super cold and on the warm side it's probably like 1-5 degrees Celsius. Barely can feel the heat but it does melt the ice from the windshield after 30 minutes.

TLDR: Heater matrix is hot, temperature flap servo moves, but no visual confirmation about what exactly it's doing. Is there a diagram of the hvac system, the flaps? How the servo is connected to the flap. There was a youtube link in this thread about it but I didn't really understand if it's the correct one. The long plastic handle is the one operating the mixer flap? Because that definitely moves, but whatever it's attached to might not be.

[cam.in.head]

so. from what you have found at least answers one question. the bypass valve allows hot water to flow into the matrix unless it has a vacuum applied to it .in which case it closes and bypasses the matrix.so a missing vacuum / pipe fallen off situation would not cause a cold matrix.
 before you had a cold matrix pipe and now they are both hot so something has changed anyway. maybee an airlock was present before ?.
so now its suggesting an issue still with the servo/ flap system somewhere.
if you have poor heat from ANY exit then it would suggest there is a mixer flap somewhere that controls the temp ( although i thought that was what the bypass valve was for ?)

[Doctor Gollum]

The HBV regulates the hot water flow, the blend flap regulates  the inside/outside air AIUI.

[jb]

Blend flap arm come off the servo motor?