Although I can't see what (but will admit I've never been inside a heater box) but you've obviously got something wrong. I leave mine on 20 degrees and Auto and it just works. The only slightly odd thing I have on mine is that if both sides are set at 20 degrees, the drivers side cools off sooner than the passenger side, If I leave the passenger side at 20 and put the drivers at 21, the air from the vents is the same temperature from both.

There really isn't much to get wrong - the most complicated bits of it is the distribution flaps, which all need to be fitted in the right way or they'd cause the thing to jam up.

The only thing I'm not 100% on is the blend flaps themselves. They're slightly V shaped, and I suppose it is possible I might have swapped left and right while putting it together, and thus the V may be upside down, perhaps upsetting the mixture of air. I'll have to take the old heater box apart and see if that's possible.

What I've just done for now is partially block up the front footwell vents, from the inside, to not look too bodged. What I hadn't realised about these is there is actually three holes that air comes out of - the main one you can see, and two smaller apertures underneath. One of these shoots air sort of INTO the centre console when the side panel is in place, which explains why the sides of the console get hot or cold... I've blocked up the main hole, and this side panel shooter, leaving just the other hidden hole that points forwards into the footwell. This has the added benefit that the near underseat vents (not the centre console rear vents) now actually do something with the increased pressure! With the airflow set to footwells and screen, the airflow is far more distributed between the footwells (front and rear) and screen. So I'm pleased with that.

Still hot on the floor, hopefully it will reduce the cooking of my feet. Might take it to work again tomorrow and see how it goes.

Nope. Still going cold up top. Dash will have to come out again. Even if I set the air to only come out of the dash, it stays cool, yet the blend flaps don't move. So something must be cocked up inside the box.

You've definitely got something wrong Nick. I've just driven down to Dover, outside temperature reported as 15 degrees and the HEVAC set on 21, Auto. Once the interior was up to temperature the flow and temperature from the face vents dropped to about ambient and the footwell vents dropped to the same.

Been playing around with the spare heater box and checking the flaps on the one in the car. I think this is just the way it is, annoyingly.

More cold air ends up going to the face vents in the dash because of the way the blend flaps are arranged. This effect is made worse by the fixed speed air conditioning. Say you have the air set to face/feet, interior is warm, the blend motors have backed off slightly. Firstly, more cold air is ending up at the dash vents, and then the A/C compressor engages again - the cold air suddenly gets quite a bit colder again, and the air from the dash vents feels quite a lot cooler. Meanwhile, the air still going to the footwells is still warm, because where it exits in the heater box has encouraged more mixing... Set it to feet/screen position and this is less noticeable, because the screen outlet is near the footwell outlet.

I suppose this is just a bit of a 'me' problem as I like the air at the face/feet position all year round once things are demisted, and while in Summer it can't be cold enough, in winter its a bit annoying as on hi/hi, it does become an oven pretty quickly. I want to be warm, not on fire.

For anyone that hasn't seen inside one, here is half of a heater box. To the left you have the footwell outlet, then the screen outlet, and the dash/face vents on the right. Blend flap right in the middle, where you can sort of see how the cold air meets the heated air coming around the core, and I'm presuming they sort of blow against each other. The probe stuck in the box in various places seems to agree. It just doesn't mix well, but I can't think of how I'd encourage it to without adding something that would likely obstruct either the blend or distribution flaps.

I don't know what I expected the heater box to look like on the inside but that wasn't it. Fascinating, thanks Sloth!

Went down to Chippenham on Friday and gave my AC system a test.

It gave me hot and cold on request and out of whichever vents I asked it to.

Sounds like something is definitely wrong with yours but looking at the image of the box I can't see what it could be. It looks like a pretty simple set up and everything looks as though it can only go in one place.

There's not anything fallen in the pipework that could (somehow) be affecting it?

I can get hot from the dash vents if I turn it up high enough, and I mean properly hot. It's just when it mixes, it doesn't do a very good job on the face vents.

I think I'm going to just live with it and stick with the screen/feet setting - that works well enough, and I really can't be arsed taking the whole dash out again.

Besides, I've spotted a new coolant leak this morning, and the EAS compressor keeps running at idle, stopping, then starting, repeat... so I have new issues to look at.

Having got home a couple of hours ago from a trip that started at 4pm yesterday and during which the outside temperature varied between 18 degrees and -1, my heater kept the car at the required 20 degrees all the time. The air temperature from the face vents was the same as that from the footwell vents. I could open a window and let the warm air out, the output from the heater would get hotter to get it back up to temperature but no matter what I did (it helps to have something to play with on a drive like that), I couldn't manage to get air at a different temperature from the face and footwell vents. The heater box may be simple but you must have something wrong.

Well, bollocks. Just when I thought I'd live with it :)

I have no clue how I've done this then...

Now the water pump is leaking, this'll have to wait. Though I guess dropping the coolant again would make removing the heater core a bit easier at the same time.

So every P38 is behaving different when it comes to heating then...
Starting from cold I'm getting warm air (from when available) at my feet and face, when the interior reaches set temp (on auto) the blower slows down and it looks like the facevent is a bit cooler than feetvent. When shutting off for a quick shopvisit and continue in, say 5 minutes, it starts blowing cool at feet and face, blower speed rises and it takes another 5 minutes before things are normal again. I think these cars are like human.

That's one of the troubles... with most other normal cars, you could just go and look at another one to see what its doing. My parents P38 for example... I could just go look at that one. But I know its climate control has all manner of faults too, so its pointless checking it :)

I think I'm probably going to live with this... it works well enough on the feet/screen position, there are other interior bits and pieces I'd rather move on to (once I've done the water pump annual replacement, anyway).