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8 & 10 cyl Bristol cars From Type 407 onwards

411 Heater Controls

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Old 13-09-18, 10:17 PM
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Location: woodhouse australia
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Default 411 Heater Controls

Hi All , I am after some advice on how my heater works on a 411 mk 2.
There are 2 controls on the dash.The demist defrost knob and the heat knob. Clearly the heat knob controls the hot water tap on the side of the heater box. does it do anything other than that?
The demist defrost knob in the demist position clearly sends air to the front screen but my question is what is meant to happen to the air in the defrost position ?
The reason i am inquiring is I get very little hot air in the cabin as I am driving along the road and get no heat out of the fresh air vents.
Strangely when the car is at idle in the driveway and the 2 speed fan is working I get lots of hot air in the cabin at the screen and the under dash trap door, but also through fresh air vents. The car becomes almost too hot which suggests I dont have a water flow problem.
My guess is that at speed the cold air is mixing with the hot air and lowering the overall temperature, which suggests a flap in the system isnt doing its job.
Should I at any time be getting hot air through the fresh air vents?

Any thoughts or suggestions much appreciated.
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Old 15-09-18, 10:29 PM
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There was an excellent article a little while ago in the BOC Journal which described the rather complicated (clever) workings of a 408(?) heater. Do you have access to back issues of the same?

The 411/12 heater is, I understand, from the viewpoint of a 410 owner, basically similar, with some clever fresh air variants, but trying to explain its workings and the clever variants feels rather too much after a glass or two, so let me know the answer to the above and I'll have some idea where to start from.

Roger
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Old 16-09-18, 07:04 AM
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Rodger thank you for that. We are off on a 5 day Bristol Rally starting tomorrow morning . Brother Geoff is a member of the UK club so should have the article.
A bit of a look at the heater today made me realize how complex the system is, It would appear that there are at least 3 flaps inside the box one of which is activated by the heat knob which also turns on the hot water tap.
As you said the 411 has the flow through fresh air vents as well. I may yet have to pull it out and apart to work it out,which i suspect cont be an easy job.
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Old 17-09-18, 02:41 PM
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You will find the article I mentioned in the Bristol Owners Club Bulletin Number 142, dated Autumn 2011 and the good news is that it actually describes a 411 heater and ventilation system, not an earlier model which is what my memory told me.

I found it extremely useful when I had the 410s heater out to replace seals etc. The heater box itself is apparently identical.

Two points

I found that there was a very simple gravity flap on the 410s heater which opens a direct path into the heater box from the air intake in front of the windscreen when the pressure in the former overcame the pressure supplied by the fan. So at speed the air doesn't all have to pass through the fan. I don't see any reference to this in the article, perhaps the 411 was different. I also discovered that the foam on this flap had long since perished causing it to close at intervals with a surprisingly loud, rather alarming and previously untraceable clang.

I see I marked up the copy that found its way into my workshop manual, at the bottom of page 27 to read "Slacken screw E ..........at the point F and retighten screw E"

Trust you have a good tour

Roger
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