There is an excellent article in the Bristol Owners Club Bulletin Number 142, dated Autumn 2011 which describes a 411 heater. The heater box in a 411 is the same as in a 410 but is relocated to accommodate a separate cold air box.
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"
One further tip. The demister vents work best when they are adjusted so that the vent exit is as close to the screen as possible. This requires more lying on ones back, the development of spider monkey arms and the deployment of much bad language.
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