Water getting in through scuttle vent?

[88V8]

That should work as long as the holes are large enough; basically the cross sectional area needs to allow sufficient air into the plenum to match what your trying to pump out through the heater. Without going to all the trouble of calculating the requirements, a pretty safe bet would be to ensure the area of the holes matches the area of the intake with the vent flap open.

The top of the heater box is about 4" dia, so has a cross-sectional area of around 12.5"sq.

Wonder how big those access covers are?

 

What I intend to do with the heater, having closed up the external inlet, is to fit a larger fan as I previously posted elsewhere, see if I can get a larger matrix made for it, perhaps block off the 'fresh air' outlets at the top although on the other hand one can effectively block them by closing the eyeballs, do away with the flap that directs the air to screen or screen + interior and close the bottom of the heater box with a fixed plate so that the air no longer spills out of the bottom to warm the transmission tunnel , thus I will be sending all the air through the tubes to the demister. The demister tube I will then split with elbows, and add a number of eyeballs in the footwells to provide the cabin heating so that there will be full-time air to the screen and part-time to the footwells.

I am hoping that the fan will be so vigorous that it wil be possible to unrestrict the demist outlets or perhaps just the driver's as the passenger doesn't really need to see where one is going.

To get air into the heater box I shall make a number of holes in the top of the box above the fan, equal to the existing X-sectional area of the intake. The access covers could make a very useful contribution here, I hadn't thought about them at all.

Interesting that Telster says it's 'no worse' - perhaps that's because of the open access cover.

 

The fan will need to be controlled with a rheostat which I may put in the H frame in the hole that the existing redundant flap knob presently occupies.

 

Anyway, that's the plan atm. The spare heater is lying under the car where my foot regularly comes into contact with it as I squeeze around the front to continue polishing the rust in the engine bay, and this reminder serves to continue the incubation of 'The Plan'.

Doubtless this will mature as time goes on.

 

One other thing to think about in the fug-stirrer concept is that if the heater is drawing air from inside the car near the bulkhead, it will probably be important to ensure a good seal between the engine and pasenger compartments. Reason is that the passenger compartment is a low-pressure area whereas the engine compartment is a high-pressure area, so engine bay fumes will readily be drawn in through any leaky or absent grommets etc. I realise this already applies, but there is a difference between fumes percolating in, and being actively sucked in by the heater.

 

Ivor