From: BOC V8 pages (members only, so you must subscribe to the BOC to read these excellent pages!)

HeHeater & Demisting

It is more than likely that the heating will not work. There are various reasons why this may be the case, but the short list of two main reasons are that the water tap and control are blocked (remove front hose and clean as required), or the internal heating matrix is blocked and corroded. If the latter, then the whole unit needs to be taken out and pulled apart (it is soldered together!) and repaired. This is a specialist job and also means that the internal air direction controls may require servicing as well. It takes several hours (if you are experienced), which explains why SpencerLaneJones charges around �350-450 to correct one back to perfection (relative term). Whilst out, the area under the heater and indeed the air intake box, which also contains the drain holes for incoming water, may be corroded or at least in a sad state. This is a good time to clean, strip, rust convert (or more), treat, seal and paint before replacing reconditioned unit. You may wish to see what BristolCars, BrianMay or others may charge for replacing one unit for another. Considering the amount of mucking about involved, my advice is to save the pennies and ask someone who has done it several times before to do it, with a guarantee. Whilst out of the car, make sure you fix the rest yourself, or at least make the service people do it for you, else you will get a surprise sooner or later. Finally, if the air flow controls or flaps are damaged, this will mean no air, or little air goes where you want. The Bristol unit, although maybe not as efficient as one would like (mine has never worked so I am using others' experience here), is highly sophisticated and compact.

Heater air intake

Q: How can you tell how much of its life a Bristol has been kept outdoors?

A: Look at the state of the main air intake box behind the heater and the bulkhead below it. Some rust through completely. This area is open to all elements and is extremely difficult to repair properly. The box catches rain water and the drains are susceptible to being clogged by leaf debris. The box can't be rust treated with waxol or similar because you'd stink the car out when you opened the fresh air vents. You can't easily paint the inside of it either unless you are doing a full strip down restore. The best you can do is vacuum it out, clean with a wire brush and poke a wire through the drains to clear them. It is worth treating the susceptible areas with anti-corrosion paint. Damage to this box results a considerable amount of water getting into the car, ruining the carpets and rusting the floor out.

The picture above is of my car, whose heater didn't work: small wonder! The original core (left) evidently had developed a leak. To stop it, somebody poured in the cooling circuit a lot of sealant. Once this was removed, it leaked. I had a new one remanufactured (right)