Something that's been slightly troubling for many years is that my heater/demister seems to have all the power of a flatulent mouse and I'd be interested to learn from fellow owners if this is normal.
To be fair It's not a pressing problem as I've almost never driven with the hood up, but the day will come when I have to drive home through the rain - so hood up and I suspect that the demister will not be up to the job. With a warm engine and switching the blower on I cannot feel a zephyr of breeze - or heat from the demister vents.
I've checked that the heater unit has a good throughflow of water, all the tubes to take the alleged hot ait to the demist vents are in place.
I guess I may need to swap the heater matrix for a new one?

Martyn

Does the heater flap inside the car close properly? The blower will send either air to the demist vents if the flap is closed or air to both the demist vents and the car interior if it's open. For max demist power the flap should be closed.
The heater matrix has felt seals at its corners where it fits inside the heater case. If these have degraded part of the fans airflow will bypass the matrix. This won't affect the flow to the vents but might reduce the temperature. The matrix itself may have become clogged over the years with debris between the fins. This will reduce both the temperature and the airflow. If the matrix is clogged internally this will reduce the temperature.

If the airflow is the main problem the possible sources are a clogged or by-passing matrix, a poorly closing heater flap or possibly the fan motor.

If temperature is the problem it could be the heater tap, the matrix clogged either internally or externally or possibly an airlock in the water lines.

One cause of poor demisting that crops up occasionally in restored cars is that the vents have been fitted the wrong way round i.e. with the vent facing into the car rather than at the windscreen.

Most problems with the heater are caused by the water pump, whilst standing there is plenty of heat going to the matrix but very little heat returning, when in motion this drops to freezing, the gap between the pump casing and the impeller is critical, whether it cavitates and little water is forced through the small bore pipes or that the gap allows the water to mainly bypass the pipes I'm not sure. The best check is to measure the gap in a water pump that actually produces heat and the one you have, my heater stopped working on a change of water pump and neverworked again until the pump was changed again.

Both front and rear clearances are important . Often the angle is machined to remove corrosion . When the impellor is reset to this new face the rear clearance to the manifold is increased . Have just machined an unidentified impellor to take up these variations ( the angle was different ) .The backset between faces is 23mm . now have 1mm clearance at the manifold to impellor . will test when the Rain stops .
PS , the modded impellor is not an l6 jag one , they end up short as well.

cheers all.

Nick, Ian and Steve

Thanks for those suggestions.
The flap that acts as an airflow diverter is a fairly good fit into its 'box' and I have some soft neoprene foam strip I can use to make it better.
I'll take a look and see what I find in the water pump.
Martyn

Hi Martyn

My heater didn't work for around 25 years until I fitted a new matrix when the engine was out in 2011/13. Now it's gloriously warm when required, enough to keep warm in Scotland with the hood down and snow lying everywhere, not that I'm inclined to do that very often! In a near 60 year old car, a blocked matrix is high on the list of possible causes.

Good luck with resolving the problem.

Cheers

Fossil

Martyn
I found my connections to the demisters had disintegrated and heat leaked away. The heaters themselves are very effective and much better than the V8250. They should cook the footwells if you open the flap.

SimonP

Well, one year on I've decided to replace the heater matrix and take a look at the blower fan motor. I've removed the transmission tunnel so's I can get at the four screws that hold the heater unit. I've removed all the plumbing from the heater box, and three of the four screws (the last is being difficult) from beneath the unit. Looking at the size of them ( see photo) I find it difficult to believe that these are all that the only fixings for the unit. I can find nothing on the forum or the manual - can anyone confirm please?

http://archive.dloc.co.uk/forum/topic.a ... Terms=1378 shows Kev Monk's restoration with a series of photos of the heater.

As far as I remember there were only 4 fixings through the base when I replaced the matrix a couple of years ago.

The screws may be for the flap.
Four bolts thread up into the base.