FRESH AIR HEATERS

Richard Johnston

Using the fresh air heater

The standard fresh air heater fitted to Saloon models is not very powerful. The recirculatory heater fitted to vans is more effective as a heater but as it recirculates the damp as well as the heat it is no better for controlling condensation. Poor heating may result from problems in the water circuits or in the air flow. This article relates primarily to the fresh air heater, and should be read in conjunction with the workshop manual.

When using the heater operating levers, note that they should always move smoothly and should not be forced if they are stiff, because the inner cables are easily buckled at the operating lever end - this is difficult to repair. It should also be noted that setting the air knob to "cold" will give cold air to the car, and also to the screen if the demist knob is on "demist" or "defrost". The "defrost" setting activates an additional flap which largely cuts off the air flow to the feet. Use of this setting reduces the overall effectiveness of the heater in cold weather. The defrost option should only be used until the screen is clear of ice, and this will be most effective if the air knob is also set to "off". Once the ice has cleared, the demist knob should be returned towards "demist" and the air knob set to "hot". Ice will not normally reform, and having the fuller flow of air results in a generally warmer car which is more effective for reducing ice on all the windows, not to mention one's feet! Effective heating in very cold weather usually means using the fan, because very little air is drawn through the heater unless the quarter-light window is open because of the weakness of the design of the air intake. With the fan off it can actually be warmer with the quarter-light slightly open as this draws air out of the car and hence warm air into the car through the heater.

The Heater Box

Removal of the box from the car
Drain the radiator and remove the two heqater hose clips and hoses. Loosen the clip which joins the heater box to the blower unit. Undo the bowden cables from the box. Undo the four set screws which hold the heater box to the scuttle. Draw the box forward and upwards to gain access to the hoses which take the air to the screen. Undo the clips, if necessary and pull off the hoses.

Dismantling
Comprehensive dismantling is not advised unless essential, as a number of the internal components are pop-riveted in place, and such a procedure is not normally necessary because most problems can be solved without total disassembly.

Remove the four clips which hold the front and back panels together. The rear panel can then be removed, but the water valve prevents the front panel being removed. The side panels (a single unit) can be loosened without completely removing the front panel by easing the front panel away from the sides except for the region near the input valve. Avoid removing the front panel unless absolutely necessary, because there is a major risk of damaging the water valve or its mounting plate.
To remove the front panel, undo the two screws which hold the water valve unit to the mounting plate which is proud of the front panel. Even with these screws undone, the valve will be difficult to remove because the pipe from the heater matrix is a push fit into the valve housing. Gentle twisting may help to break this seal. With the valve off, ease the front panel away from the side panel so that it clears the outflow pipe from the matrix (the top one). The front panel can then be twisted round the inflow pipe to ease the input valve attachment plate away from the heater matrix input pipe. It will help if the rubber ring on the input pipe between the valve and its attaching plate can be removed (careful not to damage it, as it forms the water seal). The front cover can now be drawn off with the valve mounting plate attached.

To remove the heater matrix, prise off the two clips which hold the heater matrix in place.

The heater is now dismantled as much as is normally necessary.

The heater matrix
The heater radiator fills with deposit in the same way as the main radiator and requires overhauling in a similar way. Minor sediment can be removed by back-flushing the heater matrix with a pressure hose. It is worth remembering that it is possible to get quite a good flow of water through a radiator which has ceased to be a satisfactory heat exchanger because the scale that lines the tubes is a poor conductor of heat. If this is the problem it may be possible to remove it by filling the dismantled matrix with acid, otherwise it will have to reconditioned by a specialist.

The flaps
It is necessary to understand how the heater works, so that any internal problems can be identified. The air comes in from the heater blower and some goes through the heater matrix and is heated, but some goes down the front of the heater to provide an unheated supply to mix with warm air to provide anything between cold and hot air to the car through the opening below the parcel shelf. Another supply of cold air travels down the rear of the heater for mixing with hot air to supply the screen with air of adjustable temperature. There are three sets of flaps:

The main set of flaps is controlled by the frontmost external control and this set adjusts the air that enters the car below the parcel shelf. There are two flaps at rightangles. They close off both the hot and the cold air when the operating lever is in its furthest forward position. As the lever is rotated clockwise (as looked at from the left side of the car), the flaps both turn, but the one covering the hot intake is shrouded so that only cold air is drawn at first. With further rotation, both flaps are uncovered, and the hot opening becomes relatively larger, so that at full opening the flap which originally closed the hot air hole now closes off the cold air to give maximum hot air.
The screen set of flaps are operated by the lever towards the rear of the heater box. Again it consists of two flaps at rightangles and it works in a similar way to the one just described. The flaps are closed when the lever is in its furthest forward position, but it opens as the lever is pulled anti- clockwise, as looked at from the left hand side of the car, giving increasingly hot air as its position progresses. However, at the end of the movement, the lever pulls a further flap into operation:
The defrost flap is operated by the demist control when it is set to the "defrost" position. This flap normally largely closes off the hot air to the demist side of the airflow to encourage the hot air to go to the car and largely cold air to the screen. When the defrost option is used, the flap flips over to largely close off the hot air flow to the car so that most of the hot air now flows to the screen.

Flap problems.
The main problem is deterioration of the foam seals on the flaps, which then allows air to flow when it should have been cut off. This foam should be renewed. Stiff felt, or thick cloth can be used as a substitute. The flaps may become stiff, and a little oil may be applied to overcome this. The defrost flap can become so stiff that it cannot be operated, so that the defrosting option is never available. Greasing the operating lever helps to reduce the friction, but to make sure that defrost can be used, and to improve the normal demisting by increasing the flow of hot air, it may also be necessary to prevent the flap from fully returning to its rest position by putting a small nut and bolt through the hole in the flap side. But care will be needed to make sure that the heater matrix still seats satisfactorily, as any gaps where the matrix meets the flaps unit will dilute the hot air with cold air.

Modifications
Richard Bird (Summer 1987 newsletter) suggested blocking off the cold air flows through the heater by using foam to force all the air to go through the heater matrix. This is probably desirable as a measure for dealing with a very ineffective heater, or for use in an extreme climate, but might make the car too hot if this was done on a car with a fully effective heater - I do know some cars where the hot air flow is unbearably hot!

Reassembly
The heater matrix is clipped back onto the flaps box (the front clip will be difficult if the front cover has not been removed), and the front, rear and side covers must be replaced with the side covers held in place by the front and rear covers, and the front and rear covers held together by the four clips. If the valve was removed from the heater it should be reassembled using a gasket compound to eliminate leaks.

The water valve.
The water valve can be disassembled by undoing the four bolts, after which the valve can be removed and cleaned - it tends to get filled with sludge, and this can stop the water flow. It is also prone to sticking and this not only affects the water flow but it also upsets the position of the air flaps. Adjustment of water valve: If not already done, slacken the clamping screw on the wire that links that water control valve and the main air control flap, so allowing the water valve to be moved by hand through its range of travel. It should go just tight at the point where the valve is fully closed. The tight spot is adjusted by the locknutted grubscrew, and it should be adjusted to allow the valve to open as far as possible, consistent with it fully closing.

Reinstalling in the car.
If the foam has failed on the out put to the car under the heater box, replace it with foam or felt. Place the heater box so that the screen air hoses can be reconnected, and reconnect them. Put the box into place with the rubber gasket and clip to the heater blower already in place. Secure the four set screws which hold the box to the car. The next stage is the adjustment of the bowden cables.

Adjustment of the bowden cables
This can be a frustrating job, made somewhat easier with an assistant who can adjust the levers in the car as the work progresses. As you go, check that all these cables operate smoothly and that they move correctly when the car levers are moved to and fro. Note that the cable clamps have to be very tight, and that the inner cables and levers may need lubricating. There is a danger of the inner cables buckling at the operating lever end under the parcel shelf, so it is important never to force a lever if it is stiff. (If the inner wires do become damaged it may be possible to shorten them slightly to remove the damaged portion from use - this can only be done once as the extra wire available is small - or to stiffen the wire with solder.)

Start with the cable for the demist and defrost: this is the shorter cable and connects to the rearmost flap lever. Push the wire through the cable clamp and clamp the outer cable. Push the inner cable through the hole in the wire clamp on the forked flap lever so that the inner cable is between the forks. With the "demist" lever set to "off" in the car, push the flap lever on the heater box as far forward as possible. Clamp the inner cable. Check that the special "defrost" flap operates when the "demist" lever is set to "defrost". (The heater blower may be used to help determine this.) The adjustment of the bowden cable for the main air flap is more difficult. Start by adjusting the interconnecting rod between the water valve and the flap lever on the heater box. This rod should be clamped in the water valve wire clamp and the lower of the wire clamps on the heater box lever so that when the water valve is fully open, the two parts of the air lever on the heater box are aligned with each other (Any adjustment of the water valve should be done before adjusting the bowden cables). The pull off spring connects the bottom of the outer of the two air flap levers to an eye attached to the lower and frontmost of the four bolts that hold the water valve together. Clamp the outer cable of the bowden cable in its clamp. Set the "Air" lever in the car to "off". Push the flap lever on the heater box to the forwardmost position, and clamp the inner cable in the wire clamp. Check that when the "Air" lever in the car is set to "hot", the lever is pulled through to the rear to its full extent, but the water valve has not moved, and that the water valve becomes "off" when the "Air" lever is set to "cold".

Water flow.
The water flows from the rear of the engine to the bottom of the heater and returns from the top of the heater to the bottom radiator hose via the pipe which runs along the engine above the manifolds. If the upper heater hose does not get warm fairly quickly after the engine is started from cold there is some obstruction. The most common cause is an airlock due to air trapped in the upper part of the heater. When the cooling system is refilled this airlock should be vented by taking off the radiator cap, loosening the upper heater hose clip and starting the engine (preferably when it is below normal running temperature and the thermostat is shut. If the thermostat is open or missing there can be difficulty in establishing the water flow) and easing off the heater hose. The throttle is then opened gently until the water starts to flow out of the heater. The hose is then quickly replaced while the water is still flowing. If no water flows, this may be due to 1) a closed, badly adjusted or obstructed water valve, 2) severely choked heater radiator, 3) collapsed or obstructed heater hoses, 4) missing thermostat, 5) ineffective water pump (heavy scaling, slipping fan belt) or 6) severe sludging of the water channels in the cylinder head or block.

If the thermostat is missing, the water does not circulate through the engine properly, especially at the rear of the engine, and this prevents a proper circulation through the heater.

Sludge in the engine is indicated if the engine block water tap does not yield water even if the whole tap is removed, or if the water exit on the top of the cylinder head seems choked. the only real remedy is to dismantle the engine and mechanically clear the water passages. This problem is likely to be associated with localised engine overheating and engine roughness when hot.

Water temperature
If antifreeze is not used in cold weather the water in the heater can freeze and render the heater ineffective by obstructing the water flow. The ice may also damage it. A30 engines were fitted with a very cool thermostat (72 degrees C). It is worth fitting a warmer thermostat (85 degrees C) as this improves the heater efficiency, reduces contamination of the oil and improves fuel consumption. For a really warm winter fit an 88 degrees C or 92 degrees C thermostat, but replace it by a cooler one in summer. It is also a good idea to have the radiator grill partly blanked off in winter as this reduces the heat losses from the outside of the engine and so increases the total amount of heat available to heat the car. The easiest way, in the absence of the proper muff, is to tie some canvas over the radiator grill.

The Motor and Fan

The motor only operates in one direction irrespective of the polarity of the input wires. It should turn in the anti-clockwise direction when looking down on the output drive spindle, ie from the front of the car when correctly fitted. The fan itself is the centrifugal type and when looking from the front of the car the blades should be as shown in the figure.

A motor and fan (possibly a replacement of the wrong type) were found where the fan motor operated in the opposite direction, with the blades of the fan inclined in the opposite direction. This caused the fan to operate in reverse, so that the heater did not function. The easiest solution was to obtain a motor and fan of the correct type, but failing that the fan blades can be bent to operate in the normal direction, and the direction of the motor changed by dismantling the motor and connecting the brushes and field coils in the opposite direction relative to each other. [The exterior wires are joined to wires that go to the field coils (round the outside of the motor). The other wire from each field coil goes to a carbon brush mount. The other side of the mount carries the brush. As the wires on the field coils are not very long the only practical way to reverse the motor direction is to unsolder the wires to the field coils and resolder so that the wire that presently joins the input wire becomes the one on the brush carrier, and the one on the brush carrier becomes the one that joins the input wire. This exchange must be made on both field coils.]

Maintenance of Motor
No maintenance is normally required and the motor should run quietly and smoothly, but if it is noisy, a little oil on the spindle may help. Failure of the motor is normally due to worn out brushes, and these can only be replaced by dismantling the motor. As with many electric motors, disassembly and reassembly are both relatively difficult as the case is very tight and there are a lot of awkward pieces which come loose on dismantling.

Removal of Blower from Car
Disconnect the large heater air hose and loosen the clip that holds the gasket to the heater box. Undo the bullet connector and the earthing screw, and then remove the nut and bolt connectors between the fan housing and the mounting bracket, or alternatively the nuts under the parcel shelf that hold on the mounting bracket. Installation is a straightforward reversal of removal.

Air flow checks
Air flow can be checked by switching on the heater fan. If the flow is poor, check for obstructions in the air pipes - leaves and other debris easily get trapped in the pipes, or for disconnection of the screen air pipes. Sometimes the carpet obstructs the outflow of air below the parcel shelf. Check that the airflow corresponds with the settings of the controls (see Use of heater, above).

Whilst great care is taken to ensure that the technical information and advice offered is of the highest standard, neither the author, nor The Austin A30/A35 Owners Club, or the Officers of the Austin A30/A35 Club accept any liability at law for any death, injury or loss whatsoever arising from the use of advice contained in this article.