Poor Heater Performance

Details: Written by Malcolm Eades

When the Magnette was designed and manufactured, car heaters were a relatively new accessory and many lower cost cars only offered them as an optional extra. In the Magnette it was standard kit and gave the car a touch of luxury. Now many Magnette owners complain about inadequate performance of the heater and especially its lack of demisting capability. This is partly because we have become used to the superior heaters provided in modern cars but it also reflects that: a) the system was never a great piece of design and b) performance deteriorates over time. Fortunately, a number of owners have refused to accept underperformance and have done some development work. What follows is based in part on a bulletin board thread to which the contributors were: Andy Dear, David Harrison, Trevor Jones, Geoff Pollard, Paddy Reardon and Dan Smithers, and I am grateful to them for sharing their experiences with fellow Magnetteers. The excellent pictures were taken by Andy Dear for use on the Register website.

To work effectively, the system needs to be feeding the matrix with a good supply of hot water from the engine and transferring the heat into the cabin by blowing a fast-moving air supply across it. The poor performance is attributable to a number of design or wear issues that inhibit this:

Weak fan motor
Poor fan design
Clogged or furred-up matrix
Poor water flow arising from blocked pipework and ineffective valves/controls
Poor air flow because of leakage and obstruction

The relative complexity of the control system is also a possible source of problems. There is a water valve, air flaps and a fan, all controlled from the dashboard, so linkages can get stiff or maladjusted. The picture below shows the heater box assembly removed from the car with its controls still attached. From this it is not difficult to understand why the controls can be a source of problems. There are levers, pivots and springs, all of which have to be working in unison if the right relative positions of valves and flaps are to be maintained. The flaps are not visible in the picture, but each of the levers is attached to the end of the axial rods that move the flaps inside the box. The round component is the water valve controlling flow from the engine block outlet. It is difficult to understand why the designer preferred this arrangement to the more common one with the valve next to the engine.

MAINTENANCE

Working on the heater system in situ is not easy, with many of the main components tucked well away below or behind the dashboard. If your car is being rebuilt, it is wise to do any modifications and maintenance before reassembly. If you want to overhaul your system at other times, then removal from the car and complete re-calibration is the best solution. With the components removed and stripped down it is possible to carry out the following maintenance operations:

Clean all leaf debris etc from airways and clean the matrix vanes.
Lubricate the air intake flap hinges and control linkage.
Flush the matrix through in both directions and use a kettle de-scaling solution if it appears to be affected by limescale.
Check the function of the water control valve, which is mounted on the heater box. (See below for more details).
Lubricate and adjust the cable controls and air flaps for ease of use and good seal.
Check inside the fan casing for leaf debris etc.
Check that the hose take-off connection and the waterways on the cylinder head are clear of sludge and limescale.
Renew old hoses because even if they look OK externally, they may have collapsed internally.

These operations will at least rectify the problems that have arisen over time and been caused by many years of use. However, if you want to go a stage further and overcome design problems, some modifications will be needed.

ADJUSTING THE CONTROLS

To understand how the controls are supposed to operate, it is a good idea to consult the relevant section of the Operation Manual (=drivers handbook) and Section S of the workshop manual. You will need to read these several times to understand them because they are not a great example of written English. (For example: ”For maximum heat at the windshield, place the temperature control lever in the OFF position...”) The controls themselves could also have been better labelled for clarity. It would all have been a lot simpler if the designer had not used one of the sliders to control both air flow and water temperature.

The slide controls need to be easy to use without diverting the driver’s attention from the road ahead and they should be able to travel the full range of their panel slots.

Before the heater box is re-installed under the dash, you need to set the water valve control rod, so that the position of the valve is correct in relation to the air flap controlled by the same cable. You may find that the travel available on the valve is more than the rod travel can cope with, so then you need to choose between having the valve fully open at one end or fully closed at the other but you may not be able to achieve both. I prefer a valve that opens fully, because a trickle of hot water through the system in warm weather is unlikely to be a problem, whereas in cold weather you need every bit of heat that you can get.

Once the box is re-installed under the dash, you can adjust the cables so that the position of the flaps is correct in relation to the position of the knobs. If you operate the slide knobs and observe the movement of the control cables at the other end, the push/pull movements will become obvious and you can set the flap controls in the appropriate corresponding position before tightening the cable clamps.

When everything is set, you will want to test the system. If you close the air intake flap, you can test the air distribution and heating function while stationary, with the engine running to heat the water. This is the intake setting in which the system heats re-circulated air from the cabin rather than fresh air from outside. It is generally used when you want to get the cabin warm as quickly as possible. Once you are satisfied that all is well in that setting, you can test the system on a run, so that you are getting the full benefit of fresh air flow. For safety’s sake, it is probably best to have a passenger to move the controls on a test run. It is the middle position of the air intake control lever that directs air through the heater. If you push it fully forward it just lets air straight through the bulkhead into the cabin, by-passing the heater box. In the middle position, you can check what airflow is being achieved and how effectively it is being heated in the various panel positions.

MODIFICATIONS

Experience suggests that the original design specification was not good enough to deliver the kind of performance that we might expect from a car like ours. Air-flow design was poor and much of the motor effort is lost in overcoming the inherent resistance of the ducting and airways. Look at the big U-bend that the air has to negotiate after the fan before it even reaches the heater box. The fan rotor is inefficient and does not generate enough air pressure.

Upgrade possibilities include:

Change the fan & motor to increase output. There are various options. The following have been contributed to the BBS:
1. Use a specialist company such as Coventry Classics (02476 691916) to give the whole system an upgrade. Cost based on time + parts.
2. Cannibalise a more modern system for components that deliver better performance. Geoff Pollard suggests using the motor and fan from a VW Polo Mk 4, which will fit if you modify the alloy “snail” housing on the bulkhead. Trevor Jones suggests the unit from a Metro, which has 3 speeds. (Make sure the fan is pushing the air in the right direction! Polarity may be an issue).
3. Increase the pipework diameter using 15mm copper tube to improve water flow.
4. Relocate the water valve to a more accessible position. (This will not address its inherent design problems but at least it will be easier to work on).
5. Adjust the operating rod to make the valve open wider, although this may mean it never fully closes in the “off” position.
6. Pack out the heater box to ensure all air passes through the matrix and cannot escape past the sides. Paddy Reardon used high density foam.
7. Fix some sort of sealing material around the air flaps to reduce air leakage.
8. Replace the intake flap grille. The original mesh is too fine to let enough air through. replace it with a more open material designed for air intakes (see Demon Tweaks or a similar supplier of performance components).

THE WATER CONTROL VALVE

This key part of the control mechanism is well hidden on the heater box under the dash, which means that repair and adjustment are difficult and leakage is out of sight until you notice a sodden carpet.. These valves are prone to internal corrosion, leading to blockage and difficult operation, so a strip-down and clean out are usually needed. They also often leak, a problem that you may not be able to rectify, meaning that a new valve is needed.

Two types of valve were fitted to our cars. The early version was apparently of all-alloy construction and was held together by four set-screws. To complicate matters, I believe it also had a different sequence of open/close positions. It was apparently also used on the Austin A35, which may provide more opportunities to obtain spares.

The later type (see picture - right) has a pressed steel top held in position by the folded clips that engage on the body lugs. The valves have a sloping slot in the side so that when one moves the actuating lever, the internal core is moved up or down in relation to the water inlet spout The latter needs to have a smooth surface if a good shut off is to be obtained. Clean off any corrosion here. (NB This picture shows the valve without the rod clamp fitted into the hole on the lever).

FIT AN ALTERNATIVE VALVE?

In practice, heating tends to be a seasonal issue and the control system offers (in theory at least) more temperature variation than is generally needed in practice. We all understand hot and cold and tend to operate the controls at these extreme positions. So you might consider a simpler system that by-passes the complexities of the valve/flap control and provides the simple option of turning the hot water flow on or off, according to the season. Open in winter, closed in summer. This involves fitting a simple valve that is operated manually, not by cable from the dashboard. This will leave the dashboard knob controlling only the air flap in the heater box. Earlier Morris 1000s had a simple tap like this on the cylinder head.

The MGA has a simpler cable-operated valve that could be used with a separate operating cable (See picture). As our engines are the same, this is a straight swap. You simply fit this valve instead of the water outlet elbow. At the heater box end, you need to modify the old valve connection to accept hose, or even a length of copper tube that could extend through the bulkhead for an even more accessible hose connection. This modification would mean fitting a new, separate hot/cold dash control and, again, it would mean that the original slide control only managed the air flow flap control. One of the push/pull knobs from the MGA could probably be modified to work.

Car Builder Solutions offer an impressive range of quality generic components aimed at modifiers and kit-car builders (www.nfauto.co.uk). This includes heater hose connections, valves, controls etc.

The Ford Fiesta has an electrically operated valve that has the advantage of providing some water flow away from the cylinder head even when it is in the closed position. This helps to avoid hot spots in the water jacket. However, if you want to retain some semblance of originality, this one would be a problem.

DEMIST

Poor demisting is a more pressing problem than lack of heat because we like to see where we are going and we hate to have to sit and wait while the system struggles to clear the screen! It is the function that suffers most from the poor fan because the air has a long way to travel through narrow ducts and resistance is high. It will also be adversely affected if the air flap is not closing off air flow to the lower vents.

To maximise air flow to the windscreen ducts, the top slide control should be set at “Defrost”. The “Demist” position distributes only part of the airflow upwards. The booster fan will be needed. If you find that your slide will not travel all the way to the right, or will not stay put at the extreme position, then adjustment of the control cable is needed.

The lower control adjusts water temperature but if you are demisting when you are setting off on your journey immediately after start-up, the engine will take a while to heat up, so you may as well leave the temperature control in the COLD position and keep the heat in the engine block until it is up to working temperature.

During maintenance, it is worth removing the trim panel to check that the twin demist ducts are in good condition and not blocked. It is not unknown for car park tickets etc to slip down the ventilation slots on top of the dash.