72-79 Bus Heating System

by Richard Atwell
(c) Copyright 2004-2011

badge

It's true: busses have heat just like beetles but you'd probably never know it unless you'd driven the bus away from the dealer's lot when it was new or after it had been 100% restored. When I had my 71 super beetle and finally unclogged the heating system there was actually too much heat! Before that, I had no idea VW heating systems actually worked.

Buses are harder to heat than beetles because of all the cabin space. There are several points of failure in the heating system. The parts that fail are easy to deal with and range from 50 cents for a clamp to $200 for a heat exchanger. If you don't have a gas heater you can still achieve good heat but the system will have to be sealed like it left the factory because of the complicated nature of the design.

I've colored the failure points in the diagram below in order to go over them one by one. The BA6 gas heater is downstream on the left on 74-79 models. You may just have a long continuous tube running down the length of the center of the bus if you don't have a gas heater. The early bus gas heater, BN4 was mounted in the engine compartment.

This article covers the 72-79 design but the principle is the same for the earlier models. There is plenty of documentation on the beetle specific heaters which apply equally to the 68-71 buses.


System Operation:

Air-cooled volkswagens have no radiator. No radiator means no overheating in the summer (thank heavens), no radiator leaks, no fluid purchases to make, no hoses to fail, no thermostat to replace, no water pump to break, etc. In modern cars it's also the primary way that cabin heat is produced. Heat from the radiator is diverted into a unit embedded in the dash and air from an entry near the doors is blown over the heater core through which the hot engine coolant passes. The mix of hot air to cold air is controlled by a lever on the dash for your preference.

When VWs were invented in the 30s in eastern Europe, the design goals of the day emphasized practicality even before Porsche created the beetle: rear engine and rear wheel drive for unpaved muddy roads and an aircooled engine design for the cold weather. Because of these necessities, the importance of heating was not factored into the design as an overarching goal. Not only that but heaters were considered luxury items back then!

To get around the lack of a radiator for heat, the designers used heat exchangers or heater boxes as they are more commonly called and they are part of the exhaust system. Heat from the exhaust tubes running through the heat exchangers warms the air entering the heat exchanger and the exhaust and the air entering the heat exchangers are kept separate from each other. This air is blown up to the passenger compartment by fan, either the impeller fan of the engine or a booster fan atop the engine compartment to increase airflow at idle.

Because of all of the separate components there is a lot of opportunity for heat to escape under the force of the fans especially after 25 years of age have set in.


Finding the Leaks:

heating diagram

1. Missing parts (green):

If you've just bought your bus, the PO may have removed a lot of the heating system especially if the bus was located in a warm climate or a draconian Type 1 engine swap was performed. Parts that were left in place like the accordion tubes may be blocked up to keep cold air from getting in; others may be left open.

As can be seen in the diagram above, there is a heater booster fan atop the engine compartment and two hoses that lead down to the engine tin (which may also be missing along with the engine seal). The tin is essential for long engine life so replace them if they are missing. Some models have one year only tin so make sure you know the part number and/or year of the bus your are sourcing it from.

At the engine tin are two rubber seals that keep the hoses from rubbing against the tin and becoming cut. These often rot with age and eventually go missing thanks to gravity. They can be hard to locate used in good condition but a few vendors still carry them new.

Both hoses are secured with hose clamps at each end. The clamps are different sizes because the left hose is a different diameter than the right hose.

From the tin, there is a metal heat riser tube on each side that connects the hoses from the engine compartment to the heat exchangers. Quite often, the PO has removed the booster fan and the paper tubing and simply left the heat exchanger risers open to the engine compartment. If you do this your oil and cylinder head temperatures will increase by 30-35F because the hot air from the heat exchangers will be ingested by the impeller fan.

mufflers

The heat exchangers come in several configurations depending on the year but in general there are two types: those with trapezoidal flanges (72-74 and 79 models) and those with triangular flanges (75-78). The flanges are not compatible and make it impossible to mix and match exhaust components (and there are California model only pieces to look out for also).

So far this is the path that the air takes from the booster heater. The impeller fan is far more powerful especially at above idle RPMs and the fan shroud diverts some of the air from engine cooling to the heater boxes.

Hot air exits the heat exchangers and travels into two short tubes connecting the heat exchangers to the heat control boxes sometimes called flapper boxes. This is where the controls on the dash decide whether the heat continues on through the accordion tubes to the front of the bus or exit to the outside. You would think that VWs needing all of the air they could possibly get for cooling wouldn't be wasting any of it and the VW engineers would have put extra controls in place but it would appear it complicated the system too much and VW was able to generate enough airflow for cooling purposes.

There is a small design change of interest here. On the older models, the heat exited the flapper boxes and was directed up towards the body. This had the effect is heating the engine unnecessarily so the boxes were flipped around and the bell was put on top in order to vent the heat downward. On some 78/79 models the flapper boxes have "exhaust" tubes that sneak in between the heat exchangers and engine and exit to the rear. It's always very confusing at first glance because they appear to be extra exhaust pipes.

The system sounds simple but when you count up the seals, every joint and seal is probably going to give you trouble at some future time. Because of the force of the fan and the split nature of the design you have to eliminate all of the leaks if you want sufficient heat to make it the length of the bus up to the cab.

2. Control Flaps (blue):

fan shroud flaps

Because there are two sources of airflow there are 4 flaps in the system to direct air. Two are located in the booster fan and keep air from the impeller fan, which has now been warmed by the heat exchangers from entering the engine compartment. When you turn on the fan while driving the flaps open a little because the booster fan has a powerful motor. Chances are you still have these flaps because they are hard to notice (the PO can't toss what he can't see!). If your flaps are missing then you'll have engine overheating issues just as if you had removed the hoses and left the heat riser tubes open.

The other two flaps are in the base of the fan shroud and prevent air from the booster fan from re-entering the engine compartment. Often if there has been exhaust modifications the flaps have been lost or left out. Sometimes they have unknowingly fallen off while driving. They are held in place by a simple pin so if you ever have someone work on your engine be around when they are about to put the fan shroud back on. You will notice (or never have known that you lack) a serious lack of heat if you are missing these flaps.

3. Controls (orange):

Sometimes the heat system is intact but heat fails to reach the cabin because the heater control boxes aren't working properly. Either the cables have too much slack in them in which case they have to be adjusted or the linkage between the cables and the boxes has been disconnected. The control arms of the flapper boxes are simple spring loaded levers and the cables are held in place with a barrel nut and screw. Because they are under the bus they are often very rusted.

It's also possible for the flapper box to be rusted or have a broken spring inside and needs to be replaced.

4. Rust (red):

Rust is the worst kind of problem because it's the costliest. The heat exchangers have a heavy finned block inside to act as a radiator. This helps to keep the heat level constant although the heat flow varies with engine speed. It also helps to maintain the heat when you shut off the engine for brief periods of time. The rest of the heater exchanger is a shell around this core and rusts away. The metal is the thinnest where the enveloping metal of the heat changers meets the front and back flanges and the rust eats it away the way creating a direct path for the airflow to exit to the outside before reaching the flapper boxes. Replacement is the only effective cure. Make sure your new boxes get repainted every couple of years with a quality paint to make them last longer than the originals.

5. Other leaks (yellow):

When you disassemble the system for the first time you'll notice that many of the pieces simply come off even though they are clamped. Sometimes there was a thin gasket or a rubber sealing compound in that location. The gaskets tend to become brittle when they get old.

vanagon seal

Because, they didn't know better at the time, VW sealed the flapper boxes to accordion tubes with asbestos cuffs. Asbestos is fibrous and flaky and when you mishandle it, the material crumbles. I would recommend replacing the asbestos with silicone cuffs from an aircooled Vanagon because they seal better and are better for your health (thanks to Karl von Salzen for this part swap idea). Unfortunately, the Vanagon only has one straight cuff, so you'll have to raid two of the 251-255-393D versions in order to seal both sides of your bus heating system.

Apart from the places I've marked in yellow the worst offenders are the brackets and gaskets that connect and seal the fan shroud to the heat exchangers. This is the point in the system where the air flow is greatest and has the most opportunity for loss with an otherwise working system. The brackets are held on by phillips screws from the underside. Quite often vibration causes the screws to back out and the brackets fall off venting the air to the outside. When installing new brackets use Loctite on the screws but only if there will be no chance of stripping the head later (don't use phillips screws and use stainless steel M6x12mm fasteners).

The gaskets that seal that area were very expensive when I last checked (about $15-20 each) but have since become NLA. It's such a simple piece you can cut some from a sheet of fibre board 1/8" thick to replace what's missing.


Gasket Photos:

The original gaskets for the fan shroud are no longer available from the dealer. You can cut your own from the appropriate piece of gasket material for much less than what the originals cost by following these measurements: gaskets
L specs R specs
L profile R profile

The Smell:

If you or your passenger is taking their first ride in a volkswagen and the heat is turned on and the smell is overpowering welcome to the club! More buses have this problem that not so it's worth addressing. Smells can be grouped into two categories: oil or exhaust smells and burning smells.

Oil and exhaust are obvious. You could be missing the engine hatch seal, the tailpipe of the modified exhaust could be exiting directly back instead of to the side or oil has dripped on the heater boxes and the smell is coming through a rust hole.

That burning smell is a little more elusive. The accordion tubes in the bus have a paper lining on the inside. Over time the dust and heat create a burning smell that's irritating to breathe. The only solution is to clean it out or replace it.


Gas Heaters:

Coming soon after I R&R my own BA6. Check out those specs (6,000 kcal/h = 24,000 BTUs!).

See my Tachograph article for the BA6 Heater Timer Clock and wiring and operating instructions.

When replacing fuel lines, do not bypass the take-off that diverts fuel to the BA6. There is a tee between the fuel filter and FI pump and it's important to keep the diaphragm in the BA6 fuel pump wet so it doesn't try out.

ba6 plate

In the meantime here are the part numbers for the hard to find glow plugs.


References:

History:

02/18/04 - Created
09/24/04 - Added gasket dimensions
11/24/04 - Added glow plug part numbers
09/06/11 - Fixed broken photos, added translate button, updated footer