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If you have a smaller engine (<1915 cc), you can run the stock diameter OEM heater boxes - a bilge blower assist helps flow.  A larger engine will take larger diameter heat exchangers which are costly.  Another alternative is to add a reconditioned or new gas heater - it provides heat that you'll have to turn off.  They are more costly.  In Ohio I'd think heater boxes would extend driving time and be more comfortable than NO heater.

Eberspächer was one of the original suppliers of gas heater for VW and Porsche.  They made them for bugs (BN2), Bus/Campers (BN4) and the 412 (BA4?).  They are available used but parts are getting hard to get.  Maybe $400.  New ones are also available for around $1-1.2k.

Image result for eberspacher VW gas heater bn2

Last edited by WOLFGANG

Depends on where you live and how much you would like to drive in cooler weather. On CMC's they routed the heater box flow through the L & R side 2 x4 box frame, issue with that is the heat passes through a lot of cold steel surface and by the time it exits by your feet of " defrost" vents it's just warm at best.  I have done heat routing through the vertical door jambs (roasts your elbows) Also through the vertical panel below the rear seat worked very well but no defrost (like it matters) and even did a 12v high amp forklift cab electric heater using a higher amp alternator and a deep cell golf cart Aux. battery (this worked but wasn't cheap to do and I have no longterm info as I sold that project.

The gas fired jobbies are a challenge to install, but it has been done to good effect.  Other Folks here can tell you all about how that goes.  And they will air fry your tootsies, or other parts too close to the source.  I have heater boxes on my 2332/Sidewinder lash up, and they work -- not great, but they work.  Warm to slightly hot air comes out.  Takes the chill off.  Trouble is, side curtains do not allow much of that warm air to stay around.  If you really drive in cold weather and want to be warm, then do three other things: dress appropriately, like you were going to go skiing; carry a blanket; and install heated seats.  I have heated seats and several really warm jackets, and gloves.

PS, about the standard way the heated air is routed to the front of the car, which is as mentioned, via steel tubes along the outside of the car.  Lots of time for some serious heat transfer loss.  I have taken rigid Styrofoam and made a series of slabs that cover that tube with a nice large 1/4 round radius on the outer surface.  Sprayed with undercoating, this insulates those pipes to a fair degree, upping the discharge temp at your feet and defroster.  hahaha, defroster.  Great idea, leaving a lot to be desired. .  I have the ducting at the base of the windshield (not all do) but the advice about Rain-X defogger is right on point. 

In my previous life I had good experience with heater boxes on air cooled VW beetles but that was after making sure that everything was airtight.  On my speedster I have heated seats and the bigger (1.5”) heater boxes on my 2017cc engine.   So far just using the engine blower, though you can add an electric fan from a Porsche 914 or a later VW Bus for more air flow at idle.  After listening to the experience on this site, I will likely skip the flow through the rocker panels and just have the heat dump into the passenger compartment behind the seats to minimize heat loss.  The rain-x product for the windshield sounds like a good solution.  

I ordered my VMC speedster with heated seats and heater boxes.  Heated seats are wonderful and really help when its cold, however only a relative small portion of your body is in contact with the seats, so you still need to dress appropriately.  The heater boxes work, but don't put out much until things heat up in the back, and even then I would not describe the output at your feet as hot.  Warm yes, not hot.  And if the top is down, the warm air blowing at your lower legs does help a little (and when I've driven in temps around 50F I was glad to have even a little extra warmth), but it does quickly exit the cabin.  I've not tried the heaters with the top up, because why drive with the top up.

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OK, no one's mentioned this twist yet, so here goes.

The old Vintage Speedsters came up with a slightly better mousetrap for heat ducting. They routed the heat from the heater boxes into flexible 3" plastic hose that ran under the car, but just inside the rocker panels, and connected that to the usual vents in the front footwells. (They also connected small ducts up to the defroster vents, but you could probably skip that.)

This provides pretty good, uh, 'heat'. If you go to the trouble of putting the top up and side curtains in, it almost feels like a modern car inside — after a while.

With the top down, though, not so much.

Only you can decide if a little heat with the top down is worth bothering with. In the 50's and 40's, it might mean the difference between going out or not. Or, it might mean the difference between driving with a passenger or alone.

Here in sunny California, with (in most years) mild winters, it has greatly extended the usability of our car, and I wouldn't be without it. Ironically, we may use it most in summer. Then, to beat the mid-day heat, we often start a long trip in the wee hours, when it can be chillier than you'd guess for the first hour or so.

What will call for larger than standard heater boxes is the diameter of the exhaust headers, and most engines larger than stock will require that. Decent quality one-and-a-half inch boxes are not too exorbitant. Larger than that, and look out!

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@WOLFGANG posted:

@pkdfw A convertible D vs Speedster - with taller top, windshield, windup (power) windows pretty much solves cold/water leakage issues.  Many (IM) have multi-layer Haartz fabric tops too.  You're adding weight and costs. Side curtains are difficult to seal.

Thanks @WOLFGANG

I saw the Cruzing windows, but does anyone know who can make taller windows that can help seal up farther to the convertible top?

Many of us did that ourselves as each car is a little different.  Heck, My first car was a little different from one side to the other.  Making a standard set that'll fit every car is not possible.  That's probably one reason @aircooled made sure to market his as Cruzin' windows.

Doing it yourself isn't that had and plenty of us can provide guidance, aka blatant opinions.

The main problem with air cooled cars and heat production, is that the amount of heated air coming into the cabin is solely dependent upon the engine speed.  The higher the revs, the more the engine pushes heat forward to keep you somewhat warm.

So, it should be true that smaller engines that need to rev higher to get, and keep, up to speed, should produce more cabin heat.  At least, that would be the theory.  But we all know it doesn't seem to work that way.

In my case, it is the opposite.  Higher displacement engines usually run a little more slowly, especially at cruising speeds; therefore, less 'hot' air is sent into the cabin.

And that's why I wish I would have had a fan driven heater system built into my car.

For some reason, I prefer being warm these days.

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