The Stovebolt Site Forums The Shop Area Interiors fresh air heater question

I have my Harrison fresh air heater all apart for some fixing up. Mine has a large door that blocks off the fresh air from going thru the heater core. Most of the similar heaters that I've seen in videos and in pictures do not have this door. It seems to me that it served a pretty important function. Did some of them come without this door, or do lots of people remove them. If people do remove them, why do they do it?

I believe the door you are referring to is actually a diverter that when engaged diverts air to the vents to defrost the windshield.

No, I know the diverter that you're referring to. I havr that too. The door that I'm referring to is roughly the same size as the core. It comepletely blocks off the fresh air coming in from the louvers in the outer cowl. It is operated by a link to the smaller door that is activated by the cable. When the cable closes the small door, the link opens the large door, and vice versa.

There are two styles of Air-flow heaters the early style 1947-52 and the 53-55 1st Series with the dump door.
Early.
https://www.flickr.com/photos/advance-design_parts_co/8129439492/in/album-72157631868452754/
Late.
https://www.flickr.com/photos/advance-design_parts_co/50865248532/in/album-72157631868452754/
Original hang-tag for the early heater explaining operation.
https://www.flickr.com/photos/advance-design_parts_co/29739577756/in/album-72157631872034068/
https://www.flickr.com/photos/advance-design_parts_co/29775287635/in/album-72157631872034068/
This is the Jim Carter explanation for the later heater:

With the introduction of the Advance Design Cab in 1947, a new dealer installed heater also became available. When used together this new body was referred to as The Cab That Breathes. (Here is the slideshow for the Cab that Breathes).
https://www.flickr.com/photos/advance-design_parts_co/albums/72157649468726416
All U.S. manufactured Advance Design Cabs had a row of louvers on the right outer cowl which allowed air to enter the cab interior with the help of the new optional fresh air heater. It was a great improvement over prior years!
As air was brought in from the outside it passed through the heater core. This warmed air usually held less humidity than air inside the cab due to the breath of passengers. The fresh, dryer, air helped prevent the inside of the windows from fogging during very cold days, thus GM said the cab breathed.
This excellent heating system was made even better in 1953 with the introduction of the revised airflow heater. By pulling a knob, at the left of the steering column, a door inside the heater would cover the outside air intake. Now the heater was of the recirculator design. It pulled air from inside the cab so that inside air was re-warmed. Yes, humidity did accumulate quicker and windows could fog but heating the air inside was faster on a cold morning.
The driver operated cable was pulled and a door inside the heater shut off outside air. At the same time, a one inch wide door on the case opened to allow recirculation of air to occur. A non-related lever on the case directs the heated air to the floor or above through the defroster ducts.
This ingenious idea by GM engineers now allowed the driver to have a cab that breathed or one that did not.

The heater you have with the door which blocks the hole in the cowl while at the same time opens the narrow door to the cab actually converts your fresh air heater into a recirculating heater. That narrow door inside the cab becomes an inlet for cabin air to enter the heater core while the cowl hole is closed off.