© 1998-2008 Joe Weaver





Rebuilding the

Heater Box

Reassembling Heater Box

Part 3


Install the heater core in the rear half of the box with the tubes sticking out the back. Then join the halves back together again. Be sure to help lift the top of the front half as it slides over the foam seals on the temp valve assembly. Although difficult to see in the photo, there is an indentation in both the front and rear halves for the temp mix valve control rod to ride in (arrows in left photo). The right photo shows a close-up that helps illustrate how the temp mix valve functions. When the door is to the right, as it is in the right photo, the cold air is forced to the left and through the heater core which gives full hot air into the car. If the door is to the left, the cold air is forced to the right and bypasses the heater core which gives full cold air into the car. If the door is half way between left and right, half the cold air goes left through the heater core, and half the cold air goes right. The hot air that went left, meets with the cold air that went right and mixes before and during its flow through the fan resulting in warm air from the heater. The final temperature of the exhaust air from the heater is determined by the position of this door as it is controlled by the Temp lever on the dashboard.


The left photo shows the firewall seal (with adhesive sprayed on) which seals the hole for the fan as it protrudes into the engine compartment. The right photo shows the seal installed. I kept the old seal in place also just for some added thickness to the seal.


This is the seal that goes between the fan motor and the fan housing. If you remove the fan and motor from the fan housing, it will be easy to see where this seal goes. I replaced the seal on my '64 heater box, but chose to not separate the motor from the housing on this box.


The left photo shows the box assembled. The right photo shows new heater hoses installed. The defrost/floor mix valve attaches to the fan housing as shown by the arrow in the right photo. I did not install the defrost/floor mix valve in the heater box for this car because the under dash air conditioning unit would not allow the additional length of the box to make the turn with the item installed. I needed to attach the defrost/floor mix valve after both were under the dash.


This is what the assembly looks like with the defrost/floor air valve attached to the heater box. It attaches with four small sheet metal screws that go into the outlet of the fan housing. If you do not have an under dash air conditioning unit, installing the defrost valve on the box now will make installation in the car easier.

The box is now ready to install in the car.


The old defrost/floor mix plenum, which was originally made of cardboard, is on top in the left photo--note the deteriorated condition of the end that attaches to the heater box. The new and improved version is made of plastic and is shown on the bottom. The floor output holes are different on the new model. The hole on the left side for the drivers feet in the new version exits out the front rather than the end as it did in the old version. This tends to blow the air more on your right leg than your feet. I cut a 1" hole in the end so the warm air would keep my feet warm (as seen in the right photo).


The original defrost outlets were also made of cardboard. The left photo shows two available replacement outlets. The one on the left was in my '65 when I bought the car. You'll notice it is much smaller than the new one on the right. It is small enough that it fit loosely in the dash and rattled as I drove the car. The one on the right (left photo) I had on hand for my '64 Sprint restoration, so I tried it and it fit perfectly (I ordered another set for the Sprint). Not only do these fit better, but the hole is larger and allows for better airflow. Note, however, that it is designed to fit facing to the front rather than to the side as it seems it should, as seen in the right photo. These larger ones I got from Melvin's Classic Ford Parts.


As mentioned above, the stock factory air conditioning unit was in the way and prevented the defrost/floor mix valve from being attached to the box prior to installing it in the car. The left photo shows the valve in place with the control cable installed. The right photo is a close-up. It should be noted that the air conditioning unit did not clear the defrost cable attach point on this piece (red arrow in right photo). I ended up bending the metal part so it would not hit the A/C unit, but still work properly.


The flexible air plenum duct is shown in the left photo. The side with the foam seal goes up against the bottom of the cowl, and the other side goes over the hole in the top of the heater box. This plenum is held in place against the cowl with clips, shown in the right photo. The clips attach to the plenum where my finger is in the left photo. The plenum is then pushed up to the cowl with the clips inside the hole as outlined below. It must be noted that in the absence of these clips, some have drilled holes in the cowl and screwed the plenum to the cowl. The only real problem with this is that the cowl is both an air duct and a water duct. As rain water or road water that is splashed up enters the cowl area (the slotted holes between the hood and the windshield) it runs to the sides and exits just behind the rear part of the front fenders. If holes are drilled into the bottom of this water duct, they will eventually rust out and water will then drip or run into the interior of the car from under the dash. It is possible to protect the drilled holes, but it is probably far more work than just locating the clips from a junk yard or Falcon parts vendor.


The left photo shows a clip installed in the plenum piece where the indentation is. The other clip is installed on the opposite side as shown in the right photo.


The left photo is looking up at the bottom of the cowl under the dash where the plenum attaches. The clips attach at the protrusions where my finger is pointing. The right photo shows the plenum installed. Note the clip holding the plenum to the cowl.

Installing the Heater Box and Making Adjustments

Everything is now ready for the heater box to be installed in the car. From under the dash, slide the heater hoses into the holes in the firewall as you lift the heater box up into place. If you did not install the defrost/floor mix plenum onto the heater box previously, do it now. Take care as you slide the fan motor through the hole in the firewall that the wires to the motor are not pinched in the process. Maneuver the heater box until the four bolts surrounding the fan motor slide into their respective holes. Install the nuts on these bolts in the engine compartment and reattach the fan motor wires and ground wire. Attach the heater hoses to the intake manifold, and water pump and refill the cooling system with fresh clean anti-freeze/coolant. Hook up the control cables as outlined below.


The three control cables need to be attached and adjusted as the heater box is installed. This is a relatively easy procedure. First, move all the control levers to just short of the full up position (as seen in the photo above). Then put the particular valve you are adjusting in the corresponding position. Place the cable wire loop over the control rod and pull the cable slightly to make sure the valve is fully seated (make sure the control levers don't move), then tighten the cable clamp. Check the operation of all three valves for ease of movement and complete seal when closed. Re-adjust as necessary to get a complete seal, particularly of the heater on/off valve.

The Defrost/Defog System

Installing the Flexible Air Plenum Duct

This is the end of the Heater Rebuilding Edition


Feel free to save this page to your computer for your personal use and future reference--no other use is authorized without prior written permission from me. All illustrations from the 1964 or 1965 Falcon Shop Manuals used pursuant to permission granted by Ford Motor Company. Disclaimer: This site is not intended to instruct or teach anyone in proper or safe methods of working on or maintaining any type of vehicle or use of any tool and the author takes no responsibility for the use of the information contained herein.

If you have comments or suggestions, email me at joe@joesfalcon.com