Anyone REALLY know the Heat Mixer?

[Echo]

So my '65 E has one issue that I need addressed at annual.  The issue is the cabin heat diffuser.  Even with heat off there is massive heat at co pilot and pilot under panel outlets.  With the cover closed STILL ridiculously hot.  I have looked and the firewall cable IS moving at the box with full travel.  What blocks on inside?  Is it the rectangular box or a flap inside?  I did not pull the scat off and peak in, but am at the point where I want the heat blocked 100% if the mixer won't work.  When fresh air is pulled with heat off it just blows piping hot air.  WTF.  Who KNOWS this system well and can give input on what is going on?  The engine was swapped not long before I purchased.  Is it possible that scat was not installed properly?

Bonus points for maintenance manual photos.

 

[N201MKTurbo]

Take it apart. It isn’t rocket science. There is a flap with a seal. The seal is probably dead. You need to make a new seal from silicon baffle seal material.

[Echo]

Didn't think it WAS rocket science.  OK, we shall see if this massive amount of heat is from "a seal failure".  I will say that is NOT what is causing this, but thanks for the input.

[Echo]

Is there a download of the maintenance manual somewhere on this site that I am missing?  That would be helpful.

[Echo]

Well that is in maintenance manual.  No diagrams.  No detail beyond this.  Zero.  Zilch.  Zip. Nada.  My "guess" is that the fresh air is compromised as there is no impact from opening.

Edited March 10, 2024 by Echo

[Echo]

Hey Mooney, it would be helpful if you showed a diagram of the junction box and the scat tubing providing fresh air and heated air...

Edited March 10, 2024 by Echo

[N201MKTurbo]

The diagrams are in the IPC.

[takair]

I had to replace the seal years ago because it always leaked. I don’t have pictures, pre-digital. Has held up well since then. 

[Utah20Gflyer]

9 hours ago, Echo said:

Didn't think it WAS rocket science.  OK, we shall see if this massive amount of heat is from "a seal failure".  I will say that is NOT what is causing this, but thanks for the input.

I’ve had this exact issue before and it was caused by a seal failure.  In my case the seal had come off and was allowing hot air to bypass the flapper.   I found the seal in the scat tube.  
 

There are two different heat boxes,  the flapper and the slider.  Both are simple,  you just need to open it up and see what’s going on. 

[00-Negative]

14 hours ago, Echo said:

So my '65 E has one issue that I need addressed at annual.  The issue is the cabin heat diffuser.  Even with heat off there is massive heat at co pilot and pilot under panel outlets.  With the cover closed STILL ridiculously hot.  I have looked and the firewall cable IS moving at the box with full travel.  What blocks on inside?  Is it the rectangular box or a flap inside?  I did not pull the scat off and peak in, but am at the point where I want the heat blocked 100% if the mixer won't work.  When fresh air is pulled with heat off it just blows piping hot air.  WTF.  Who KNOWS this system well and can give input on what is going on?  The engine was swapped not long before I purchased.  Is it possible that scat was not installed properly?

Bonus points for maintenance manual photos.

 

I don't have maintenance manual pictures, but I have pictures of mine before I put the scat hose back on. I still want bonus points though. 

I had this problem when I got my M20E. Previous owner had disconnected the hoses to the heater intake box attached to the fire wall d/t constant heat coming in the cabin. You can see the foil caps he made to cover the 2" and 1.5" scat tube inlets. You can also see the scat tubes hanging there. The larger scat tube comes from the muffler shroud while the smaller scat tube comes from the back of the doghouse. I lubed the cable and the flap mechanism really well with triflo. The second picture shows the inside of the box. When I pull the heater 'on' the round flapper pulls up and closes off the the small opening from the dog house and allows hot air from the muffler shroud into the cabin. When heat is off (3rd picture) flap is down pressed against the cabin inlet which then forces air from the smaller doghouse hose to travel thru the muffler shroud hose.  You can see in the pictures  the baffle material attached to the firewall side of the flapper. The baffle material seems to be in decent shape and does a good job of closing off that intake tube. I don't get any heat when it's closed. 

Hope this helps.

 

-David

[Echo]

47 minutes ago, 00-Negative said:

I don't have maintenance manual pictures, but I have pictures of mine before I put the scat hose back on. I still want bonus points though. 

I had this problem when I got my M20E. Previous owner had disconnected the hoses to the heater intake box attached to the fire wall d/t constant heat coming in the cabin. You can see the foil caps he made to cover the 2" and 1.5" scat tube inlets. You can also see the scat tubes hanging there. The larger scat tube comes from the muffler shroud while the smaller scat tube comes from the back of the doghouse. I lubed the cable and the flap mechanism really well with triflo. The second picture shows the inside of the box. When I pull the heater 'on' the round flapper pulls up and closes off the the small opening from the dog house and allows hot air from the muffler shroud into the cabin. When heat is off (3rd picture) flap is down pressed against the cabin inlet which then forces air from the smaller doghouse hose to travel thru the muffler shroud hose.  You can see in the pictures  the baffle material attached to the firewall side of the flapper. The baffle material seems to be in decent shape and does a good job of closing off that intake tube. I don't get any heat when it's closed. 

Hope this helps.

 

-David

Bonus points earned David.  That was super helpful.  Appreciate everyones responses.  Majority think it is the seal.  Plane is in having panel updates completed.  I will pull that scat line and check it.  Scott

[Echo]

Found it.  This is for 66-68ish

 

[Echo]

Thickness on that RTV?  1/8"?

 

[Echo]

JB Weld to secure it?

[47U]

5 minutes ago, Echo said:

Thickness on that RTV?  1/8"?

 

 

4 minutes ago, Echo said:

JB Weld to secure it?

I think you mean 1/8” baffle seal?  And a high-temp RTV to secure it, (unless there’s a rivet or screw… I have no knowledge).

[takair]

57 minutes ago, Echo said:

JB Weld to secure it?

As I recall…it is a sand which..bolted or riveted…. Don’t recall sealant or glue.  Remember, anything you put in there is subject to high temp and thus you will smell it for a while…

[Echo]

Of course.  Rivet not happening by this guy so will wait until annual.

[Ragsf15e]

This is one of those things that seems more complicated than it is to fix.  Just open up the top of the mixing box and look at your valve.  The problem and solution will be readily apparent and relatively easy.  Take pictures while you’re there.  Mine was fixed by the hangar fairy and he’s usually not very talented.

[Shadrach]

On 3/10/2024 at 9:00 AM, Utah20Gflyer said:

I’ve had this exact issue before and it was caused by a seal failure.  In my case the seal had come off and was allowing hot air to bypass the flapper.   I found the seal in the scat tube.  
 

There are two different heat boxes,  the flapper and the slider.  Both are simple,  you just need to open it up and see what’s going on. 

This is solid advice.

@Echo post a picture of the mixing box on the cabin side of the firewall. Need to determine what kind of set up you have. On my 67F, the fresh air circuit is opened and closed by means of a sliding plate. It gets lubricated at annual with triflow. I use kroil or mouse milk on the cables depending on what I have handy. The heat circuit control is on the engine side of the firewall and is a butterfly valve. When the heat circuit is in the off position, all of the hot air is diverted out of the box, and into the lower cowl via scat tube. As the heat control is moved into the on position, the butterfly valve allows heat to enter the cabin side, mixing box, when the heat control is in the full on position, all of the heat is diverted into the cabin side mixing box. This puts a tremendous amount of heat into the cabin. This will yield output temps in excess of 200° at the center vent even in cool weather (YMMV With the modded cowl). Definitely something you want to address now with warm weather right around the corner.

[Shadrach]

On 3/10/2024 at 10:24 AM, Echo said:

Bonus points earned David.  That was super helpful.  Appreciate everyones responses.  Majority think it is the seal.  Plane is in having panel updates completed.  I will pull that scat line and check it.  Scott

David’s mixing box looks a bit different from my 67, but is similar in function.

I don’t have the best images on hand but made flow charts for both engine and cabin side of the firewall.

Engine side:

 

 

Cabin side:

 

 

 

 

[Echo]

5 hours ago, Shadrach said:

David’s mixing box looks a bit different from my 67, but is similar in function.

I don’t have the best images on hand but made flow charts for both engine and cabin side of the firewall.

Engine side:

 

 

Cabin side:

 

Thanks Ross.  Very helpful!  I will take some photos.  Of course I agree that it needs to be repaired for summer flying.