Cabin Heat Won't Shut Off Completely

[moontownMooney]

I was flying today and noticed that the air coming out of the lower central vents (fed by the starboard side scoop, shares ducting with cabin heat) was warmer than the air coming in through the roof scoop ceiling vents.  I knew that this vent line is shared with the cabin heat so I immediately suspected that the cabin heat was ever so slightly mixing in.  I shut the cabin vent push actuator and confirmed that there was a very small amount of quite warm air coming from the same vent.  Is this normal?  Any thoughts on likely culprit?  Just an issue with the baffle controls in the vent line or something more nefarious?

Thanks in advance!

[jamesm]

If it is original cabling  and hasn't slipped my guess would  be the  housing is worn   (on the firewall on my 'C' model) there is  air mixing /diverter  (butterfly)  valve  assembly has probably elongated the pivot point in the hosing  over all of  the years. It was bad design at least one mine there were no bushings the pivot point was the housing it self. I have since then put busting for the pivot point. works great.

Hope this helps,

 

James '67C

[N201MKTurbo]

It is either the cable rigging or the seal has deteriorated. Take the hose off the top of the mixer box and look at it. You will figure it out.

[carusoam]

Do you know where the airbox controls are in front of the firewall?

this is an area that changed a lot over the years.

in '65, they were slide gates, later on a butterfly valve with rubber bits that need replacement.

slide gates stop sliding...

hot air coming from the engine can really ruin a nice flight in July.

which parts does your bird have?  Got a pic?

Best regards,

-a-

 

[Bartman]

Mine has the slide gate.  A little cleaning of the slide and lubrication at the end of the cable did wonders for my summer time comfort.

Now my overhead vent is another story... It works as long as I open it on the ground by turning the knob and pulling on the vent flap simultaneously, but not in flight with just the knob.  Its much more involved to fix so I'll do it when I do some interior work sometime.

[Shiny moose]

My 67 F had same problem when I purchased it, I have the butterfly. My problem was simple to fix, someone had put a screw in the box that was to long and preventing the flapper from closing all the way. Change out the screw. and its was like having AC ( not really but no more heat when I don't want heat) when I want heat, it's there with a vengeance


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[moontownMooney]

I appreciate everyone's advice.   Sorry it has been a bit, buy you know... life...     

So I've been able to get some more pictures and info.  A related piece of information is that there was a seal lodged in the vent ducting behind the direction control baffles of the center vent (located in the lower center console, behind the johnson bar for manual gear).  It is just visible through the vent in the attached picture.  The seal was very soft/pliable and I was able to pull it through the vent with some needle nose pliers and the seal is pictured below with some postage stamps for size reference.  My particular in cabin mixing box (pictured below) has the sliding gate valve for the cabin vent actuator and the off-center butterfly valve for the cabin heat actuator.  I was able to use an endoscope threaded up the vent ducting into the mixing box to get a picture of the butterfly valve without taking anything apart (two other images).  I was also able to remove the scat-hose from the box in the engine cowling to reveal the other side of the butterfly valve.  

With all that said, it is not clear to me where this seal is supposed to go.  I have to imagine it is part of this butterfly valve assembly and was blown through the ducting to where it was lodged.  With that said.  I can't figure out from the parts manual if this is true, how the seal would have been part of the assembly but able to be blown free in this manner.  According to the parts manual the valve seat assembly is part 640212-027, but I can't find any other info on this part number on google.

I will say that the amount of hot air coming through the vent when the cabin heat is closed would seem consistent with the butterfly valve closing but a seal being missing.

Any thoughts?  Is this foam rubber disc part of the cabin heat valve assembly or something else entirely?  If I haven't said it elsewhere my M20F is a '68 model.

Thanks in advance...

[carusoam]

Bingo!

Great pictures too.

Somebody once posted a picture of the seal assembly. I believe there are two metal disks and the rubber seal disk is between them.

It looks like your valve disks have become separated...

PP thoughts only, stuff I learned around MS...

Best regards,

-a-

[cliffy]

Just to add- What is on the "Engine Fire" check list?

SHUT OFF THE CABIN HEAT!

If you can't shut it off you've got a real problem if you're flying in it. 

Small, seemingly insignificant items sometimes have serious consequences.

[Hank]

11 minutes ago, cliffy said:

Just to add- What is on the "Engine Fire" check list?

SHUT OFF THE CABIN HEAT!

That is not "on" the Engine Fire checklist, that "is" the Engine Fire checklist!

[moontownMooney]

I wanted to close out this thread. We finally got our cabin heat fixed. We had resigned to waiting for it to be fixed at the annual, and our mechanic said he fixed it and charged us for it, but in reality made it worse. In the end there were two or three problems which were a little more unusual and beyond the standard "missing seal" cause so I thought I'd detail here. At the start we certainly were missing the seal (a replacement had not been adhered properly to the valve plate/flap and blew off, becoming lodged in our vent ducting. At annual, the mechanic did a sound job of reinstalling, adhering the seal to to the valve and trimming precisely. As I mentioned however, the heat was actually worse at this point. Inspection revealed: (1) The bushings on the flap valve axle were badly degraded or missing entirely, allowing substantial slop in the valve (1/8 - 1/4" gap). (2) The actuator cable sleeve had become unanchored from the firewall and/or nearby structure again allowing substantial slop in the valve. (3) This one is a little less firm, but precisely trimming the seal to the valve flap radius and completely adhering the seal to the valve flap prevented the seal from being able to accommodate even the slightest gap or misalignment. As such, the new seal was trimmed to slightly overhang (1/8") the valve flap and it was adhered only on the inner 1/2 to 2/3 of its radius, giving the seal more flexibility to accommodate small gaps or misalignment. Thanks to all who contributed to this thread! My heat works... now on to the air conditioning !

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[carusoam]

Great follow-up Moontown!

The power of MS is awesome.

Best regards,

-a-