Hot Air... Fixable or Expected?

[Zippy_Bird]

Hi All,

This past weekend my spouse and I flew two hours in our "new to us" 1966 M20E during which she complained of hot air bleeding into the cabin via the passenger side lower vent.  I subsequently worked both the "Cabin Heater" and "Cabin Vent" controls but only managed to find the "melt your face" setting.  With heater ON and vent OFF, it was crazy hot as expected.  With heater OFF and vent ON, it was hot air.  Not expected.  With heater OFF and vent OFF hot air was still bleeding in just not as much.  That was the best I could manage.  For comparison, the pilot side left knee has an adjustable ball vent that blasts cool outside air into the cabin.  It is very nice very cool air.  The difference in temperature between these two sources is unmistakable.

Question to the forum is this... With the Cabin Heater valve "OFF" and the Cabin Vent valve "ON" should I be getting air temperatures and air flow through the floor vents roughly comparable to the air coming in through the pilot's side ball valve?  If so, then something is afoot with the air mixing that I need to go chase it down.  On the other hand, if this is a "feature" of the 1966 M20E, then I'll move onto other squawks and and enjoy it during the winter!  :-)

 

Thanks,

Zach

 

 

[kortopates]

Sounds like the heater/hot air door isn’t completing closing or sealing. Have your mechanic investigate.
Everything is fixable given enough time and $.
Luckily this isn’t an airworthiness issue so it’s up to your discretion to correct or defer.


Sent from my iPhone using Tapatalk

[Boilermonkey]

Often the blend door seals degrade and need to be replaced.  It's never perfect, but the air should be cool when the heater valve is closed.   I sometimes have the opposite problem of too much cold air blending when the heater is open and vent closed.  Messing with the seal helps.  

[Ragsf15e]

33 minutes ago, Zippy_Bird said:

Hi All,

This past weekend my spouse and I flew two hours in our "new to us" 1966 M20E during which she complained of hot air bleeding into the cabin via the passenger side lower vent.  I subsequently worked both the "Cabin Heater" and "Cabin Vent" controls but only managed to find the "melt your face" setting.  With heater ON and vent OFF, it was crazy hot as expected.  With heater OFF and vent ON, it was hot air.  Not expected.  With heater OFF and vent OFF hot air was still bleeding in just not as much.  That was the best I could manage.  For comparison, the pilot side left knee has an adjustable ball vent that blasts cool outside air into the cabin.  It is very nice very cool air.  The difference in temperature between these two sources is unmistakable.

Question to the forum is this... With the Cabin Heater valve "OFF" and the Cabin Vent valve "ON" should I be getting air temperatures and air flow through the floor vents roughly comparable to the air coming in through the pilot's side ball valve?  If so, then something is afoot with the air mixing that I need to go chase it down.  On the other hand, if this is a "feature" of the 1966 M20E, then I'll move onto other squawks and and enjoy it during the winter!  :-)

 

Thanks,

Zach

 

 

Definitely fixable.  Reasonably thrifty hangar elf can open up the air mixer (big orange tubes coming into a metal box attached to engine side of firewall) and see what’s happening.  Mine has a butterfly valve, yours may be a slide, but it’s definitely fixable.

[Shadrach]

Valves are adjustable and seals replaceable. I do not get any hot air in the cabin with the heat circuit valve shut. None, nada, zero.  In addition to feeling the temp change from the heat circuit, I can also faintly here exhaust noise when it is open even a smidge (This was not the case prior to ANR headsets).  The heat circuit of the ventilation system should be in good condition and adjustment for flight. If it is not, you have zero chance of combatting a CO leak in the air.  Better to be cold and conscious than toasty and passed out. 

[DCarlton]

I've had similar problems on my '67F.  Mine was bleeding through the center vent near the flap handle.  Yes it would melt your pants if fully open.  Studied the diagrams, removed, cleaned, reconditioned the sliding valves on the passenger side.  Can't remember the details.  You're getting some good advice above; it's fixable.  I try to avoid using those functions though; just use the vents on the pilots side and overhead.  Rarely use the heat.  

[Ragsf15e]

23 minutes ago, DCarlton said:

I've had similar problems on my '67F.  Mine was bleeding through the center vent near the flap handle.  Yes it would melt your pants if fully open.  Studied the diagrams, removed, cleaned, reconditioned the sliding valves on the passenger side.  Can't remember the details.  You're getting some good advice above; it's fixable.  I try to avoid using those functions though; just use the vents on the pilots side and overhead.  Rarely use the heat.  

Ha! Try “rarely use the heat” up here in Washington! Last year I had a flight with -21C air temp!  That very warm air was barely adequate! 

[0TreeLemur]

6 hours ago, Shadrach said:

Valves are adjustable and seals replaceable.

I've got the same issue.  It's on my to-do list, but hasn't risen to the top yet. 

When I first flew my J in January, the heat was nice.   That first flight I pulled the air on, with heat off, and the air was still warm.   After five minutes or so, it cools down to "not hot, not cool".

It seems that the hot air valve leaks pretty badly.  Once an Alabama hangar once becomes a place where one can spend much time (latter half of Sept.) I'll take a look at that.

 

[gmonnig]

Super weird because I have the complete opposite problem. My plane is so cold in the winter time. The mixer opens up as it should but for some reason I don’t get much warm air, certainly not hot air. I am actually limited to how high I can fly due to the cabin getting too cold. 

[Ragsf15e]

That is interesting.  And you’ve opened up the mixing box to see how the valve works?  And the scat tubes aren’t pinched somewhere?

[Zippy_Bird]

Thank-you everyone for the information concerning function when in "good working order" and advice on troubleshooting and fixing.  Now I am confident on where to look and what to expect when operating.

My gratitude to the forum!

Thanks,

Z

[gmonnig]

On 8/30/2023 at 11:44 PM, Ragsf15e said:

That is interesting.  And you’ve opened up the mixing box to see how the valve works?  And the scat tubes aren’t pinched somewhere?

I have. This tread even reminded me to go out to the airport and remove the scat tubing from the cowl front to the muffler shroud to check for blockage. Nothing blocked, all new tubing throughout the entire plane. It's a mystery to me but I want to get it figured out before the cold comes. 

[kortopates]

1 hour ago, gmonnig said:

I have. This tread even reminded me to go out to the airport and remove the scat tubing from the cowl front to the muffler shroud to check for blockage. Nothing blocked, all new tubing throughout the entire plane. It's a mystery to me but I want to get it figured out before the cold comes. 

The only thing that could explain this is missing or disconnected hose perhaps somewhere in the system. Follow the entire circuit starting with the hose coming off what I beleive will be the rear baffling leading to the first heat exchanger on pilot side, then the cross hose to the front of the right heat exchanger and then lastly out the right exchanger to the air mixing box and ensure the heat control cable is working properly,

[gmonnig]

5 hours ago, kortopates said:

The only thing that could explain this is missing or disconnected hose perhaps somewhere in the system. Follow the entire circuit starting with the hose coming off what I beleive will be the rear baffling leading to the first heat exchanger on pilot side, then the cross hose to the front of the right heat exchanger and then lastly out the right exchanger to the air mixing box and ensure the heat control cable is working properly,

Definitely nothing disconnected it's all been gone through and new. Everything firewall forward is 3yrs old with less than 300hrs, everything behind the panel is 20hrs old. While doing all my avionics I made sure every scat tube was replaced. Maybe it's normal but my other planes have been face melting hot. Just thought it was a Mooney thing. My Comanche could melt the windshield if you left the defrost on. 

[Shadrach]

14 hours ago, gmonnig said:

Definitely nothing disconnected it's all been gone through and new. Everything firewall forward is 3yrs old with less than 300hrs, everything behind the panel is 20hrs old. While doing all my avionics I made sure every scat tube was replaced. Maybe it's normal but my other planes have been face melting hot. Just thought it was a Mooney thing. My Comanche could melt the windshield if you left the defrost on. 

Something is wrong. It needs to be re-examined. I did some experimenting a few winters back at various mixture settings and OATs. Not sure how the system on your 65E differs from my 67F.  Bottom line is that it’s rare that I want for more heat. At moderate OATs with full heat, I can get nearly 200° air out of the center vent. Even with single digit OATs heater output temp is 150° To 175°.  It’s a good thing the J bar is on the floor in flight because it would be absolutely untouchable after a few minutes in front of that vent.

 

[gmonnig]

6 minutes ago, Shadrach said:

Something is wrong. It needs to be re-examined. I did some experimenting a few winters back at various mixture settings and OATs. Not sure how the system on your 65E differs from my 67F.  Bottom line is that it’s rare that I want for more heat. At moderate OATs with full heat, I get nearly 200° air out of the center vent. Even with single digit OATs heater output temp is 150° To 175°.  It’s a good thing the J bar is on the floor in flight because it would be absolutely untouchable after a few minutes in front of that vent.

 

There's nothing to examine since it's all new. My Lycoming is new, so maybe the exhaust heat exchanger is different? The Mixer is the only thing in the system original, so maybe I just replace the flapper. I do think there's a slight difference in the 65. My friends 66 does not have the dump scat off the mixer. I've wondered if plugging that up would make it hot like your temps. During the interior refit, everything was sealed up. The overhead vent duct was sealed up along with intake vent door on the roof of the plane fixed. So there was a lot of cold air leaking in but that has been fixed. 

[Shadrach]

10 minutes ago, gmonnig said:

There's nothing to examine since it's all new. My Lycoming is new, so maybe the exhaust heat exchanger is different? The Mixer is the only thing in the system original, so maybe I just replace the flapper. I do think there's a slight difference in the 65. My friends 66 does not have the dump scat off the mixer. I've wondered if plugging that up would make it hot like your temps. During the interior refit, everything was sealed up. The overhead vent duct was sealed up along with intake vent door on the roof of the plane fixed. So there was a lot of cold air leaking in but that has been fixed. 

Mine has a scat tube that points straight down and dumps air above the right cowl flap.  “All new” does not equal correctly installed and properly adjusted. Yours is mechanically different than mine per the IPC (see attached). However, we are both dealing with the same energy output. Do you get strong airflow with the heater open and the vent closed?

[gmonnig]

36 minutes ago, Shadrach said:

Mine has a scat tube that points straight down and dumps air above the right cowl flap.  “All new” does not equal correctly installed and properly adjusted. Yours is mechanically different than mine per the IPC (see attached). However, we are both dealing with the same energy output. Do you get strong airflow with the heater open and the vent closed?

Correctly installed? I don't know how anyone can mess up the install, it's a very simple system. I'm gonna look at the flap again when it's not 100degrees in the hangar. I don't have the slider valve, it's the circular butterfly valve that gets lubricant twice a year. With all the hoses off, it seemed to seal good and divert the heat from dump overflow to the cabin. I'll mess with it some. Not arguing just stating that this plane has never been "Hot". 

[gmonnig]

53 minutes ago, Shadrach said:

Mine has a scat tube that points straight down and dumps air above the right cowl flap.  “All new” does not equal correctly installed and properly adjusted. Yours is mechanically different than mine per the IPC (see attached). However, we are both dealing with the same energy output. Do you get strong airflow with the heater open and the vent closed?

Actually just had a lightbulb moment from the lower diagram. I don't get "strong" airflow with just the heat on. That's why I initially thought I had a blocked/pinched hose from the cowl to heater muff. It's not blocked but the baffling that should be touching the lower cowl doesn't really seal. The high pressure air that should be forced into the engine baffle inlet is probably blowing by the lower cowl seal. Well, I think I found the answer

[Shadrach]

2 hours ago, gmonnig said:

Correctly installed? I don't know how anyone can mess up the install, it's a very simple system. I'm gonna look at the flap again when it's not 100degrees in the hangar. I don't have the slider valve, it's the circular butterfly valve that gets lubricant twice a year. With all the hoses off, it seemed to seal good and divert the heat from dump overflow to the cabin. I'll mess with it some. Not arguing just stating that this plane has never been "Hot". 

Fair enough. Another solution would be to find a mechanic that will tell you that “they’re all like that” and call it good.  
I’ve seen this method employed successfully if not temporarily to resolve many maintenance issues over the years.

[Shadrach]

5 minutes ago, gmonnig said:

Actually just had a lightbulb moment from the lower diagram. I don't get "strong" airflow with just the heat on. That's why I initially thought I had a blocked/pinched hose from the cowl to heater muff. It's not blocked but the baffling that should be touching the lower cowl doesn't really seal. The high pressure air that should be forced into the engine baffle inlet is probably blowing by the lower cowl seal. Well, I think I found the answer

Let us know if your theory proves correct.

[N201MKTurbo]

Just for a little theory.

The hot air from the muffler always flows. It cools the muffler. It is either dumped overboard through the hose that goes nowhere, or is diverted to the cabin. There is usually a separate door that allows pressurized fresh air to enter the cabin. These two functions have to be troubleshoot separately. 
 

If you don’t get enough hot air, or you can’t shut off the hot air, it is the hot air diverter. If you don’t get enough fresh air, or you cannot shut off the fresh air, there is an issue with the fresh air door.

The hot air box, usually has two seals, one that closes off the overboard exhaust. If this seal is bad you won’t get enough heat into the cabin. The other seal, seals off the cabin port. If this seal is bad, you won’t be able to turn off the heat.

Most of these seals can be replaced with a properly cut piece of silicone baffle seal material.

[67 m20F chump]

On my 67’ the cable had slipped.  It moved just enough that the door would always let heat into the plane.

[Shadrach]

50 minutes ago, 67 m20F chump said:

On my 67’ the cable had slipped.  It moved just enough that the door would always let heat into the plane.

Given the volume and temperature delta between hot and ambient side. A leak on the hot side is much more obvious.

[Echo]

Flew my new to me ‘65 E home from ND last week. Definitely got a hot air all the time situation that will need resolved. Serious heat from the under panel vents. Not serious cool. All heat all the time. Felt ok at 9500. Not so much at 3500.