Cabin/Carb Heat System

[Bryce]

When changing the oil on a 0-300 engined F172 I found the air hose on the starboard side exhaust heat exchanger had fell off.
When trying to understand what effect this should have given ,if any, during the previous trips and that it didn't effect the operation of carb heat application I couldn't understand how this cabin heat system works.
This air hose supplies heated air to an open topped Y connection located on the firewall which I presume feeds the cabin with air but another air hose hose from this Y connector goes back to the carb air mixing box.
On the other side of the carb heat mixing box a seperate air hose connects to the port exhaust heat exchanger.
Can someone give a simple explanation how this all works.
Many Thanks
Bryce

[Brad Brady]

When changing the oil on a 0-300 engined F172 I found the air hose on the starboard side exhaust heat exchanger had fell off.
When trying to understand what effect this should have given ,if any, during the previous trips and that it didn't effect the operation of carb heat application I couldn't understand how this cabin heat system works.
This air hose supplies heated air to an open topped Y connection located on the firewall which I presume feeds the cabin with air but another air hose hose from this Y connector goes back to the carb air mixing box.
On the other side of the carb heat mixing box a seperate air hose connects to the port exhaust heat exchanger.
Can someone give a simple explanation how this all works.
Many Thanks
Bryce

Voodoo .....Brad

[jrenwick]

Is it all hooked up correctly? I have a '55 170B, which should be the same as some early 172s, and it isn't connected as you describe (if I understand what you're saying). The IPC for your model should give an idea of how the hoses are supposed to be hooked up.

On my airplane: The "Y" connector you describe on the firewall is a mixer for heated and fresh air into the cabin. The upper part of it is connected via 3" SCAT tubing to the rear of the engine baffle. Cool air comes into the cabin through it. The lower part connects to the rear of the right exhaust shroud. The front of the right exhaust shroud has a SCAT tube to the engine baffle ahead of cylinder #5 -- fresh air for the cabin heat.

The left muffler shroud is for carb heat, and it has a smaller SCAT tube to the carburetor air box. The intake for that is in front of cylinder #6. The cabin heat and carb heat systems are on separate mufflers and aren't interconnected in any way.

[Bryce]

Thanks Brad and John


I have included a photo from 2003 (before we fitted a FM oil filter adaptor).
This was taken from the port side and the carb heat scat hose from the port exhaust heat exchanger can be seen connected to the carb heat mixer box.
On the other side of the carb heat mixer box a scat hose connects to the Y connector and from this connector another scat hose connects to the starboard exhaust heat exchanger.


http://www.flickr.com/photos/27011563@N02/4150335159/

Bryce

[JHKeeton171]

The way I understand that set up, with the carb heat in the cold position, both left and right exhaust heat muffs provide cabin heat when cabin heat knob is pulled out. When carb heat is pulled on a diverter valve is closed in the rear of the airbox and the left heat muff air is now directed to the carb intake and the right exhaust muff still goes to the cabin. When the engine is uncowled on the early 172s you will note that the carb heat control has two cables going to the airbox; one for the flapper closing off the air from the filter and the other to the diverter valve halving your cabin heat. Clear as mud, I know.

[jrenwick]

That makes sense. It's different from any stock 170 setup.

[Bryce]

I will check the heater control cables when the covers are next removed .

Never really studied this set up before and didn't realise there would be a direct connection from the carb air box to the cabin.

Just uneasy now about a fire/smoke route but I suppose the cabin air control damper would restrict this if closed .

[n2582d]

Bryce,
Here are some illustrations from the IPC of how your system should be set up:

This system looks to me like you get more heat than the later C-170 system because, when the carb heat is off anyway, hot air is being ducted from around both the left and right muffler instead of just the right one. When the valve plate (#6) is horizontal you are getting maximum hot air. When the valve is vertical it is blocking hot air from going into the cabin. The hot air from both mufflers (with the carb heat off) is ducted out the vent on the top of the valve body assembly when that valve is vertical.

My question about the system concerns what happens when one selects carb heat. It seems to me that the right muffler does not provide any heat for the carburetor or, as you Brits would say carburator, regardless of the position of the cabin heat valve (#6). On first glance it would seem that hot air for carb heat is diluted because, when the cabin heat valve is closed, the hole on the top of the valve body assembly is directly venting air from the engine compartment to the carb air box. But this doesn't happen because the pressure from the left muffler ducting is higher than the pressure at the cabin heat valve. But then wouldn't the hot air from the left muffler ducting just go out the cabin air valve box instead of melting ice in the carb when carb heat is selected? I guess because there is a vacuum at the carb heat box with carb heat selected, especially at low power settings when one usually uses carb heat, the hot air from the left muffler goes into the engine rather than out the vent hole of the cabin heat valve. If I understand it correctly then, carb heat works because the pressure at the cabin heat valve assembly is higher than at the carb heat box but lower than the pressure from the left muffler heat ducting.

So having the ducting from the starboard side fall off shouldn't have made any difference for carb heat. The danger is that you might get exposed to CO poisoning when you select cabin heat. ...But maybe not because the pressure from the left side would be greater ... .

[GAHorn]

I'm SHOCKED that system was ever approved! Imagine a carb-bowl fuel leak allowing fuel vapors to be directed to the cabin via that direct connection from the air box to the cabin heat mixing valve.

[Brad Brady]

I'm SHOCKED that system was ever approved! Imagine a carb-bowl fuel leak allowing fuel vapors to be directed to the cabin via that direct connection from the air box to the cabin heat mixing valve.

George,
I agree. When Bryce called the airplane a F172 I thought he had a (Reims) aircraft not a 172 F, like mine....when I saw the pictures I knew what he had. I've always marveled that that system was approved. But I will tell you something....It heats the cabin so well that you can not open the heat up all the way even when it is "0" outside.....Brad

[c170b53]

I'm always a bit slow (so maybe I'm missing the mark on the question) but this airbox is quite simple. Hot air passes through the aft chamber in the box to heat the cabin. For carb heat the forward flapper cuts off ram air, the aft flapper that allows air to bypass from left muffler to augment heat will cut off left muff heat from escaping the box and directs it to the carb. There's a hole in the aft section bottom to allow any fluids to drain. I guess there could be fumes on a rich start up but that's aviation isn't it?

[n2582d]

I'm always a bit slow (so maybe I'm missing the mark on the question) but this airbox is quite simple. Hot air passes through the aft chamber in the box to heat the cabin. For carb heat the forward flapper cuts off ram air, the aft flapper that allows air to bypass from left muffler to augment heat will cut off left muff heat from escaping the box and directs it to the carb.

Jim,
I'm the slow one here. I've never seen this sort of setup before. I didn't realize that there is another valve aft of the forward flapper valve in the carb heat box.

[c170b53]

In your post of the ipc, the aft interior flapper's bottom edge can be seen in the hole above the number 31. Item 31 points to the arm attached to the flapper, to which the cable attaches.

I've wanted to reply to a remark made by George Horn in a thread of which I'm at a loss to remember. Basically he said that there's a few with a considerable amount of knowledge here and he mentioned my name as one of those people who know a bit. Although at the time I contemplated using aspirin to combat the swelling, the sensation was short lived as I quickly realized and came to terms with how far off the mark George was again. Quite frankly George knows significantly more stuff than I do as do many others here know more than me, often buckets more. I try to retain it all but I'm always faced with something new comes in, out goes something old. I'm sometimes envious of those with time in. I'd love to have Miles number crunching neurons but alas thats not my fate. I'd love to have Gary's sense of ha-ha but then I'd probably drive my coworkers nuts. What we have here is a website full of experiences, knowledge and the occasional attitude to keep it interesting and the longer I'm here the more I realize how little I know. which keeps me coming back or more. And lastly, simple stuff, I'm all over it!

[bagarre]

I wonder if this approach could be used to get more heat in the 1952&prior 170s.
Splice the right heat duct to provide cabin heat when carb heat is not applied.
You'd probably still have to change out the heat valve on the firewall to accommodate the larger airflow (2 2" ducts spliced into 1 3" duct)

If the splice had a valve as well, you could ensure the carb got full heat when applied.

Approval would be interesting, I'm sure.

[Bruce Fenstermacher]

I've often wondered the same and always had planned to "upgrade" but never have. The easiest and least amount of variation from the standpoint of avoiding changing the heat to the carb would be to use the early Y at the carb heat box and feed it with both 2" ducts as per stock on 48 and A models when the pancake mufflers were used. Then I'd modify the Y to use 3" scat from it to a 3" valve at the firewall.

The thought process was that it is not a function or failure to have enough heat but a failure of the system to be able to move the heat into and distribute it about the cockpit. Without measuring I'm pretty confident 2 -2" scat tubes can deliver more volume than the 3" is capable of handling. And the 3" is capable of supplying more air that the original 2" duct.

An alternate method would be to convert the right side muffler heat muff to the later B model config with 3" scat from the front baffle all the way through and out to a 3" valve on the firewall. This would be essentially like the later B model with the exception of the distribution ducting inside the cabin. Then I'd feed the carb heat with the 2'" scat from the left muffler essentially as the stock config would have been except using the left muffler rather than the right.