cabin heater scat tube size

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cabin heater scat tube size

Postby 1moretw » Thu Aug 04, 2022 11:17 pm

Well, here we go again. My 1952 170B It is my understanding that the original setup for the heat tube from the muffler to the cabin heat distribution input was 2 inch. Over the years, someone changed mine to accept a 3 inch Scat tube. My IA insists that it must be changed back to 2 inches since that is the way it was built. We can't find anything in the logs that documents the change to 3 inch. When did Cessna go to a 3 inch output from the muffler shroud to the heat distribution box? And what argument can be used to convince the IA that it is a legal modification? More heat seems like a good idea here in North Central PA.
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Re: cabin heater scat tube size

Postby flaglor » Fri Aug 05, 2022 4:54 am

You need a different IA.
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Re: cabin heater scat tube size

Postby voorheesh » Fri Aug 05, 2022 5:19 am

How was the system modified to fit the larger tube? Was either the exhaust system or firewall modified? If so there should have been a basis for approval (Form 337) or at least a maintenance record. You might attach a picture. I would suggest asking your IA if it can be field approved, assuming there are no safety issues. (Remember, exhaust systems should be secure and not modified to risk leaking). If approved parts from a later model were used, that also might be basis for field approval.

I recall seeing some 337s where an IA found minor modifications during inspections and documented them as minor alterations, determined to be airworthy, not previously recorded, or words to that effect. That might apply if the only difference from type design is the actual tubing size.

I remember a 170 from up north where the prior owner cut a large hole in the firewall to accommodate a larger hose. The new owners had to change it back plus repair the firewall. Couldn’t find anyone to sign the annual. Someone mentioned the larger setup did not result in any increase in cabin temperature.
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Re: cabin heater scat tube size

Postby Bruce Fenstermacher » Fri Aug 05, 2022 9:47 am

3" wasn't used till '53 when the entire heat system was changed. You would have 2" if you have the correct system for your airplane. We need pictures to determine what you have.

Changing to 3", if done in an airworthy fashion using standard practice would be a minor alteration in most eyes.
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Re: cabin heater scat tube size

Postby Richgj3 » Fri Aug 05, 2022 10:28 am

You will notice in the picture posted in the thread about baffles by hilltop170 that the firewall cabin heat fitting/control is a later style than 1952. The scat tubes look like 2 inch but I can’t be sure.
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Re: cabin heater scat tube size

Postby Richgj3 » Fri Aug 05, 2022 10:36 am

Picture of the cabin heat firewall fitting on my 52.
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Re: cabin heater scat tube size

Postby Bruce Fenstermacher » Fri Aug 05, 2022 1:16 pm

voorheesh wrote: Someone mentioned the larger setup did not result in any increase in cabin temperature.


More heat depends on how it was done. There have been more than one 170 converted to "3 scat by replacing most if not all of the heat related distribution network. There is no reason to modify the muffler system to do so other than to do it all properly either the correct right hand muffler would have to be replaced with the correct part for the later system or the current Hanlon Wilson modified by removing the end baffling flanges to meet the spec of the correct muffler.

If the '3 scat was not run from the front baffle, through the baffling at the muffler for it, then 3" scat to an appropriate modified 3" manifold at the firewall, I can not see how inserting just a 3" scat in the system would increase heat. Just one piece would not increase the volume of air that could move through the system
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Re: cabin heater scat tube size

Postby Bruce Fenstermacher » Fri Aug 05, 2022 1:18 pm

1moretw, we really need pictures of the entire system to evaluate whether it should be changed or not and whether the mod might rise to the level of a major alteration requiring approval.
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Re: cabin heater scat tube size

Postby hilltop170 » Fri Aug 05, 2022 6:54 pm

Richgj3 wrote:You will notice in the picture posted in the thread about baffles by hilltop170 that the firewall cabin heat fitting/control is a later style than 1952. The scat tubes look like 2 inch but I can’t be sure.


I had a complete C180 heater system installed in my 1951 170A on a 337 Field Approval here in Alaska. The only exception was we used 2" scat from the engine to the heat control valve because we did not want to modify the exhaust system. Also, the motor mount restricted access to the control valve on the cold air/top side.

The 3" connections on the 180 control valve were necked-down but even with the 2" scat, there is adequate heat and cold air available in the cabin with the two outlets in the firewall header and outlets in the side panels at the door posts directing air to the back seats.

p.s. For the detail oriented folks, Del Lehman later installed the guillotine shut-off valve that was omitted in the original control valve installation shown.

7572204E-A50E-42A1-B3B8-6370B0E331BA.jpeg
C180 heater control valve with 2” scat
Last edited by hilltop170 on Sat Aug 06, 2022 5:48 pm, edited 2 times in total.
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Re: cabin heater scat tube size

Postby 1moretw » Fri Aug 05, 2022 7:12 pm

Will try to get pics and post Sat evening.
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Re: cabin heater scat tube size

Postby voorheesh » Fri Aug 05, 2022 8:39 pm

The oversized cabin heater I mentioned earlier was on a plane out of Montana I was involved in nearly 20 years ago. The prior owner had modified it by enlarging the firewall opening and fabricating his own cabin heat box. The exhaust system was also modified. He claimed that it did increase the flow of hot air, but he was still cold. There was no way to get that thing approved and the new owner had to find replacement parts and pay for a stainless steel patch to return the firewall to spec (Major Repair/difficult to identify approved data, but it got done). Expensive. It makes you wonder what people are thinking sometimes.
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Re: cabin heater scat tube size

Postby c170b53 » Fri Aug 05, 2022 9:01 pm

I’d venture some, are better at ranching than wrenching. :D
I’d guess there’s a few planes with or have had a parking spot next to a tractor, mine likely one.
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Re: cabin heater scat tube size

Postby GAHorn » Sat Aug 06, 2022 8:47 pm

That was a good “catch” by Del Lehmann. (Del never fails to impress.) Without a cabin-air shut-off valve the TCDS/Production certificate was violated (even tho’ approved by field approval). Goes to show that what my old Chief of Maintenance at my first corporate job said was true .. “Field Approvals can be Fool Approvals”.

Without a cabin-air shut off a failed heater-valve/failed-muffler could pump carbon monoxide into the cabin even with the heater valve shut off…and smoke from the engine compartment could enter the cabin also.
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Re: cabin heater scat tube size

Postby 1moretw » Mon Aug 08, 2022 2:03 am

heater2.jpg
Finally, I have mastered the art of attaching pictures. (and without a grandchild helping). The pic of the heat input on the firewall is obviously modified to be a larger size by the tin snip markings. The output from the muffler is the "funnel" going from about 4 inches down to 3 inches. Finally, the input to the firewall is a 3 inch donut. And of course the muffler output was connected to the firewall adapter with 3 inch SCAT tubing. No matter what we do, we need to add an adapter to the muffler shroud so that we can run a scat tube to the firewall. I want it to be a 3 inch adapter for more heat but my IA says that it must be 2 inch since that is the way it was originally built. Obtaining a 337 is out of the question so no need to suggest that. As an aside, the mufflers are Hanlon Wilson and the shroud which held the 3 inch output was completely bastardized. The shroud was not the "fish mouth" type. So, the real question is, how does one make the 3 inch system legal or can it be given that this a 1952 B model.
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Re: cabin heater scat tube size

Postby voorheesh » Tue Aug 09, 2022 12:45 am

If your IA is unwilling to document this as a minor alteration (limited to a change of duct size/similar type and material), I don’t see how you can make this legal. Hopefully, the cabin heat hose and connections are safe and not in danger of exhaust leaks or contamination of cabin heat. The photos show the original parts have been modified (probably not to acceptable standards for your inspector). You mention the shroud was “completely bastardized”. That sounds like something an IA would be justified in requiring it be corrected before signing off. I’m sure others on this forum will have more expertise than me, but I don’t see how this is “legal”. I recommend you follow your IA’s advice on the current problem. If you need more heat, see if you can find replacement parts that can be installed properly either by STC or Field Approval. And hope you get your airplane back up and running soon.
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