Why is carb heat air unfiltered?

rframe

pǝʇɹǝʌuı
Was cleaning my airplane last night and wiping down parts and staring at the carb heat box and was struck with the question: why is carb heat air unfiltered?

I see no reason why a screen and or filter couldn't be on the hot air intake just like the cold air intake, to keep bugs/dirt dust out of there.

Am I missing some obvious reason why it has to be unfiltered air?
 

highspeed

Well-Known Member
Perhaps because filters clog/get dirty. For the amount of time carb heat's used, and the importance it plays when needed, having it fail because of a cloged filter would be pretty crappy.
 

rframe

pǝʇɹǝʌuı
That's a good point, I know of mechanics who tell me of people who haven't changed their oil in 200 hours because it wasn't convenient, so having a carb heat air filter that isn't visible might not be a great idea.... lazy people.
 

Roger Roger

Paid to sleep, fly for fun
Many aircraft do have a fairly coarse screen on the carb heat/alternate air opening. Not so much a filter but it keeps loose hardware and bits of safety wire from making their way into the intake.
 

trafficinsight

Well-Known Member
Some do have screens. But if the air filter is clogged the filter over the carb heat probably would be as well, and the idea is to also have an alternate intake air source.




Sent from 1865 by telegraph....
 

Matt13C

Well-Known Member
Just a guess, but a filter is one more place for ice to form before you have a chance to add heat to the air by passing it through a heat exchanger from the exhaust.
 

Stoneage

Well-Known Member
We like to think the RPM drop we get when pulling on the carb heat during run-up is from the less dense heated air; well some of the RPM drop is, and some is from added resistance of moving the air through the carb heat system. A filter would add to this resistance and increase cost, weight, maintenance and add another failure point. Simple is good - if it works and un-filtered carb heat works. Just remember it is unfiltered air and do not use when starting the engine (it won't heat the air until the exhaust is hot and the startup is where the prop first has the chance to kick up dirt and does not have the oppourtunity to blow the dirt away before the carb heat is engaged).
 

rframe

pǝʇɹǝʌuı
We like to think the RPM drop we get when pulling on the carb heat during run-up is from the less dense heated air; well some of the RPM drop is, and some is from added resistance of moving the air through the carb heat system.
Dont think I'm buying the resistance thing. The ones I've seen are pretty much wide open holes.
 

CFI A&P

Exploring the world one toilet at a time.
/\ I concur. Pulling the carb heat cable moves a "door" in the carb heat box blocking the filtered air and opening the unfiltered air passage, also known as a large hole in opposite side of the carb heat box. Doesn't really cause mechanical turbulence in the systems I've seen. Then again, I haven't flown/ worked on every airplane in existence, so there maybe an application out there with such a system.
 
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