Hot air dump valve closed when carb heat is on?

[M20 Ogler]

If I’m reading the drawing correctly the hot air dump valve is closed when the carb heat is on. Then it’s open when carb heat is off. 

Why is a dump valve needed?

[cctsurf]

Two things:  To direct the hot air only into the carb when carb heat is called for (and not just allow it to be dumped overboard), and so that the extremely hot air has somewhere to go when the carb heat is off.  I believe the earliest models of the carb heat system didn't have the dump at all.  And If I recall correctly, there is a service bulletin that called for modification so that they didn't melt things or make other problems.  I believe the forced air also pushed the hot air into the carb even if it was turned off without the dump valve.  

[carusoam]

Things to look for…

The health of the butterfly type valve is prone to wear…

The small bits pieces that go with it are also prone to wear….

The latest carb heat installations also used an exhaust tube to direct the excess unused heat overboard….better.

The engine always produces heat, the heater muff is always hot, and some hot air is alway being dumped into the bottom of the cowl via this valve…

Without the valve… carb heat would be less effective… drawing warm air from inside the cowl…. And mixing with the hot air coming from the muff…  So… a valve is needed to do the job…

If the valve was permanently closed… carb heat would be on all the time….

 

Check on the health of the bits and pieces, adding the proper hose should have some proper information to support it…

PP thoughts only, not a mechanic…

Best regards,

-a-

[Prior owner]

When the dump valve is open (carb heat is off), the heated ram airflow being directed against the flapper door is reduced, thereby reducing the pressure on the flapper door, thereby decreasing the likelihood of hot air leaking past the flapper door seal (which isn’t the greatest seal design) into the carburetor throat…???

With this design, the heated ram air in the muffler shroud that feeds both the carb heat and the cabin heat, will always have the cabin heat receiving a roughly equal amount of airflow, whether the carb heat is on or off…  Perhaps the valve is there to provide the cabin heat a consistent amount of airflow???

 

 

[cctsurf]

1 hour ago, PilotCoyote said:

Perhaps the valve is there to provide the cabin heat a consistent amount of airflow???

My understanding is that it provided a consistent amount of airflow over the muffler and that they had problems with heat muffs melting or something like that before adding the dump valve.

[SheryLoewen]

If there wasn't a hot air dump --Hot air would leak into the airbox/carburetor and cost you power.

[carusoam]

Great insight from the Loewen camp! (As always) 

Thanks for sharing the details.

Best regards,

-a-