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Heater Valve Safety ?

Larry DeCamp

Well Known Member
My standard aluminum Vans valve came with circa 2009 RV3 kit. When the valve is open to admit hot air from the heat muffler, it also opens to the FWF air under the cowl. The intake to the heat muff on the baffle intake ramp is also generously vented to the underside of the baffling.
1-Why is the hot air pressure relieved so much by venting at both ends of the heated air source ?
2-What keeps CO and fumes out of the cockpit ?
3- Are the new stainless units referenced in the stainless valve post any different regarding contaminated air ?
 
The inlet to the heat muff should come from some place with fresh air. This is usually means a hose is run from an opening on the baffles to the heat muff, often from just above/behind cylinder #3. This will also provide more airflow than if the inlet of the heat muff is just left open since the pressure will be higher, in addition to preventing CO2 etc from being introduced into the air going into the cabin.
 
Pressure differencials?

Thanks Brad. Without putting manometer probes in the heater scat tube and under the cowl FWF, How would I know the clean heated air is higher pressure than under cowl air in front of the firewall Also, why dump heated air into the Engine cooling air destined for the cowl exit. Ie, why not an air tight heater air path from the intake to the cockpit ?
 
Without putting manometer probes in the heater scat tube and under the cowl FWF, How would I know the clean heated air is higher pressure than under cowl air in front of the firewall

Because if it wasn't, the engine would not be cooled. The pressure differential that you are not sure is there, is what makes cooling air move through the cyl fins and cool the engine.

BTW, this configuration is pretty much the same as probably every general aviation airplane you have every flow.
 
Air must flow through the muff to prevent that section of exhaust from overheating and failing prematurely. I have seen many bulged/cracked exhausts underneath heat muffs. We call these "ball park franks". They plump when you cook em.
 
Thanks Brad. Without putting manometer probes in the heater scat tube and under the cowl FWF, How would I know the clean heated air is higher pressure than under cowl air in front of the firewall

As Scott mentioned, it has to be that way in order for the engine to get any cooling. Think of it this way - the air in the top area is being introduced and pressurized by ram air coming in through the cowl inlets due to the forward motion of the aircraft. This air goes through the cooling fins into a larger area underneath. Larger area = lower pressure.

Also, why dump heated air into the Engine cooling air destined for the cowl exit. Ie, why not an air tight heater air path from the intake to the cockpit ?

Some do exactly this. When my cabin heat valve is closed, there is zero airflow through the heat muff and no air expelled from the upper to lower plenums. Others believe that doing this will destroy the exhaust. Maybe it will, but mine is still fine (and not swollen like a hot dog) after 600+ hours.
 
No one answered your question about stainless steel. In the event of a fire, aluminum can/will melt thru, leaving a hole in your firewall. SS has a much higher melting point.
 
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