brian

brian

Well Known Member
The heater on my -6A is a muff over one exhaust pipe, then the air goes up to a flapper valve on the upper part of the firewall. The heater control valve opens to allow thie heated air to go into the cabin. However, I noticed that the side of this flapper control box has an opening on the side of it. When the heater is "on", the flap covers that hole and opens the hole into the cabin. When the heater is "off", the flap covers the hole into the cabin and opens the heater output to the side hole. It appears to me that, in summer when I don't need more heat under my cowl, this is dumping heated air into the upper cowl area.

How are you others dealing with this? I thought I should either rivet a plate over the side hole, to block airflow entirely when the heater is "off", or rivet a flange over the hole and run some 2" SCAT tubing down to the lower cowl air exit.

What do you all think about this?

thanks,
brian
 
I was trying to increase my airflow through the oil cooler and over the cylinder fins this summer so I blocked the hole (on the flange mounted on the baffle) with duct tape. It actually helped decrease my CHT's and oil temps a bit.
 
On our -10 it dumps it right over the fuel pump :mad:

We will be trying to rework this soon as the fuel pressure issues are annoying. We taped over the inlet, warm day, no fuel pressure problems:rolleyes:
 
I do the same on my RV4 now as I did on my RV7. I put a piece of metal tape over the hole at the baffle so no air goes into the heater hose. Keeps my feet from sweating and temps stay lower under the cowl when it's hot out. This time of year I simply peel it off and I'm now winterized.
 
I did a quick search as this has been brought up before, but my search skills failed me. In prior discussion, closing off the inlet brought up concerns of possible damage to the heat muff. I recall that Van's does not recommend that the inlet be closed off for this reason.
 
Once a year you should remove your heat muff and inspect both the muff and the exhaust pipe where it rests. With this requirement in mind: remove and inspect the heat muff for the summer months and reinstall it in the fall. Remove all hoses and cap off the back baffle flange and use aluminium tape to seal the heat box.
I do not fly in the winter and so I have not used a heat muff for the last four flying seasons.
 
Air needs to continuously move through the muff to prevent heat damage to the muff, exhaust system, or ducting that could lead to CO troubles in the future. Either it gets dumped overboard or into the cabin. I don't think that adding a duct to the opening to have it dump closer to the cowl exit would hurt anything. Whatever you do, don't block the exit from moving air. YMMV
 
Some thoughts...

Two thoughts (untested ideas, just thinking out loud):

1. Route the waste air directly overboard:

Mount a 2" tube flange to the waste air exit of the cabin heat selector valve and run a scat tube from there to the cowl exit area. This will at least dump the heated air directly overboard without increasing the temp inside the cowl.

This will, however, not help with respect to wasted airflow and pressure drop in the cold air plenum. It may in fact worsen it slightly because the cowl exit area likely has a lower pressure than the area where the cabin air valve resides on the firewall, thereby increasing airflow through the cabin heat waste air circuit and increasing pressure drop in the cold air plenum.

2. Partially constrict the opening in the engine baffle to reduce flow rate:

Partially constrict the opening in the engine baffle where the 2" scat tube connects such that air still flows through the cabin heat circuit but at a reduced flow rate. This is a tradeoff compromise between the stock system and the idea of removing it entirely for the hot season as has been suggested. By constricting the opening an appropriate amount (found through experimentation?), there will still be some cabin heat available when selected, there will still be sufficient airflow through the shroud even when cabin heat is not selected to prevent its accelerated deterioration, and there will be a lesser pressure drop in the cold air plenum thereby improving engine cooling.

The constriction could be easily implemented as an aluminum plate with a hole, which attaches to the forward side of the engine baffle over the existing 2" hole. The smaller the hole, the lower the flow rate and pressure drop. Some experimentation would determine a good size for the hole. The stock design is 2" diameter. Half the stock area would be 1.4" diameter, and quarter the stock area would be 1" diameter. Different constriction plates could also be changed out seasonally, if needed, with very little effort (4 screws accessible with only the top cowling removed).
 
GregM said:
Air needs to continuously move through the muff to prevent heat damage to the muff, exhaust system, or ducting that could lead to CO troubles in the future. Either it gets dumped overboard or into the cabin. I don't think that adding a duct to the opening to have it dump closer to the cowl exit would hurt anything. Whatever you do, don't block the exit from moving air. YMMV
Click to expand...

Now I'm concerned about something else. Because this plane is quite cold in the winter and quite hot in the summer (hot cuz it picks up its cabin vent air from the top of the engine baffles under the cowl, rather than from a NACA duct on the side of the fuselage, as on later models, but that's a fix for later), I decided to try to make the heating system more efficient. To do that, I got some of that motorcycle racing exhaust wrap, and I just finished completely wrapping both pipes going into the muff, as well as several layers around the muff and a layer around the SCEET duct going from the muff to the control valve.

From what you said, and what a couple other posters said about damaging the muff from too much heat, now I'm wondering if this is going to be a bad idea.

The heat from the heater, even in spring and fall, has been quite marginal. Last March, I flew from VT toward Sun N Fun, and it was pretty bad. I was shivering and shaking badly the whole way to the first fuel stop in MD, and I had to spend a couple hours in the FBO just warming up.

Has anyone ever tried making the heater more efficient by using exhaust wrap? Do you folks see a problem with doing this?

thanks,
brian
 
No Wrap

Wrapping or coating exhaust pipes is not recommended by Vetterman. Check with your exhaust system supplier.
 
Heater

I am installing the same exact heating / fresh air system the I have in my mooney, you can apply all heat or mix heat with fresh air instead of full heat.
When the heat is in the off position it does not dump over. I have heard of others using this type set up without dumping with no I'll effects.
 
I installed a longer heat muff and it seems to have made a difference. Haven't flown in real cold weather yet though. I believe I got it from Robbins Wings. If you have the room it may help but not all installs allow for it.

Next time I see Koger I'll tell him he should have built a warmer plane!;)
 
summary

Larry Vetterman was kind enough to contact me, and we had a thorough discussion about this. I just wanted to summarize it here, so no one reading this later will follow any of the mistakes I've made here.

First, as a couple people mentioned, wrapping the exhaust is a very bad idea. The exhaust is a major heat sink for the air-cooled engine, and it needs to get air.

Dumping the heat from the muff inside the cowl when the heater is off is no problem, because that air is not nearly as warm as the air coming off the cylinders.

If I need more heater, Larry suggests adding a second muff on the other side, then feed the output of that one into to the input of the first one. And he said, as someone else mentioned, see Rick Robbins at robbinswings.com for muffs.

Many thanks to all for your help,
brian
 
The heater flap on my RV-6 is just below the recess on the firewall. It either diverts hot air from the upper cowl via the heat muff on one exhaust to the cabin or dumps it into the lower cowl and overboard through the exhaust exit.

It seals well enough not to bleed into the cabin during the summer however it doesn't produce a lot of heat during frigid north east US winters. I stuffed the heat muff with wire wool last winter to slow the airflow down and provide more heat transfer area. It increased the cabin air temperature slightly.

Jim Sharkey
RV-6
 
brian said:
The heat from the heater, even in spring and fall, has been quite marginal. Last March, I flew from VT toward Sun N Fun, and it was pretty bad. I was shivering and shaking badly the whole way to the first fuel stop in MD, and I had to spend a couple hours in the FBO just warming up.
Click to expand...
FWIW, my -6 came with dual heat muffs and dual cabin heat. The stock original one exhausts into the center of the cabin at the firewall, and at some point a previous owner added a second that exhausts directly in front of the passenger. In cold weather, with both open, you can fly without jackets or sweaters. It's quite cozy.

And that's in addition to the Robbinswings muff on the crossover tubes for the carb heat. Heat does not seem to be in short supply in my aircraft.
 
The amount of heat being vented into the lower engine compartment by the cabin heat valve, even though force-fed by the scat tube, would appear to be minimal compared to the heat produced by the full length of the exhaust system, plus the air passing downward over 350-400 degree cylinders.
 
CFI1513840 said:
The amount of heat being vented into the lower engine compartment by the cabin heat valve, even though force-fed by the scat tube, would appear to be minimal compared to the heat produced by the full length of the exhaust system, plus the air passing downward over 350-400 degree cylinders.
Click to expand...

More to the point, the heat being vented into the lower engine compartment by the cabin heat valve comes from an exhaust pipe in the engine compartment. The fact that this heat is transferred via scat tube from one area to another within the engine compartment doesn't change the total quantity of heat that is in there.
 
Normal and racing configurations

Yes this was covered previously. I have the control cube on the firewall with the alternate outlet piped to the cowl outlet. When racing I install a cover plate inside the rear baffle over the air feed hole. As stated previously this also stops the stealing of cooling air from the engine behind cylinder #3 and the CHT for cylinder #3 is reduced.

Bob Axsom
 
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