The International Cessna^® 170 Association

Gary-
I have flown my 1958 C-180A with the firewall guillotine valve (behind the heater valve) both open and closed (with the heat off) on the same flight long enough for the CHTs to stabilize. I did not see any change in CHT with a 6-probe EGT/CHT.

With the firewall guillotine valve open (the normal position), the same amount of cool air is taken from above/upstream of the engine whether the heat is on or off.

Richard, That's interesting. As I see it the only way to effectively "shut off" the fresh air is to have the guillotine closed and the cabin air/cabin heat butterfly valve in the cold air position. Good to know it doesn't make any difference in CHT's to have the guillotine open with the cabin heat valve off.

The fresh air valve I pictured in my previous post is mounted on 1966 and later 180's. With this fresh air valve Cessna uses a different firewall valve--one that just goes to the exhaust muff.

Gary-
The guillotine valve shuts off ALL air to the cabin no matter how the heater control valve is set. With the guillotine valve open, you either get cold air from above/upstream of the engine or heat or a combination of the two.

Fresh air and cabin air changes are both good things, I can't imagine not wanting air circulation in the cabin especially since temp can be modulated for comfort with the heater control valve. Except of course if smoke or fire is coming thru the heater/vent system.

I looked at a buddy's late model 180 yesterday and concur the right side boot cowl air vent/flapper valve scat tube does go the the heater header. On my old 1973 A185F, someone had disconnected the hose from the flapper valve and capped the header port. The hose was open-ended to the cabin for an additional fresh air vent to the cabin. I liked having a separate source of fresh air.

I wasn’t clear in my last post. My thinking was about limiting the amount of cooling air “bypassing” the cylinders through the 3” fresh air duct. With the guillotine closed and hot air selected fresh air is ducted overboard and the hot air from the muff has nowhere to go. (The owners manual warns against configuring the valves this way because the ducting from the muff might get too hot. Not sure if that is really an issue with modern SCAT ducting which can withstand +550°F vs. original CAT ducting which can handle +350°F). With the gullotine closed and fresh cold air selected there is no flow of fresh air from the rear baffle duct. Theoretically more air flows by the cylinders in this configuration. In this position hot air is ducted overboard at the firewall valve.

I agree with everything you just said. Are you having CHT problems?

No CHT problems. Just in the planning stage on my baffling. Don’t want to cut a bunch of 3” holes in my new AirForms baffling and then decide I should have gone with the boot cowl vent setup instead. So far I’ve collected the later style right muffler with the associated funnel and heat muff, the later style firewall mixer valve, and fabricated the manifold for the backside of the new Del-Air stainless firewall. Still need to find or make a spring for the gullotine slide and find or make some front vertical baffles as discussed here.

hilltop170 wrote:Dave-
I had a C-180 heater system installed on my 1951 170A with the Hanlon-Wilson mufflers. It is pretty much the same as the -B model heater I think. The heater valve was put on the left side where the old one was. New side panels with heat ducts had to be fabricated. It came out very nice and works great but was unable to fit in a defroster outlet due to lack of space.

Hey Richard,

I am fabricating the '53+ heat system for my 1948 ragwing and have rookie questions:

1. Do you recall what thickness of Al sheet you used for the panels and ducts?
2. Did you use 2024 T3 for a particular reason? I'm thinking 6061 T6 would have adequate strength and the improved corrosion resistance is quite useful in our damp climate (Ireland).

Thanks,

Gareth

Gareth, for ease-of-shaping you could use SO (soft anneal) sheet for this non-structural purpose (instead of T3 or T6)... just a consideration.

gobrien wrote:
Hey Richard,

I am fabricating the '53+ heat system for my 1948 ragwing and have rookie questions:

1. Do you recall what thickness of Al sheet you used for the panels and ducts?
2. Did you use 2024 T3 for a particular reason? I'm thinking 6061 T6 would have adequate strength and the improved corrosion resistance is quite useful in our damp climate (Ireland).

Thanks,

Gareth

Gareth-
Memory says .032 for the side panels, definitely not .020. .040 would not be too thick, just heavier. Cessna used flimsy plastic on the later planes with that heater system. The duct on the firewall came out of an early C-180 so whatever Cessna used, I would guess .032. I would not add ANY holes in the duct pointing up under the panel, you want all the heat aiming down and aft.

Don’t know much about grades of sheet aluminum, I would not use hardened sheet due to cracking potential in the bends, some of them 90°. Corrosion resistance is good.

Also, 3” ducts will flow more volume but the inlet side to the heat muffs behind the cowl intakes are 2”. Where we placed the C-180 heater valve on the firewall would have interfered with the motor mount with 3” so we reduced to 2” all around.

Thanks Richard,

That helps, especially knowing the 2" supplied adequate heat. I'm planning to leave the original 2" valve in place and feed the existing scat into the new ducting. I've deleted all the suction equipment so hoping that will leave enough space for a demister. Given 2C is a LARGE temperature-dewpoint spread around here for 6 months of the year, I'll need it!

Gareth.

Reviving an older thread due to the content here…

Has anyone successfully installed a later 172-style fresh air vent on the right/copilot side of the fuselage as part of a heater upgrade?

TCU76109 wrote: ↑Sat Apr 27, 2024 3:31 am Reviving an older thread due to the content here…

Has anyone successfully installed a later 172-style fresh air vent on the right/copilot side of the fuselage as part of a heater upgrade?

You’ll have to be more specific (at least for me to understand) what you mean..

Are you meaning the fresh-air “eyeball” vents which serve the rear pax? If so…that has nothing to do with the heater system.

Since I’m working on a 1964 172, I was wondering if that air system had been installed successfully since it allows for both hot and outside air to be mixed in the cabin.

If you look at the IPC for the 1963-1974 Skyhawk, figure 138, item 41, the external air vent on the passenger/copilot side of the aircraft. It allows in fresh air with a control which can be mixed with the hot air through the firewall.

It seems though that installing the later heat duct (Figure 55, item 25 in the 170B IPC) with the hot/cold valve on the right side of the aircraft works well enough since there doesn’t seem to be much concern about shutting off all airflow other than in an emergency. I was asking the question because it would seem that sometimes you’d want the airflow shut off completely, but it sounds like that may not be the experience of most 170 owners.

TCU76109 wrote: ↑Mon Apr 29, 2024 3:37 am Since I’m working on a 1964 172, I was wondering if that air system had been installed successfully since it allows for both hot and outside air to be mixed in the cabin.

If you look at the IPC for the 1963-1974 Skyhawk, figure 138, item 41, the external air vent on the passenger/copilot side of the aircraft. It allows in fresh air with a control which can be mixed with the hot air through the firewall.

It seems though that installing the later heat duct (Figure 55, item 25 in the 170B IPC) with the hot/cold valve on the right side of the aircraft works well enough since there doesn’t seem to be much concern about shutting off all airflow other than in an emergency. I was asking the question because it would seem that sometimes you’d want the airflow shut off completely, but it sounds like that may not be the experience of most 170 owners.

The 170B ‘53 and later…. And All the 172s…. mix cold/hot air at the firewall mixing valve. While there was a re-design of the mixing valve…There’s no particular advantage to using the later 172 valve I don’t believe…. And it will be harder to fit that later, larger valve to the 170 firewall, I think.

<EDIT> OK…. I went back and Re-Read the older part of this thread… I see you are likely asking about the right “boot cowl” additional fresh-air which leads to the cabin manifold….not what I was thinking.

Yes, the boot cowl inlet.

It seems that since it’s not a common discussion to upgrade to the later system with the cowl inlet that the 170B heat/air system does the trick.