C&D Heater Experience & Heater Control

[n86121]

OUT WITH THE OLD - Years back I started to get uncomfortable with heating my T337 by tossing a match into a 45 year old fuel air bomb under continuous ignition. Its original janitrol heater was getting long in the tooth and its ignition rather twitchy. Would it start this time? Would it not? What exactly WOULD happen when I flipped that switch?

I didn't actually close my eyes each time, but to say I never flinched would be a lie.

Finally ye olde janitrol didn't pass the 'ever-harder to get done' pressure decay test, so it became time to look at alternatives.

Extracting the heater from a 337 reminds me of a molar I once had pulled. The molar was easier and as I recall, less painful. Another reason to tech-out of constant pressure decay testing.

The easiest thing to do would have been another janitrol heater, but that would have imposed almost the same ongoing hassles. C&D offered (offers) an AD free unit that doesn't require the same ongoing fuss, so the decision was made, go with the more modern C&D heater.

IN WITH THE NEW
The C&D wasn't exactly a direct replacement for the janitrol. The local shop that had never done one before spent some not inconsiderable time and money fabricating a bracket for the combustion fan, etc, but eventually got it all in.

Could it have been easier? Probably a youtube opportunity next time one gets installed.

CHASING A GREMLIN
Then the heater's output varied all over the place. Maddening. All the bench tests would pass, but in flight the heat would vary all over the place. Went on like that literally for years. We only finally (recently) tracked the issue to an intermittent cyclng switch, which finally did me the favor of going POOF! My theory is it would only act up with the heat and vibration of flight, but how to readily test that? I guess one could have strung a series of test lamps into the cabin, but that would have not been trivial, even with FAA willing to 'blink'for a few minutes. So it was just a big sigh of frustration for quite a while, until I chased it down (with C&D help).

OPERATING INSIGHTS
Perhaps unlike most installations of combustion heaters, the C337 feeds the heater's ventilation and combustion with ram air, instead of what would be a much more consistent fan. There just isnt room for one. That means a LOT of ram air, which varies a LOT with airspeed and (pressure) altitude.

THERMOSTATS The heater has three internal temperature limiters:

1. DUCT - There is a bi-metallic thermostat in the pilot's foot well duct which you adjust by the TEMP lever over the passenger's knees. Moving TEMP lever all thee way down (backwards from any rational design) bypasses the footwell thermostat entirely to always on. TEMP can theoretically limit duct temp from about 75F (UP) to +200F (FULL DOWN).

2. CYCLING - A cycling switch on the combustion heater itself limits the heater's normal internal operation to about 250F, and

3. OVERHEAT - An over-limit thermostat that shuts it off if the combustion chamber gets too high, around 350F. If it gets too hot, from lack of enough ram air for example, the OVER temp light comes on and the heater shuts down. It takes a few minutes for this sensor to re-cycle before you can restart the heater.

MANAGING COMBUSTION AIR = To control the amount of air going to the combustion side you use a pull lever over the passenger's knees. Pulling out incrementally closes a duct controlling how much ram air goes into into the combustion fan then heater. IN is fully open, and OUT is fully closed.

(I am convinced the Cessna designer of these controls either had a sick sense of humor, or some rare form of dyslexia to make them as counter-intuitive as possible).

A. GROUND OPS - On the ground you leave it open (IN), to get enough air from prop wash to activate the air pressure safety switch and have heat on the ground.
B. FLYING - Once flying you partially (1/2) close it so the combustion flame isn't running too lean, and doesn't get pushed past the heat chamber, and blown out the heater exhaust, and
C. THIN AIR AT ALTITUDE - As the air gets thinner (50% at 15k ft!) you start opening this again to get enough thin to the combustion side. You are literally adjusting the airside mix for the combustion side of the heater.

TEMP INDICATOR HELPS - So i can readily adjust these airflows I have put a small 200F battery-powered LCD digital temp gauge on the panel, with its sensor dropped into the defroster vent. Because the heater has so many dynamics, all of which can take a few minutes to level out to a desired result, I find this much easier than guessing and chasing it around.

AIRFLOW - The AIR lever over passegner knees controls total airflow through the heat exchanger ..at its back. Now that's an odd design. If you let too much air go through the heater too fast, it has a lower outlet temp because its has less time to heat. If you restrict it slightly the air stays in the chamber longer and gets much hotter.

I am not sure which one delivers more 'heat' to the cabin, as you are effectively controlling the volume and temp at the same time. in this case the 'lower' duct temperature of unrestricted airflow may actually be delivering more total heat to the passenger cabin.

If it is not very cold outside when testing the heater on the ground, because you are literally turning a blow torch on and off, by the time the footwell switch can cycle the outlet duct temp is already hovering around 145F at the min setting.

Of course, now that we've finally got it working correctly, it's 75F out in Washington DC. Just going to have to climb!

And that's probably more than anyone really wants to know about combustion heaters...

Any other questions or to share your experiences, insight's on these, well here is your opportunity!

[edasmus]

Wow, very interesting. Thank you for sharing! Glad I don't have one in my 337G!

Good Luck and may you get many hours of warmth!

Ed