Help - intermittent combustion heater

[MikeZ]

HELP please. My 79 P337-H combustion heater has become intermittent. Last trip from Florida to Upstate NY was almost hellish (cold). Heater fires, gets warm, works on the ground, goes out after 5-10 minutes in flight. Will re-fire, but not stay on, sometimes seems to not want to fire.

The heater was JUST replaced with an O/H unit. The old one had identical symptoms. We also just replaced the floor mounted thermostat. All accessories with the heater are new including a new fuel pump.

We are just checking and servicing the combustion air blower, which has not been serviced yet. This may or may not work (we'll find out).

Short of pulling and replacing every single wire in the system, can anyone suggest a magic bullet?

[Jerry De Santis]

Mike, a while back there was quite a bit on the heater on either this site or Peter's. Check the archives on both.
Jerry

[MikeZ]

Tech service said that the newly overhauled heaters have tighter limits on the pressure switches for combustion air flow to ensure that the unit gets adequate flow. They advised replacing the combustion air blower, which is now done. Combustion air flow seems much more poweful. I'll post whether this solves the problem after my next long flight.

thanks.

[Bob Cook]

Mike

I believe this is probably the cause... not much left <G>.

Did you get the new heater chamber to remove the AD or did you just overhaul the old one? Where was it serviced?

Not much fun running around without heat!

bob

[skymaster]

mine popped a breaker. now afraid of using it. my 73 p gear pressure switch has required a couple pumps to get green light/retraction. after cycling no problem. does the pressure switch have a tweek or do the o rings leak air during colder weather. J

[MikeZ]

All combustion heater parts are now new. We even replaced the defective Overheat light (eyebrow panel). It's still happening. Combustion heater runs for 30 seconds, then overheat light comes on, heater cuts off. Some useful diagnostic there.

Last flight from upstate NY to Florida was on the coldest day of the year. Thankfully I had sun in my face the whole time, kept the cabin warm enough.

I think we have a direction to go.

Figure 13-1C, page H2 of Parts manual shows an "Inlet Valve", #4. This bypass allows air from the cabin air blower to divert around the heater. IF this valve is stuck open internally, or the actuator lever or cable is not correct, it just might be allowing air to bypass the heater even in part. This would cause the overtemp sensor to show the heat exchanger gradually increasing in temperature due to inadequate air flow.

You have to wonder what kind of engineers designed these systems. I can't imagine it being more obscure for a little airplane. It seems every part has multiple functions and is interconnected with other systems.

Still love my plane though, but I want it to keep me warm

Next update will be after we check the Inlet Valve.

[Bob Cook]

Does the orange light come on (over temp) when the heater shuts down ?

If it overtemps then it must be combustion air (not enough). The thermosat should cycle the heater. If you run the thermostat all the way down does the heater cycle at all ?

Perhaps you are looking at a wiring problem and not the heater. A chaffed wire ?

90% of the time they just don't fire. This one is really strange since you have replaced the heater in its entirety.

[MikeZ]

Bob: Yes the heater OH light does come on. There's a new combustion air blower as well.

To my knowledge, the thermostat does not measure heater coil temperature, it measures airflow temperature in the cabin floor area under the pilot's toe (where the toe valve is). So the thermostat would cycle the heater based on airflow in the ductwork, but if the airflow is not adequate over the coil, then the thermostat would be calling for heat, the heater would be firing, but the coil would overheat.

Since the gremlin happens every time with the same pattern, i.e. runs for 20 seconds, light comes on, heat shuts off, I would doubt an intermittent short, because then run time would vary. On the other hand, it seems to run longer on the ground. sigh.

Maybe it is some vibration induced problem.

I'll let you all know what the inspection of the intake valve turns up, in the meantime I'll bring a blanket and hot drink on my next trip north. Thanks.

[MikeZ]

Hey all.... finally got it, after mucho dinero.

Analyzed system drawings and deduced that inadequate airflow over coil was causing overheating. Traced all parts and cables and found that the front engine pressurized air temp control runs two cables, one to the diverter for air in the face above the controls, and one to the inlet valve at the pilot's toe in the firewall. This cable had come off the panel control. When the valve remained in the "warm" vs "hot" position, forced cabin air bypassed the heater coil, therefore the coil was not getting ariflow and overheated.

So now I have a new heater, and every part and control is new or OH as well. Not bad for a 20 year old plane to have a new combustion heater. Too bad it wasnt really necessary. Such is life.

regards,

I'll fly warm again.....