Aztec Heater Can't Keep Up

N6537A

"79 "F" Model Aztec with Janitrol Heater. We live in Minnesota and to justify the plane need to be able to fly it year-around. Entering our 4th winter with this plane, and have NOT figured out how make it warm (or more accurately, not cold) in the cabin, even in mild by Minnesota standards, temperatures (mid-teens Fahrenheit). Heater puts out heat, although air volume feels a bit anemic. Have tried all the combinations of front/rear/defrost imaginable, with no great solution. It's not "see your breath" cold, but after a 2-hour flight, you are definitely cold-soaked. Let's just say there is no shortage of "Spousal Input".

Curiously, a few days ago, I was flying some currency approaches with my co-owner. No sun & 18 degrees F. When we hit the glideslope, reduced power and started down, both of us felt a noticeable rise in cabin temp. If it were just me, I'd shrug it off as perception or an anomaly, but we both felt it. Anyone have any ideas? Maybe the long Aztec nose was "shading" the fresh air inlet on its underside? Reduction in airspeed? Not sure, but we're pretty desperate for ideas as to how to increase the cabin temp or this Aztec is headed down the road, which would be a shame as we like almost everything else about it.

Any thoughts, ideas, insights most welcome.

empannin

Some thoughts for you...

1) can you pre-heat the plane on the ground with an electric heater or even one of the portable, low cost diesel heaters?

2) I think the heaters work by modulating fuel flow to keep a temp set point. (by cycling the fuel solenoid). If your airflow is low the system will still work but the heat delivered will be much less. The system uses two fans, a combustion blower for the flame and a circulation blower for the cabin.

I would check for issues with the brushes for the circulation blower and any obstructions in the ram air system. or post blower distribution system. you should be able to snake in a flexible inspection camera system used for inspecting pipes, etc.

Another consideration is do you have any door leaks or cabin air leaks. Can you fully close off the fresh air system? I ask because air will only flow with differential pressure. If the cabin air is "high" then the fan will be ineffective.

For comparison, below 10k feet with the system working fine and on high you should be in t-shirts and shorts after 2 hrs... You are right to question the system performance and there is enough expertise in the forum we will get you sorted out.

Merry Christmas!

planewrench

Merry Christmas n

Realistically that heater should cook you out of the airplane. So either you have an air flow issue or you have a very poor combustion process going on in the heater itself. First off I don't need to tell you how to operate the cabin air flow controls but it's imperative that the airflow controls for the front and back be open minimum of halfway or full to get air flow into the airplane. If you have those closed down and calling for full heat with the temp knob pulled all the way out you run the risk of doing an over temp and that has to be reset on the ground. But that doesn't sound like your issue.

My first test would be to have the first three full knobs on that lower panel all the way out which basically is calling for full heat front and back and then see what you get. Carl

N6537A

empannin & planewrench...Thank you for the feedback!

"you should be in t-shirts and shorts after 2 hrs." I wish! I'd settle for a lighter jacket.......

We have had the control knobs set right, although we've definitely "played with" shutting the back off (second knob from left) to try and get more to the front seats when no passengers involved, leaving the first and third (front seat & temp control) max out, with little or no improvement.

Your statement "The system uses two fans, a combustion blower for the flame and a circulation blower for the cabin." has me intrigued. Question: When the 3-position toggle switch for the heater (off, fan & heat) is set to fan, I hear a fan running. I assume that's the circulation blower only. When switching from "fan" to "heat", should I hear an additional fan kick in? If so, I'll try that (on the ground, heater side-panel removed).

Additionally, I have determined that the cabin fresh intake (cable operated door over the incoming end in the nose, IS in fact sealing. Also, I've sealed off the fresh air intakes in the dorsal ahead of the tail. While not airtight, the plane isn't particularly drafty (one-piece windshield re-sealed, door seal seems OK, etc.

While I used to operate a Mooney with a heater that would cure pottery, this is my first gas-fired, remote heater experience, and it feels to me like its an air volume issue more than an air temperature issue. One last data point: There's a vent directly to the outside world behind the third seat. POH says that's to vent the air that the heater flow is displacing out (kind of like the '73 Bronco I had that would give no heater or defrost output unless you cracked a window). On a cold trip to Fargo last week, the second seat passenger reached back and said that there was cold air coming IN that vent, even though we were flogging the heater for all it was worth.

Thanks for thinking about this!

Keith

N6537A

planewrench

Keith where are you located I'm here in the twin cities if you're in the upper Midwest. Carl

planewrench

And no you won't hear the combustion air blow. You're hearing the regular fan for air. Combustion air blower's mounted on the heater itself. Carl

N6537A

Carl- I’m at Crystal (KMIC). Heated hanger on the east side. Love to have you look at it of you’re at all close.

Or happy to fly it to you!

Tbanks

Keith

planewrench

Hi Keith my airplane is based over here at the lake Elmo. I'm due for a drive over to Crystal havent been there for a while since I worked on Eric's airplane let's plan on getting together sometime and let me see if we can figure out what's going on with that poor thing. Drop me a text at my phone 651-338-2275 and we will connect. I live here in Forest lake close to the airport here. Carl

jolexa

Nothing to add. If you meet up. Let me know (also KMIC east side D11)

empannin

Keith,

the combustion blower is only on with the heat on AND the circulation blower on. You would not hear it but it must be working at some level for the system to heat at all.

I think your air volume is low for reasons unknown. Carl would be a huge + in troubleshooting.

I would hook up a GPU and run it on the ground. Is the airflow better or worse?

Technically the air flow from ram air should be more than sufficient in flight and the circulation blower is redundant. One theory that matches symptoms would be if the ram air tubing in the tail was disconnected from the ram air inlet.

If this was the case the blower would be pulling air from the tailcone and the tailcone air is likely negative pressure vs the cabin. You can test this running on the ground by temporarily covering then ram inlet with a piece of paper. It should suck flat if it is working.

You should be able to visually see these connections in the tail too.