Janitrol heater and front engine fuel flow
 

I have a Janitrol heater in my T337D that I have never used, but it is serviced at annual. It is usually quite comfortable on a sunny day up front and no heat is needed. however I did use it this last January on a flight to Santa Barbara from Livermore because I had a passenger and they weren't comfortable. On the flight down, I noticed that the front analog fuel flow gage was twitchy. wobbling by about +-0.1-0.2gph. I could detect some engine sound changes with it as well, so I think it was real. In subsequent flights without the heater the analog gage is rock solid, and no engine sound shifts, so it seems like it has something to do with the heater drawing fuel. From the service manual it looks like the fuel is drawn from the front engine fuel strainer, so its part of the fuel loop for the front engine and therefore a possibility. I spoke about this with my A&P and shared a video I took of the behavior. He said he hasn't seen anything like this before, and wasn't super concerned about it based on the magnitude of the wobble, but suggested I post here to see if others have seen this behavior and if so what is the cause or fix for it?. The heater started and ran as expected, producing plenty of heat. I should add that I usually run LOP, so front engine fuel flow is around 9.5 gph in cruise.

I run my P337H at about 12.5-13 GPH per motor (maybe a bit more for the rear), and see no change when I run the heater. I think (could be wrong!) the fuel that goes to the heater is unmetered, but its a fairly miniscule amount IMHO, at most .5 GPH. My buddy put an 8000BTU diesel heater on his sailboat last winter and it uses about 3 gallons per day. Heaters are pretty darn efficient.

Concur - fuel to janitrol un-metered....but from memory
 

I have had a shadin for decades, but recall eons ago when I did have Ye Olde Original Fuel Flow meters.

As I recall, they worked by pressure, to indicate GPH.

If you have a fluctuating flow into the janitrol, for example as it cycles on and off, perhaps that would induce some minor change in fuel pressure, which would show up on the original gauges as minor changes of variable fuel flow?

Yes, they are pressure driven. They sense the difference between the fuel system pressure and manifold pressure in my T model. So its not a direct measurement of flow. I think that a pressure fluctuation on the fuel side due to the influence of the heater is what I am seeing. Because I am running LOP, any fluctuation is noticable as a power change. If I was running ROP, I think it would be much less noticable. I have JPI unit with fuel flow meters as well. Its harder to see this kind of wobble on a digital readout. The one case where analog beats digital.