My findings were consistent with what I was finding on Youtube
did a test in my shop this weekend using a 20 lb bottle that cost $20.00 to fill. It's not an exact test but pretty close Sat I ran the diesel heater it used about 1.25 gal for 24 hours the next day very similar temps it used the 20 lb bottle. On both day's the temps was 55 deg in the shop, outside temps at night were about 13 deg
That can’t be a scientifically sound empirical test. I gotta do the math.
Propane is 92,000 BTUs/gallon
Heating oil is 137,000 BTUs/gallon
There’s 4.4 gallons in a 20lbs cylinder. Or $4.50/gallon.
Your diesel is $4.40/gallon.
There’s 50% more heat in a gallon of diesel.
That’s 66% less expensive than propane.
and cost 1/3 of the price of propane
It’s 2/3 the cost per BTUs of propane, not 1/3 the cost of propane.
diesel heaters apper to be very efficient when fired low but are gradually less efficient the hotter they are fired.
I’m not sure you’re referring to efficiency or consumption. Higher consumption doesn’t necessarily mean things are less efficient. Just that more heat is being made.
Lp heaters vary fro %60 to nearly %100 efficient
I don’t think there are any honest efficiency ratings for vented propane heaters above 94%. The nameplate efficiency for those that claim higher do not factor the kWh of the powered venting and circulator(s) required to extract that much heat.
Many RV furnaces and vented but unpowered propane heating appliances are 60% efficiency range and most modern hydronic furnaces are 91-94%.
Check out the Chinese diesel heaters they are very efficient
85% efficiency.
20 lb bottle that cost $20.00 to fill. It's not an exact test but pretty close Sat I ran the diesel heater it used about 1.25 gal for 24 hours the next day very similar temps it used the 20 lb bottle. On both day's the temps was 55 deg in the shop, outside temps at night were about 13 deg and the day didn't go above freezing. Diesel in my area is running about $4.80 per gal.
I don’t know what propane heater you are using but that sounds like a very inefficient heater.
“
not an exact test but pretty close”
It’s not close at all except in your circumstances.
I know I probably sound argumentative or difficult but I don’t mean to be. I just want to make the point that cost per gallon, and cost per time period, should not be used to make
efficiency statements because the other factors in each circumstance determine the efficacy (efficacy
not efficiency) of fuel used and the specific appliance it is used with- all else being equal.
If you have an infrared heat ‘thermometer gun’ you can probably make a wildhat observation of efficiency changes with a diesel heater at various output temperatures. You
may or
do theoretically lose a small percentage of efficiency at higher heat volume settings because the air movement over the aluminum heat exchanger canister is only able to receive so much heat per volume; the output will become hotter but only due to more BTUs in the burn chamber- which of course will raise the combustion gasses’ temperature. But I’m not certain that equates to a significant change in overall efficiency by
percentage. 
