We have a brand new house, state of the art windows, insulation and HVAC. The house is kept at 14 degrees C and the outside temps rarely go below -10 degrees C and hovered around -2 to +5 during the month in question. The house is roughly 2000 square feet.
During the last month, the furnace has used 386 liters of propane costing us over $300.00. Even though it rarely runs. It is rated at ~50,000 BTU. This house's insulation is excellent. Without running the furnace at all, the house will stay at between 13 and 14 deg C overnight with just the heat from the appliances, lights and Dogs.
My question is... Why would the brand new, state of the art furnace cost far more than heating the house with ceramic space heaters? I have soaped the connections from the tank to the house and found no leaks. The HVAC guys that installed it refuse to come out and check the system, saying that it is fine. I find it odd that the furnace is using this much propane, approx 11 liters a day, translating to about 258,000 BTU per day. Something is definitely not right here.
I understand that this is not enough information to pinpoint the problem, but suggestions as to where I can look would be greatly appreciated.

The only thing I can offer is to make sure the system was converted to run on LP. If your running LP(propane) through a furnace that is set up to run natural gas that's your problem. If the system was converted you will have warning stickers on the furnace showing the unit was converted to LP with the conversion chart.

Is the 50K BTU input or output? At 11 liters/day, you are using about 3 gallons of propane. Looking at your outside ambient temperature ranges for the month, this may not be that excessive. You may have "state of the art" windows but even they are only R-3 +/-. Depending on how many and where located makes a difference and cubic ft. volume not sq. ft. makes a bigger difference. 2000 ft.sq. with 12 ft. ceilings is 50% more volume than 8ft. Ceilings. It looks like your heater , if 50K output, ran 5 hours per day or roughly 20% of the time, 12 minutes per hour. At those temp ranges, doesn't seem that bad. What is shocking is the price you are paying for propane! I live in North Georgia and in October added 200 Gallons of propane to an owned tank for $260 USD. You paid $300 for 90 gallons. Where do you live?