A client recently told me that during the depths of last winter, he was spending upwards of $1000 per month on propane. My first thoughts were that the house was too leaky, or not insulated, or that the ductwork was shot.
But, through diagnostic testing, I realized that the real problem lay in the appliances he was using to heat his home. One propane water heater tank heated the radiant floor in the basement, a propane furnace heated the rest of the house, the domestic water was heated by a separate propane water heater tank, and there was a propane water heater tank that heated the floor and water in a detached studio. Three propane water heaters and a large propane furnace, all heating the air and water in a large house.
My recommendation was to simplify and step away from the heavy reliance on propane. I suggested that he get rid of the two water heaters for the domestic water and the radiant floor, and instead install a 98% efficient propane boiler with a split heat exchanger. This keeps the radiant water system on one side of the heat exchanger, and the domestic water supply on the other. Making these changes also solved the propane leak, blocked air supply, and back drafting problems that I found during my testing.
Then I suggested that he tie an air source heat pump into his existing furnace. The heat pump uses the same technology that keeps a fridge cold, but it pulls heat from the outside air and releases it into the inside, and is tremendously efficient at doing so. The furnace would kick on when the outside temperature dropped below the rating for the heat pump, generally when it is colder than about 20F (depending on the model and size).
For the final piece if the puzzle, I recommended that he remove the propane water heater tank for his detached studio and install an air source heat pump water heater in the garage adjacent to the studio. This would easily feed the low demand of the small radiant floor, and would also suffice for the occasional shower or hand washing.