Clifford says his propane boiler takes far less fuel than a diesel boiler to produce the same amount of heat. Considering physics, how is this possible? ...
That depends on what you mean by "less."
I notice Clifford mentions above that propane has more BTU per pound
than diesel, which is true. However, it has less
BTU per gallon
, as you have already noted. The two substances have very different densities, so a gallon of propane weighs just a little more than half what a gallon of diesel does.
So, yes, it takes "less fuel" measured by weight
. It takes more
fuel measured by volume
I don't know about you, but virtually every place I have ever purchased either diesel or propane, it has been sold by the gallon, not the pound. So I find it more convenient to compare fuels by volume.
Right at this moment, the average nationwide prices of the two fuels are virtually equivalent on a per-BTU basis, but that's comparing road diesel to off-road LPG, and average retail price for diesel vs. bulk delivery price for LPG. In practice, we have found that LPG runs us around $3 per gallon on the road, which makes diesel, even with the road tax, the clear winner at the moment. Since we have cylinders rather than a tank, we often get gouged for LPG even more, since many outlets have a $20 minimum for cylinders (we usually talk them into filling both our diminutive 11-pound bottles for that price), which puts the price closer to $5 per gallon.
If you compare off-road diesel pricing to LPG, the numbers get even better. Red diesel is probably as easy to find as true bulk-priced LPG. We can't use it (no separate tank), but we at least get all the federal road tax (26 cents a gallon) back at the end of each year for our heater usage. That makes diesel's advantage even more clear cut.
If you are trying to save money on heating fuel over the life of the coach, diesel is a better choice (at this writing). If you are trying to save weight on heating fuel, LPG is a better choice above a certain amount (below that amount, the extra weight of the tanks negates the savings -- I'd have to do more research to know where the crossing point is).
As far as "efficiency" is concerned, this term relates strictly to how much of each BTU in the base fuel is translated to usable BTU of heating in whatever you are trying to heat (inside of the coach, water, soup, whatever). Generally, it is easier to extract more of each BTU of LPG than diesel at the burner
. However, more of each BTU of LPG is lost "up the flue" than for diesel due to the nature of the fuel and safety considerations. For this reason, it is usually more efficient to use propane for something like a cooktop (and diesel cooktops are notoriously finicky anyway), whereas it is usually more efficient to use diesel for something like a boiler (hydronic or otherwise). Direct forced-air furnaces fall in the middle and are something of a wash; you need to compare them model by model.
None of which means anything without taking into consideration the totality of the system. Efficiency is lost in every step of the process, not just at the burner. So it's not possible to make sweeping statements like "diesel is more efficient than propane" or "gasoline is less efficient than natural gas." So much depends on the design of the rest of the system. For example, resistive electric heat is the most "efficient" of all (every kWh consumed is turned directly into heat), but it is still likely the most expensive option and of limited practicality in most circumstances, so the 100% efficiency is meaningless.