Which Espar Heater?

To answer your original question it is not viable to use the hydronic to heat your bunk because you also need a fan running and that sucks a lot of juice.

If you insulate your bunk well and have a sleeping bag or electric blanket -40C shouldn't be a problem with the D2. At that point getting the truck started could be a potential problem. No longer than 8 hours, full winter front, and have the hydronic running for at least 2 hours.

 

Not complete true. I found D2 to be weak device, it can make air in cab a bit warm, but you can't sleep without blanket though.



D4 looks more preferable for extreme cold weather due to bigger BTU output and less stress to device itself.
Take a look: what makes D2 scream like crazy on BOOST mode, sucking 2.8Amps from battery and 0.07USGal/hr of fuel, producing 7,500BTU/hr. To make almost similar 6,800BTU/hr D4 needs only second gear out of four (MEDIUM), only 1.1Amps from battery, even though it consumes similar amount of fuel.

Price difference on Thermoking is about $500, on other retailers up to $900.

 

That's a good point, thanks for showing this. One possible drawback is you don't have enough steps.in between, which us a common complaint with the d2.