Oh they get more than the radiator warm, they get the radiator, lower hose, thermostat, all HOT. Thus a lot of heat goes right on into the block. I have tested mine so far as low as 3 degrees out, I started it up in 2 cranks effortlessly with a 5 second glow at this temp with my trusty heater. W/out the heater this would have been a life/death startup involving a loooong glow and lots of cranking. I was amazed at how well it worked. It even melted all the snow off the front 10" of the hood or so, from all the heat rising up and off the radiator etc! I'd say the lower hose heaters will work great down to probably -20 to -30 degs.

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'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)