Originally Posted by macdoe

Yes, That's correct Kerry. I think the idea with this aftermarket "lower rad hose" type heater is basically the same as a factory blockheater except for the location. The o.e block heater is right in the freeze plug hole in the engine block as you mention and the aftermarket heater in the lower rad hose. I think the coolant in the lower rad hose is heated with an element (same as o/e block heater) and because it is low in the circuit allows for convection heat to travel up the hose and throughout the block, maybe even throughout the rad itself ?.

Like I said before it did a better job keeping the block warm for me up in the cold last winter than the o.e unit that was in there before.
I should mention that since posting previously I have gone through the service records of that particular car and found it to have had a new block heater installed in 2004 and burnt out in 2009. I am not sure what brand of block heater it was that was installed so I really should'nt use the term o.e for my particular circumstance in hindsight. It could have been a cheaper jobber unit put in to replace the original at that time I don't know.The car in question is an 85' model so the original if replaced in 2004 lasted for 19 years.(pretty good)

I was a little cocerned with the element affecting flow of coolant in the lower rad hose for summer operating temps. I kept close watch of the guage this summer and noticed no difference in operating temps.
I think the newer rad hose heater was quicker to heat up the coolant and therefore more efficient.
I noticed my power consumption bills get pretty expensive in the winter when plugging in 2 of these old diesel block heaters. I will definately be using a timer this winter. The lower rad hose unit I expect to perform better/cheaper with a timer since it is quicker to heat as mentioned.