D5 pumps differ from DDC in regard to their impeller casing material. D5s have a metal case which transfers heat from the stator and electronics to the water so they don't really get hot. Also the D5 is more of a high flow low pressure oriented pump so it is not driving very hard even at 2gpm levels. The DDC uses a plastic casing which insulates the electronics from the water. In addition DDC pumps are more pressure oriented with a lower max flow rate. This makes DDC pump to drive more toward the max flow rate which is where maximum current occurs. Also the DDC pump electronics are cramped and when sitting on it's base have a hard time dissipating heat build up. Last but not least, the DDC1 was a lowere watt pump, the DDC2 and later generation 18W models did't come in until later. I have always wondered if they were really meant to only be those lower wattage and the 18w model is really just a modifed version and perhaps pushing thermal limits a bit.
All pumps dump most of their heat into the water, probably 60-80% of the power consumed is dumped, the difference is how hot the electronics get which doesn't translate at all to how much is pit into the water.
Heat in DDC pumps is highly dependant on resulting flow rate and restriction. A highly restrictive loop (1GPM) will run much cooler than something low in restriction with high flow rate (2+GPM).
Personally, I would at least make sure you have good moving air blowing under the base on all DDC installs if you plan to run full speed all the time. With a 35x if you throttle down regularly, you can probably do without. D5 no worries on heat. heatsink on DDC always a good thing, probably not necessary, but cooler electronics and less heat dumping into the loop is always a benefit.
DDC pumps are also a bit more efficient with a top.
So a combination of efficiency and casing materials means DDC pumps have less heat dump (into the water) for the same amount of pumping power.
At like speeds D5 is a little quieter:
Both good pumps though..each with their own benefits.Edited by Martinm210 - 2/5/13 at 10:02pm