Depending on the layout, I'm trying to change how you look at these things. Your EWT is your loop field out, BUT your heat pump IN. Your LWT is your loop field in, BUT your heat pump OUT. So, on most of our systems that would be a temperature difference running through the heat pump. This is a very short run in most cases - say 10'. Ignoring the fact that it is plumbed to 100's of feet on the other end. If you have an internal building loop feeding a few heat pumps, then it may be eventually picking up building heat if left to run long enough. So if your heat pump isn't running, there is very little that would change the temperature in that short of run outside of instrumentation error or some plumbing setup I can't seem to fathom right now. It is not so much - why is it coming back colder than what is heating it back up before going back down? When I see this, I think instrumentation error as I know how off some tridicators become over the years.