So, I have an analogy I usually use for people who want to give heat to their outside chickens who are already acclimated to the weather. I tell them to put on all of their winter gear, go out in the cold for an hour or two, and then come back in, leaving all their gear on for an hour or so to see how uncomfortable it could be. In your situation, I would suggest the opposite. Since your ducks are not properly acclimated to the cold (meaning they have not had a chance to grow their down coats),after spending the night in your warm, comfy house, I would suggest that you go outside without your winter gear for a couple of hours, maybe hang out near the heat lamp or heat plate for a bit and see how comfortable that would be. The point is, consistent temperatures - whether warm or cold - are far better for your birds than in and out. If you want them to be warm, then keep them in all the time rather than putting them out in the cold without proper acclimation.