The problem I see with the warming huts is that the calves become accustomed to the heat and won't do well if something goes wrong with the system. If you know a cow will calve in extreme temperatures, its best to be there and towel them down after mom is finished. After their first drink either bury them in hay or put a jacket on them for a few days. I have one of my Jersey cows timed to calve in Feb. when temps are around -22C/-8F. This year its looking like it will be much colder and I will use a jacket. The calves will be out romping in the snow as soon as they get their land legs. The danger is when they are dropped outside during birth. If that happens, nothing will save them if you are not there to get them inside. Two to three days tops in a small area I can man handle them in, and then out they go, is how I do it. Just make sure there is shelter from wind and bedding to snug up on. Now, if we are talking dairy calves without a mom, then the huts make some sense as the natural parameters have already been changed as the calves don't have ready access to warm milk when its needed. I still wouldn't heat the hut, but use a jacket. I am a bit out of the norm these days as I believe that barns are a human invention and animals were not evolved to live in them. I do not have the vet coming out for pneumonia and other bronchial diseases like many farmers around here do. I have also noticed that all of the new dairy barns being built tend to be open air now; not completely enclosed.