I had always thought that was the point of the EcoGlow--it doesn't put off heat and so they have to go underneath it to warm up. It's more natural for them that way because it's how it would be if they were raised by a broody. They get used to being away from the heat much faster than with other heating methods, plus they already are on the day/night cycle so there's no weaning them from 24 hour light, either. I think if you gave them some time to adapt, they would be just fine with the EcoGlow even in a cold room. It's supposed to work for temps as low as 50, so as long as it's warmer than that, they should be fine. Honestly, that big window might be part of the problem--it being right at their level would allow for drafts and chickies in particular are sensitive to that.
I agree, though, that it wouldn't be room enough for 20 chicks, at least not for long. Also, once mine started getting on top of it, I was having to clean it all the time and the raised lettering makes that difficult sometimes. Other than that, though, my EcoGlow was worth every cent to me! Can't wait to use it with more chicks this May!