Dedicated low-flow systems are designed specifically to have a blowdown valve so that you don’t have any water standing above the frost line in the wells. QED had a bit of problem at a site in Juneau, Alaska, where the small, quarter-inch diameter tubes would freeze very quickly.  But as long as we had them installed where there was no water above the frost line before we started to purge and sample, we had no problem whatsoever getting them to work.