Winter eggs and freezing

Hey everybody, i was just wondering if someone could answer this for me, i have 10 Rhode Island Reds that i make a good liitle bit of money off of selling eggs. Now that winter has really come around i am getting lots of frozen eggs, allot just crack and i can tell, but today some of the eggs felt heavier than usual so i cracked one open and i saw that there was a little bit of the white frozen, like a little thread, not even solid just off color from the rest of the white and crystaly. Is that allright to sell or should i just get rid of all eggs that are even a little bit frozen. Also, is there another way to tell if the eggs are frozen or a little bit frozen? BTW i cant get heat lamps on the are where they lay, i usually get them before they are frozen but it was super cold the last few night and im sick and off school so i slept in. Thanks!!!