Most mosques are freezing cold when there is no service. They use carpets with electric heaters inside to warm it up a little bit. The real problem is the images. There can be no proper prayer where there is pictures of people. Many Muslims even put away family pictures when they pray at home. So, they will have to find a way to cover the pictures that were unearthed when the mosque was converted to a museum. Erdoğan sadid it would take 6 months of work to turn it fully back into a mosque though the first service is to be held next Friday.
Last year Erdoğan himself shut down the calls for Haghia Sophia to be turned into a mosque by saying that as long as people don't fill the Blue mosque for prayers there was no point in turning Haghia Sophia into a mosque as well. There are so many mosques in that district that its impossible to fill them up regularly beyond special services.
EDIT: Just read an article saying that they will try to use a special lighting system to darken the images of people during prayer. Might turn out to be an interesting application.