Halstead Ace High leak?? Please help!!!?

I seem to have a leak from my 'Hell'stead ace high, which causes it to lose pressure. I am no plumber so I will do my best to describe where the leak is coming from. I think it is coming from the heat exchanger, or maybe the small disc-shaped sensor just to the left in front of it. Any help would be greatly appreciated, as my kiddies are getting cold!!!