Glad to hear it's resolved

Hi just reading your post and had a thought the crystals that you replaced are there to stop water ingress into the system seeing as you said it happened on frosty mornings I would say you have water in the system which may be freezing or have corroded internal workings giving you your fault just a thought you may have cured the problem but still have the fault because you haven't purged the system of contaminants hope this helps .