I have a sixty year old cast iron, in floor radiant heating system that developed sludge starting last December. The screens on the mixing valve clog up every few days with a gray sludge that is the consistency of peanut butter. It is worse on the boiler side than the return side but both of them clog up eventually. Could this be caused by a sealant that was added (not sure anything was) and if so, what can be done to clean out the system. The water looks clear and there isn't any sign of rust.