@TomC: I don't think anyone here is opposed to trichlor or inline feeders. The issue is that trichlor adds 6ppm CYA for every 10ppm FC and you have to increase target FC as CYA increases. This can be a little hard to grasp, setting a CYA level and maintaing the matching FC is eaisier to teach. A common trichlor scenario is the Green August Pool. A great many pools turn green late in the summer because thier owners are trying to maintain a FC level that's too low for the CYA that's built up.

Your experience of losing CYA over the winter is also common. There are bacteria that consume CYA and release nitrogen compounds sometimes as ammonia. This CYA consumption can happen quite quickly once FC is depleted and the bacteria is allowed to grow. You may or may not have observed that sometimes it take alot - ALOT of chlorine before chlorine will hold in the spring. This is often the result of ammonia formation.