Aluminum liners, while legal for use with gas appliances, may experience several problems. They are not durable and can corrode (or be chewed) easily. The photo shows a hole in the raincap, likely chewed by a squirrel. There was a bird nest in this flue. The photo also shows that the liner is in direct contact with the concrete crown. This will prematurely corrode aluminum. The picture also shows that the mortar crown has deteriorated and there is an open hole into the chimney, where there was a squirrel nest. We replaced this liner with stainless steel.