MY HOT WATER COMES FROM MY FURNACE THROUGH A HEATING COIL. SLOWLY THE HOT WATER VOLUME IS GETTING LESS. I HAVE HARD WELL WATER. CAN I USE MURATIC ACID TO CLEAN OUT THE COIL OR WILL I DO MORE DAMAGE THAN GOOD? THE SYSTEM IS 20 + YEARS OLD. I DON’T KNOW IF IT HAS BEEN DONE BEFORE, I MOVED IN TWO YEARS AGO.