I have a commercial customer with a split air conditioning system for a room they store micro-film in. The air handling unit has cap tubes for the metering device. The evaporator coil has a history of icing up intermittently, it seems to happen in extremely warm weather. It is only a 1 ton system. When it ices up it forms a large block of ice. I just wondering if this is happening because cap tubes can not compensate for changing load conditions since it does not happen all the time. Hopefully someone with more experience working on cap tube systems can shed some light on what may be wrong.