We have a 30 year old home that has a high efficiency State, direct vent hot water tank.  We also have a air to air make up unit.  Last week we had to call in a gas fitter as the tank would not ignite.  What we found was the external vent was iced over.  Does any one know how to stop the vent from icing up?  The other problem that we have been having is back drafts.  We have the air to air exchanger and keep all of the external vents clear of debris, but we still have problems with negative air flow.  Any ideas on how this can be corrected? 

Thanks JEK