May be an overkill, but I tried Python with the Field Calculator:



ArcMap 10.1 How to make mass (in bulk) update by attribute value
We have a Navien on demand water heater. The intake and outlet vent pipes run out through the poured concrete foundation and have a plastic grill on the end.
If the intake vent is blocked in any way the water heater will not turn on as a safety feature.

I have to add another tighter mesh over the intake in the summer or paper wasps build nests inside the pipe.  If I leave the mesh on then in the winter it gets blocked with snow and ice.

I need some sort of vent cover that blocks insects and does not get blocked by snow. Here is a sample with the "as delivered by the contractor" on the right and with a crude but workable filter for summer use on the left.



Any ideas? 

Edit: the manufacturer suggests adding a PVC T valve at the end of the outlet.  The number of elbows allowed is governed by the length of the run and number of previous elbows in the run.
I'm going to try a T valve and the filters shown in the answer below.
