The PowerGuard for WaterFurnace and is considered by WaterFurnace to be the OEM replacement MERV 10 air filter for all their Geothermal Comfort Systems that use a 28 x 30 x 2 pre-filter.  The MERV11 version has been discontinued.

The replacement filters used with WaterFurnace units that use a 28 x 30 x 2 pre-filter must use a filter that is a full 28 x 30 .  If you use a smaller filter, such as many of the 27 1 2 x 29 3 4 filters sold by other manufacturers, your filter will not seal properly, and you could get unfiltered air leaking around the edge of the filter, this is called bypass.

The PowerGuard MERV 10 air filter is approximately 28 x 30 x 1 3 4 and is the size recommended by WaterFurnace for a proper fit.

Common WaterFurnace Geothermal Heat Pump Models
that use the 28 x 30 x 2 MERV 10 pre-filter:

Envision Series:
Single Speed Vertical Units: NDV036
Dual Capacity Vertical Units: NDV038

Legend Series:
Vertical Units: LSV042, LSV048

The manufacturer suggested replacement interval is 4 to 6 months, depending upon conditions and lifestyle.