It must be an HP18 to be a heat pump. An HS18 is a straight air
conditioner. That system was rated around 8 SEER.
Those high efficiency pleated air filters ar highly restrictive to air
flow. This will cause the compressor discharge pressures to run high
in the heating season, often high enough to trip the high pressure
switch. It will be worse in mild weather, which is why you are having
trouble this winter. Pleated filters often reduce air flow as much as
20%. Go back to the cheap blue filters. If you must have pleated
filters, put in a new system with a variable speed indoor blower unit,
as they generally are a more powerful blower.
Prices for a new system will vary with brand, size, efficiency,
contractor, and area of the country. If you live in LA, it will
generally be more expensive than if you live in the boondocks. If you
need duct mods, that drives the prices up. My prices vary from $3500
to $10,000 depending.......