I was writing my post while yours came in, so I just saw your post.

Everything worked fine last year even after installing the heat pump (although low speed was marginal). Something changed over the winter that added 2-3 psi of backpressure. I had thought that the "floc" might have munged up the filter media, but the difference between running the filter on "filter" and "bypass" is only 1 lb.

Perhaps something got damaged or plugged in the heat pump over the winter. I did cover the open connections with screen to keep critters out. I gravity drained the heat pump (I don't have a shop vac), so maybe there was some freeze damage if there was water left inside. I did use a garden hose to run water through the heat pump in both directions before hooking it back up this spring. Do you thing that might have damaged something? (I'm having trouble getting hold of the heat pump mfr.)