You went through 10 Grundfos pumps in 28 years? Sounds like iron-impeller pump on the potable side or something?
Polaris has a pretty good track record overall, but I don't have a lot of direct experience with them. (Ping BadgerBoilerMN on this- he posts here frequently, and has done quite a few heating systems based on Polaris HW heaters.)
There's little downside to oversizing the heat exchanger other than a bit of pumping power. Since they are fully counter-flow the delta-T between the potable & system side (sometimes called the "approach" temperature) doesn't have to be much- most are BTU-rated at a 10F difference. But installing one rated for 2x or even 3x the BTU rate cuts that difference by quite a bit, and may even be more inline with your actual radiation flows. I suspect (but don't know without doing the full analysis on the actual pump and pumping heads of each zone) with one zone valve open you're getting at least 2-3gpm of flow and with all of them open you may be getting 8-10gpm. See the BTU rate & min-flow requirements of this series.
Polaris has a pretty good track record overall, but I don't have a lot of direct experience with them. (Ping BadgerBoilerMN on this- he posts here frequently, and has done quite a few heating systems based on Polaris HW heaters.)
There's little downside to oversizing the heat exchanger other than a bit of pumping power. Since they are fully counter-flow the delta-T between the potable & system side (sometimes called the "approach" temperature) doesn't have to be much- most are BTU-rated at a 10F difference. But installing one rated for 2x or even 3x the BTU rate cuts that difference by quite a bit, and may even be more inline with your actual radiation flows. I suspect (but don't know without doing the full analysis on the actual pump and pumping heads of each zone) with one zone valve open you're getting at least 2-3gpm of flow and with all of them open you may be getting 8-10gpm. See the BTU rate & min-flow requirements of this series.