First off, thanks Dana!
I'm stuck with the 110, I'm going from a 140,000 btu boiler and a 80,000 btu water heater. I'm surprised the 110 is going to be too big considering it only has a 14 gallon internal domestic tank to heat water plus heat the house too, I'm running a recirculating pump for the domestic also. But , then again My ignorance gets me shocked a lot these days and I'm certainly not saying you are wrong to have a smaller boiler but this is what I got! I'm feeling like the customers I had replaced the old conversion burners with a Lennox pulse........25% the physical size, they just couldn't get around the much smaller size and btu.
I've got a pre 70's 3000 sq. ft house with 900 of it in the basement. Over half the house is over crawl space, single story with cathedral ceilings, 18 ft in the middle.......lots of windows (410 sq. ft.) just to give you an idea. I had originally done an online heat calc and came up with close to 30,000 btu/hr. If my math is right I'll be running close to 3000 ft of pex. A couple of the zones are only 280 sq. ft but they both also have 110 sq. ft of double pane windows. Don't think it's a grandiose home, I've got lots of work to do!
The original plan was to go with a grundfos alpha 15-55f pump with high flow Honeywell zone valves going to the radiant manifolds. Would that be my closest save? I've dug a pretty good hole to bury myself in.
Your old boiler had different radiation (maybe enough to balance with the boiler output, maybe not) and more thermal mass, and you probably had only one or two zones(?).
A boiler with a MINIMUM modulated output of 28,500 BTU/hr for a house with a 99% design temp heat load of 30,000 BTU/hr is a LOUSY match, even if it were operated as a single zone. It's guaranteed to never modulate! When you compound that by chopping it up into 6 zones, it's going to short-cycle itself into low efficiency on zone calls, and into early grave. With any of the TT -110s with low-mass radiation you're guaranteed to only have reasonable burn times 5% of the time at your design heat load & zoning, and that's only if you dial in the outdoor reset curves to perfection.
If you're "...stuck with the 11o..." you won't be stuck with it for too many years- you'll short-cycle it to death, unless you either reconfigure the heating system topology to include a significant amount of buffering thermal mass, or run it as a single zone. As a single zone with the reset curve dialed it'll run reasonable burn times and far fewer cycles during calls for heat, but it's probably not what you were looking for.
There are no combi-heaters out there other than tank-types that will work with your zoning as-is unless you add a buffer tank, at which point it makes more sense to go with a tank-type combi heater.
There are several boilers out there that modulate down to under 10K, some go as low as ~7,600 BTU/hr out (eg, the
HTP UFT-80W, there are a few others ) which would do just fine with your zone setup, and would have plenty of burner to serve up domestic hot water with an indirect HW heater as the priority zone. Even though it's oversized for any single zone, it's reasonably sized for the whole house load, and which you get the outdoor reset curve dialed right you'll be guaranteed to have overlapping zone calls, and it'll burn almost continuously throughout the winter.
Water velocity in the thin manifold and cavitation issues are the least of it. I can't/won't spec a pump for you even if I had the full system description to calculate the pumping head and flow requirements from, but somebody with the right design software tools could (for a price.) This is not a design-by web-forum level of fix. There are folks posting on this forum who could do the necessary re-design for you and specify the "save" well enough for your heating guy (or a dedicated DIYer) to manage it. But it starts with the real room-by-room heat loads and the room-by-room radiant component details that are already installed.
If I had to guess, a tightened up 3000 foot pre-1970s 2x4 house in Boulder would have a heat load closer to 45-50K @ 0F (Boulder's 99% outside design temp) than 30,000 BTU/hr but that's not going to be enough to save you here. If you have a fuel use history on the place, let's run fuel-use against heating degree days to at least nail down a reasonable upper bound on it.