What I'm trying to wrap my head around is what place in Seattle would have a heating load OR hot water load that would call for TWO 175KBTU burners???
Even running the heating system at high temp and only pulling ~85% efficiency that's still 280KBTU/hr output, which is a bit more than 10x the heat load of my not-superinsulated house at Seattle's outside design temps, and enough to supply a good half-dozen 2gallons per minute showers.
Turn off the recirculation pumps- they're just a waste of energy designed to wearing out the burners by short cycling, especially if the plumbing on the loops isn't insulated.
Does this place have a dual gusher-head master shower with 8 sidesprays or something?
Assuming more conventional bathing facilities, how much gas did you go through last winter? (to come up with a reasonable estimate of the whole-house heat load at design temp.)
What type/how much radiator/baseboard/radiant-floor/whatever do you have for heat emitters?
zl700: "The wearing of valves and flow sensors indicates excessive flow to satisfy the heating side, common installation mistake."
Tankless heaters are designed to tolerate fairly large delta-T- larger than most radiation is set up for. But rather than running a primary/secondary to be able to run a high delta-T/low flow on the tankless (and tweak flow & delta for the actual design heat loads) it seems some people just pump like hell at a 10-15F delta, often at 2-3x the design condition load to limit the output temp to something less scalding for DHW draws. To get 40KBTU/hr at a 15F delta takes an already hefty 5.3gpm, and many systems are probably running 2x that with some monster pump driving it.
"Reasonable" tankless based systems would rarely need more than 2-3gpm with a higher delta-T. Deltas of 80F (or even a bit higher) are not a problem for a tankless- it's exactly how they were designed to operate (40F in, 120F out), even if that large a delta would stress some mod-cons. At 2gpm @ 80F you have ~80KBTU/hr, which is more than 2x my 0F outdoor temp heat load. At 2-3gpm I'd expect flow sensors to last a very long time,since that's well within the bounds of the peak flow ratings, and very much like shower flow rates, lower than tub-fill rates, which a tankless should be able to sustain almost indefinitely.
Higher output temp required for high delta-T combis has negligible effect on combustion efficiency- condensing is all about the return water temp first, followed by firing rate. The sweet spot on firing rate for most condensing burners is between 1/4 & 1/3 of full fire, but if the return water is much above 120F you can forget about condensing efficiency on a tankless, even if firing at the optimal rate. Tankless designs can usually count on very low entering water temps for getting the condensing efficiency, and can be designed for higher excess combustion air than mod-cons, which results in a lower dew point on the exhaust gases. (Mod-cons can be condensing at min-modulation even at 130F return water temps, but I doubt that's the case for most tankless HW heaters. Cood be rong, offen am.)
I have a kludgy tankless based combi running at fixed ~2.5gpm on the tankless, with a min-delta of ~30F, but it can hit ~60F or more during heavy DHW draws. To boost shower performance ther is a drainwater heat recovery unit kicking back 20K+, but by limiting it the tub fills are slower, and count on the buffer tank a bit. By having a buffer tank never short cycles, the tank has a fraction of the standby loss of a recirculation pump. It's not perfect (not even close), but it's been tweaked for the actual loads, and I expect the tankless to hang in for quite awhile since it's never being abused, not on flow, not on firing rate. (It's max input is 185K, and it's usually firing between 1/4-1/2 full fire under all uses.)