The slimy stuff mostly sloughs, rather than grows on vertical smooth-bore copper drain, around which these are built. IIRC the anticipated lifespan at which it's efficiency has been reduced to 75% of it's day-1 performance is on the order of 40-50 years, based on studies that started in the 1980s when they were first being analyzed. (I don't know if they're continuing to monitor, or if they just trust the regression analysis on the first 5 or 10 years or whatever it was. Manufacturers' fine print usually gives either a 40 or 50 year lifecycle, but they don't footnote what they mean by that.) It does lose performance over time, but it's a very slow ramp. It'll outlast any solar thermal installaion.
The BTU x square footage estimates are JUST the sort of crap that leads to unbalanced room-to-room heating and oversized boilers, which EXACTLY why you want a manual-J type heat load calculation. I'm saying "...20 btu a foot times 1200 feet..." absolutely ISN'Ta reasonable approach to heating design, even if it might come close in some instances. Every good heating design starts with a real calculation based on real measurements. Heating design & heat loss software packages from boiler manufactures aren't all that difficult to use, and produce good results. Don't waste time and money on a contractor that wants to save an hour or two of measuring and data entry to deliver a potentially less-than-satisfactory result. Even if they charge a few hundred for the service to run a real calc it saves on radiator & boiler costs (not to mention aggravation) to get it right the first time. For $50 you can do your own with cheapie packages like
HVAC-Calc . SlantFin used to have a pretty-good downloadable freebie that overshot the whole house number by ~25-30%, but gave the right balance room-to-room- I'm not sure if there's an archive where you can dig it up (IIRC it was called Hydronic Explorer.)
Assuming 24KBTU is the right number (and rule of thumb estimates like that usually overshoot by 1.5-2X, possibly more since you used blown insulation rather than batts and have newer-tighter windows), yes, if the min-modulation of the boiler is over your design-day heat load it's oversized (by quite a bit, actually, but there are very few boilers available in the US with min-mod less than 10KBTU.) A perfectly "right" sized boiler would be running at full tilt at your outside design temperature to keep up. If your real heat load at 6AM on the coldest day of the year is a (more likely) 18-20K, a boiler with a 20K max output would be the right sized, and one with 24K of output at max fire would be modestly oversized. A Solo 60 puts out even more than 2x your heating-guys likely overestimate. But with mod-con boilers, it's the MIN modulation that has the biggest effect on as-used efficeincy- smaller is always better, since it results in longer burns (==less standby loss), fewer cycles (==less flue-purge & ignition cycle losses), and generally greater comfort.
The water volume in the heating system adds thermal mass, and determine the minimum burn length affecting both comfort and efficiency. In mid-April your overnight heat load might be as low as 3KBTU/hr, and your mid-day load half of that. With a lot of thermal mass the boiler can run long enough to be truly efficient, the room temps will be more even, and the time between burns much longer.
On those days your hot water heating load may exceed your space heating requirements by 3-5x, but if it's set up with a tiny internal tank you'll get a lot more cycling losses than if you go with an insulated external tank. The size of the tank + the full-on boiler output determines just how long you get to shower (or how big a tub you can fill in a reasonable amount of time.)
In a net-present-value financial analysis both fuel-inflation and the cost of borrowing are factored in to determine whether it's cost-effective over any specified period. If the present-value of the item is more than the cost of the items over the time period you're considering, it's cost effective. If not, it's not. Some things are never cost effective, even when analyzed over the complete lifecycle of the product. If you're anticipating a huge energy-inflation over the next decade, that will skew the result by a lot. But while energy costs have been volatile since the 1970s, the cost per kwh or therm/ccf hasn't had a clear positive trend- electricity has gone down in inflation-adjusted pricing, natural gas has been both up and down, but with the exploitation of coal seam reserves and recent massive shale gas discoveries beginning to be tapped in the energy hungry northeast, the demand-pressures driving the price higher between 1995-2006 have been largely relieved. A carbon tax would introduce both a step-function on immediate price, and a demand pressure as coal-fired power generation is displaced by gas-burners. Predicting future fuel prices has been, and continues to be, a fools errand (played out daily in the commodities market.) I'm not placing bets in either direction on the price of gas OR electricity, since the cost of readily-implemented efficiency measures are cost-negative even in 1-5 year time frame let alone a couple of decades. (That is, broadly in the market, if not at your house.)
What's not in a NPV calc is the externalities of things like air & water pollution associated with energy production, etc. and what value needs to be placed on those.
Buying the dual-exchanger tank is an upfront cost that gives you an option if/when a signficant thermal array makes clear economic sense. It's a matter of how much you want to pay for that option. Mind you, solar pre-heat unlikely to make more than 100therms or ccf of gas use per year for a 2-person family. If you have a reasonable southern exposure you can probably get that much or more fuel use out of a few thermal air panels at less than half the installed cost.
OTOH, in Olympia's climate and hydro-heavy grid, going with a hydronic air-source heat pump like the
Daikin Altherma for both heating and hot water would have a lower carbon footprint and lower upfront cost than a Solo-60 and a solar array, as well as lower operating costs. See:
http://www.daikinac.com/DOC/PCAWUSE09-09B%20-%20Daikin%20Altherma%20Brochure%20-%20Daikin.pdf
They're not cheap, (but neither is a mod-con + solar DHW) but it might be a better way to go long-term depending on your goals. Your outdoor design temps are high enough that you can probably even disable the backup resistance heaters on the thing and still meet your design-condition load. (I know of an Altherma installation in Maine where the design temperature is about -15F- they definitely need the backup heaters, but you won't- your design temp is 25-30 F warmer than that.)