choosing an indirect fired water heater
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Water softeners are notorious for shortening the lifespan of sacrificial anodes. The water conditioner may have contributed to the early demise by doing it's job a bit TOO well, rather than not well enough at some point in it's career. Annual inspection/swap-out is likely too aggressive a schedule, but every 3 years wouldn't be out of line for softened water. The glass liner is the first line of defense, the anode is only necessary after the integrity of the glass has been compromised.
Tankless coils are fairly low performance even when new, and all will scale up eventually, and are the least efficient way to heat hot water. Oil boilers are also pretty low efficiency (~40%) in hot water heating only mode, even with indirects (though tankless coils are usually even worse), and at the current price of oil and electricity, even 18-20cent electricity, a (comparatively inexpensive) electric tank can be a cheaper way to heat the hot water. As long as the embedded coil can sustain flow (as opposed to not being able to deliver full temperature at high flow), it can be used to pre-heat water going to an electric tank during the heating season, and just turn off the boiler during the summer, letting the water flow through unheated, then in winter the heat purged from the boiler by hot water flows will improve it's average efficiency by lowering the average standby loss.
If you install an indirect, (or even if you don't), install a heat purging control/economizer such as the Intellicon 3250 HW+ or the similar Taco unit, which will improve the boiler's overall performance by pre-cooling the boiler near the end of calls for heat, and heat-purging it at the beginning of new calls for heat (down to the programmed low-limit), which cuts significantly into standby loss.
At 40% hot water heating efficiency a gallon delivers (0.4 x 138,000=) 55,200 BTU to the water.
With 0.90 EF electric tank each kilowatt-hour delivers (3412 x 0.9=) 3071 BTU to the water.
So a gallon of summertime oil is equivalent to (55,200/3071 =) 18 kwh
At $4/gallon that's the same as heating hot water with ($4/18=) 22 cents/kwh electricity. If the client's electricity is much cheaper than that (which it is, for most of CT), an electric tank in series might be the better option. Even if oil prices drop, say it hits $3, that's still the same as 16.5 cent electricity, which is close to the current CT average.
No matter how the water is heated, at these energy prices it pays to put 5/8" wall closed cell foam insulation (not the cheap 3/8" goods found at box stores) on all of the accessible hot water distribution plumbing, and all near-tank plumbing, including the nearest 6-10' of cold water feed and the T&P valve and outflow. If the client is a showering rather than tub-bathing family there's decent payback with drainwater heat recovery heat exchangers too.
Last, (maybe least, since it's a stretch well beyond hydronic heating or hot water issues), anybody heating with $4 oil who can heat a large zone with a ductless minisplit heat pump would see a simple payback of 5 years or less on the heat pump, maybe even 3 years with the $1000 rebate subsidy being offered by the state, even with 18-20 cent electricity. In a CT climate a better-performance unit would deliver an average heating coefficient of performance (COP) of over 2.5, and in some instances even 3. At a COP of 2.5 each kwh delivers (2.5 x 3412=) 8530 BTU of heat into the room. A typical 3x oversized 85% boiler has an as-used AFUE of less than 80% (not counting distribution losses to unconditioned/semi-conditioned space), and at best would deliver 110,400BTU of heat into the space per gallon.
So the worst-case mini-split against best-case boiler, the gallon of oil is equivalent to (110,400/8530=) 13kwh used in the mini-split, so $4 oil in the boiler would be equivalent to ($4/13=) 31 cent electricity in the mini-split which nearly 2x the actual cost of electricity. Even if oil hits $3 it's equivalent to heating with 23 cent electricity, which is still well above actual electric rates. Units with an HSPF of 9.5+ will average well above 2.5 for annualized COP. If sized large enough they can handle 100% of the shoulder season loads, with a COP greater than 3.5 when the average daily temps hit 40F and higher. (Seen any sub-$2 oil recently?) During the low-duty-cycle shoulder seasons an oil boiler at typical oversizing is at it's worst efficiency, and without a purge-control will be averaging 60%, maybe less (but maybe 70% w/purge-control.)
In CT winter is warmer than in eastern Idaho, comparable in temp to Boise/Twin-Falls, but cooler than the Willamette Valley see the Metered COP averages in Table 33, p. 50, (p.63 in .pdf) in this document.
Mini-splits and multi-splits cost about $2.7K/ton, installed, in our part of the world, and before swapping out a functional but low-efficiency oil boiler, for the same money even a 3-head 2.5-3 ton multi-split ought to be at least considered, using the aging oil-fired beast as the design-condition backup and for those relatively few mid-winter hours when the dew-point is too close to the air temp to get much capacity out of heat pump spending half it's time in defrost mode. (Defrost on a ductless cuts only modestly into efficiency, but does affect it's capacity under low-temp, high humidity conditions, say when it's both 15F and snowing.)
When spending $3-4K/year in heating oil, or propane it pays to look at all options. A fuller-implementation multi-split can cut the oil bill by more than 80-90%, replacing that portion with $1-1.5K in electricity. It doesn't take a century to pay off the ~$8-9K installation cost for a 3 ton multi-split.
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