Tankless Coil

[EquipTech]

I have a 10 year old Slant Fin boiler with tankless heater. I'm on a well with hard water,at one time experiencing "black water" from apparent shale contamination. No solids,just a blackish film for a time. It has since cleared up,but I noticed over a couple years my hot water wasn't as hot as it used to be,it cooled quicker,and the pressure was far from adequate. Assuming correctly the coils interior was plugging with debris,I shut the boiler down in the fall,removed the coil,and thoroughly cleaned it with muratic acid. Knowing what this acid is capable of,especially on something with some years on it, I pressure tested it overnight. Being a mechanic, I accomplished this with the proper fittings and the help of a radiator pressure tester. It held 50 psi perfectly.
I have the HI/LOW/DIFF settings at 160/190/25. My hot water now measures 175,180 degrees F. which is plenty hot,scalding actually, and the pressure is terrific, which is the way it used to be when new,but it doesn't last long at all.
There in lies my problem. My wife showers longer then I,but I'm a Vietnam Era vet,so still take my Navy showers. Even with that,I can feel the water cooling quickly,my having to cut back the cold water mix continously,until the cold is completely off,with the water warm at the most,and this is with the boiler firing up for hot water about 2 minutes into a shower.
Assuming the coil is fine,my settings are correct(per InspectAPedia),and all the peripheral parts are good or working properly, I can't figure it out.
Any suggestions would be appreciated. My wife is getting very irritated with me.

[Dana]

First, muriatic acid it too powerful an agent to use for de-liming coils- try to avoid repeating that. (White vinegar pumped through it for an hour or so with a small pump should do it.)

Tankles coils will crud up both on the boiler side and potable side over time, which limits the rate of heat transfer, even if you have a big enough burner and thermal mass in the boiler that it "should" be able to keep up. De-liming the potable side fixes half the problem, but SFAIK there's no good solution for the boiler-water side. The hotter the water, the faster it will lime up.

If the boiler is in good shape, adding an "indirect" hot water heater that uses boiler water in a much larger heat exchanger inside a hot-water heater tank. If you install it with ball valves in some taps for deliming the heat exchanger on the boiler loop, it should give you good service forever, even if you have to de-lime it every 5 years or so. If you do it that way you can usually set a lower low-limit (or even cold-start) the boiler, reducing standby losses. It can literally double the summer hot-water heating efficiency, and you'll never run out.

They're not cheap, but it's cheaper than divorce lawyers or years of marriage counseling.

[Tom Sawyer]

I agree with getting rid of the coil and going with an indirect.

Muriatic acid is generally exactly what we use to de-lime coils. Very effective, very dangerous. Has to be pumped in using a specialty ceramic pump.

[EquipTech]

Thanks for the replies so quickly. The first answer is roughly what I thought I'd get concerning how to fix the problem,ie: going with an add-on. I've been considering that. I know the coil is clean,both inside and out,and the reason I used the acid is because I've always understood that's the normal product used,and was suggested by both my oil supplier and rep at a large plumbing/heating supplier I deal with for parts. And as noted,I checked for damage,which there was none.
The boiler is putting out plenty of heat. I just can't figure what is different now. The reason I cleaned it was because the coil for that particular Slant Fin are hard to come by,so rather expensive,in the $300. range.

[rrcur]

Any chance a mixing valve, either the one directly after the coil, or the one in the shower valve itself, is malfunctioning? Does the temperature fluctuate at any of the sinks in the house?