This is typical behavior for a scaled-up coil. There could be, however, another cause. Do you have a good service company? Oil-fired boilers should be checked and cleaned annually, no question about it.

I've only personally dealt with boilers that are, or would be over 50 years old at this point so I have to ask you some questions about how this is set up. Your hot water is supplied by an indirect tank heated by an third zone from the boiler, or is there an another coil in addition to the space heating coil? If the former, you should not have scale build-up on the coil. I an not saying it did not happen. It cold be caused by a leak leading to excessive water turnover in the boiler. That would be hard to miss but most of the damage may have been done before you moved in and then something fixed. If you have one coil and your DHW performance is poor, your space heat may be suffering as well. I don't think that I have to point out that if that is the case, you really need to get this fixed to avoid excessive oil outlays. (If you have one coil supplying hot water and heat with no heat exchanger somewhere, you have some big problems.)

If you have an indirect tank, is there a temp gauge on it? What does it run when you are not stressing it as opposed to use? I suppose there could be something wrong with the tank so that you are filling at the top or drawing from the bottom. I don't know how they are built or how they may break.

The coils/boilers that I am familiar with are not tricky at all to remove and clean with hydrochloric (muriatic) acid. You kinda hafta know a little about what you are doing so you don't overdo it and dissolve the metal. My parents went through a period between village water supply shift to harder well water from spring source and installation of a water softener to fix that. During that period, the coil was removed by (very good) service people and brought back with the annual service. My dad got tired of being without hot water for a half day and bought a stand-by coil. that they swapped out. He figured that it might wear out and need replacement eventually anyway so he might as well buy one "now" and have the convenience.

Ah, I see now that you probably don't have an indirect since you wrote about that option in the future. I missed that.. Are you thinking about an electric "tankless" I don't think that is going to be your best option. If you have hard water, don't do it. it will be a more difficult task than keeping the boiler coil clear. Even if you don't have hard water, I don't think it is your best option.

Are fuel oil , propane and electricity your only options? Just crunch the numbers to see which gives you the best outcome. If it is electricity, get an electric tank or figure out if a heat pump water heater makes sense. (If electricity is less expensive, it will be to heat the house as well so you might want to get an electric boiler if you like the HW baseboard.)

If fuel oil is a less expensive source, and you have hard water, get the coil cleaned and install an indirect tank if you want a permanent fix. (Either that or install a water softener. My parents never regretted their decision. you might need to clean the coil every few years, but it will not be nearly as often.)

You know, if you are in a temperate climate, and this is a hot-start boiler that makes your basement too warm in the summer, you might be better off turning it off and running an electric (HPWH?) tank in the summer. That is especially truer if the basement is humid. Go back to making hot water with the boiler in the winter if the investment in an indirect tank is not too much to recoup. The boiler is idle most of the time under those conditions and it costs a lot to keep it warm. You could consider having your plumber install a flat plate heat exchanger to heat water in your electric tank so you don't have to invest in the indirect and have that take up space in addition to that water heater..

Do you know how much your paying annually for domestic hot water? That would be an interesting number to know to help makes some decisions.

I personally don't care for the set up you have. I think getting a separate electric hot water tank is the way to go. You should get a quote on how much it would cost to wire a 220V service line and piping for a water heater. Then you can abandon the indirect coil.

Explore all the options before making a decision.

The pbs tv show, Ask This Old House, season 15 episode 15: addressed your situation.

Mike's question is a good one. In order to know how much to invest in any efficiency improvement, you need to know how much you are spending. With nat gas, that would be easy with summertime meter readings. With oil, it can be problematic. You get few fills in the summer and maybe only one that overlaps with heating season in the fall or spring. Your fuel oil company probably has records going back to when you started with them and before if the previous owner used them. You can ask them to send you a file with your delivery information. Watch out for rare deliveries that did not fill the tank. The oil company also has hot water factors that they use in their use/delivery software to keep track of summer use. It will probably be, however, a crude overestimate. (Delivery drivers are not that busy in the summer and they don't mind topping off tanks. That is especially true if it is a large company and the drivers don't double as service techs in annual check/cleanings as used to be common.)

I am thinking (hoping) your water heater has its own zone and aquastat. If not then it needs it or it will never operate properly.

We will have to wait for the OP, but it did become pretty clear to me that this is just an on demand coil, no indirect, since an added indirect tank was mentioned as an alternative to tankless.

Here is something else that I missed at the beginning. Replaced inlet and zone valves due to hard water? There should not be enough water going into the system to cause that kind of problem. There is a leak somewhere unless someone is a compulsive system bleeder. On the other hand, if the system is getting air into it so it needs bleeding routinely, that is a different problem to be fixed.

Thanks all! Very sorry I probably don't have my terminology correct - or making up my own :)

Yes - DHW is a coil WITHIN the boiler. As I understand it - there is no "zone" for that. It just sucks heat out of the boiler directly. DHW out of boiler immediately passes through a mixing valve and up to service the house. No storage tank. I do have 2 separate zones for heating - forced hot water baseboard.

Since there is no zone per se for DHW, it is often battling the calls for heat. There is no priority for DHW over the heating zones. This is evident and can cause low temps for DHW when heat is on, but that's not even what I'm talking about in my OP. Mostly I'm talking about the performance of DHW on it's own.

Haven't taken the time to research DHW heating cost vs. heating, but I agree this is a good step. Even if/when I know this - would still need to estimate cost to replace existing tankless coil as an option vs. other options (indirect zone of existing boiler to storage tank, electric water heater, etc...)

Boiler is serviced every year. Have records that indicate such before we bought house as well so confident it's been serviced all its life.

The inlet valve to the heating zones (pressure reduction valve? - sorry I can't recall the exact term) - I have to flip this occasionally and can hear rush of water filling up the zone(s). Am told this is due to it clogging from hard water and occasional flipping temporarily loosens. I should also replace this. BUT this is a separate issue - although might be indicative of problem that could affect DHW performance as well.

Yes - the goal is to ultimately explore all options. For now - first step is to determine why DHW performance is often abysmal.

Perhaps the other way of asking my question is the following: If given this scenario:

- no calls from either heating zones

- no other calls for DHW - only 1 (let's say the bathtub)

- should the DHW out of the bathtub run hot in perpetuity?

I also wonder and assume that the mixing valve could not be functioning properly given the scale buildup we typically experience.

Thanks all for this discussion!

"- should the DHW out of the bathtub run hot in perpetuity?"

Yes!