High and low limit on the boiler

Your tankless coil should have a mixing valve in order to temper that hot water at the boiler room to 120°. Any boiler with a tankless coil can cause scalding if there is no tempering valve. Steam boilerrs operate at 210°F and some of them are equipped with tankless coils.

The concept of the tankless coil is over 120 years old. When folks had coal heaters, someone sold a coil that could be placed in the fire box. Since the heater was already operating all winter why not add HOT to the indoor plumbing. (that was a new thing back then)

Eventually It became a economy thing for some folks. You only need to buy one appliance and get both heat and hot water from it. It cost less than an oil fired boiler and an oil fired water heater purchased separately. Oil heat was always less expensive than electric so some folks with electric water heaters would add a side arm coil to the oil fired boiler.  Later on, when the new homeowner moved in and converted to gas, and there was no room for a water heater tank in the refinished basement's boiler room, you would just get a gas heater with a tankless coil. And now you own one, for whatever reason the installer chose to do that. (you may have room for a separate water heater… I'm just giving an example from years of experience)

I see that you mentioned you have. tempering valve while I am typing this comment…. GOOD THING! 

So the worry about 200° water getting to your faucet or shower is a non-issue. The fact that you need to keep the boiler so hot in the off season is a concern for being energy efficient. When you get time and $$$, you may look into changing that water heater to an indirect and setting your boiler up to be a cold start boiler. That can reduce your gas bill by about 15% in the summer and about 4% to 6% in the winter. That is because the 200° boiler that is connected to a vent that goes outside and sucking heat with it during the off cycle will no longer stay hot when you are not calling for heat. You can also add a thermal purge to re-coop more off cycle heat into your home or water tank.

But for now ,if you don't want to take cold showers, you need to set the low to at least 185° for starters and see if that helps.  

Oil was relatively cheap back in the day maybe $.25/gallon when I started can't remember.

Some boilers work ok with a tankless and some are awful. We used to have tons of the Ever hot external tankless coils. They are still available, but they probably don't sell many.

Back in the day you would drill and tap a Snowman to get the top tapping for the 2" boiler water connection for the external tankless.

In this day and age, a tankless certainly isn't the best choice but I know people with only 2-3 people in the house that learn to live with it and get by ok, I have 3 relatives that still use them.

One house has one person but did have 4-5 at one time. The other 2 houses have 2 people each, one of them used to have 3.

Tankless coils are rated at 200 deg boiler water.

Running 200deg is not something i like doing.

Mixing valve are now much improved over the old ones. A lot old tankless coils never had a mix valve.

I used to set up Weil McLain 68 and WTGO with tankless at 160° Low and 180° High with 20° Diff, and most of these boilers would never run out of hot water.  You must put the 3 GPM restrictor on the HOT out of the coil just before the Hot inlet to the Mixing valve.  That would usually give you 4 GPM of endless DHW @ 120°F. I would always include 2 boiler drain valves and 2 ball valves so you could do the maintenance.  That was long before the Wall Hung Tankless stuff we have today.  The cleaning chemical was ACID called Hercules SIZZLE. And was done about every 5 to 10 years depending on water conditions.   I like the much safer vinegar based stuff we use today, but it must be done more often.   

Most plumbers do not know how to do this, that’s because they would rather condemn the tankless and install an Electric or Gas water heater.  There was more money in that job. Your Oil Company was an expert at doing this Tankless Coil job and maintaining them because they would sell more oil that way.   And the price of operating an electric water heater was more expensive , but the homeowner didn't find that out until the job was done.  Then it was too late. 

It's never been easier to add flush valves and mixing devices to a tankless coil. Tankless water heater, or Combi.

The beauty of Sizzle is it could be used as a drain cleaner, or powerful delimer.

Just don't let it spill in the truck.