It is not completely clear how your domestic hot water is heated. It the same boiler responsible for that? After that is clear, the steps you should take will easier to explain.

Oops, sorry was a little unclear. The domestic hot water is heated through a water tank/heat exchange with the boiler. This was the water heater that I mentioned I had turned the thermostat off.

And yes, it's the same boiler.

O.K., that makes it more clear. Unfortunately, I have no experience with that kind of water heating so we'll have to wait for someone that does to answer. I am fully willing to believe that the boiler acts independently of the water heater to maintain its temperature. I am also willing to believe that it can be controlled by the water heater in some systems.

It may be advisable to reduce the boiler temperature in the summer as long as you have plenty of hot water at the lower temp. I know of one control system, installed in the 1970s, that modulated the boiler temp with the outdoor temp and had a timer, to save fuel.

Please have your oil boiler system serviced now if it was not done since last winter, and annually. They require more care than gas-fired systems. Ask your neighbors who they like for fuel oil service and supply since there are likely others nearby who use that fuel. The service person can explain its operation. Did you find the service company;'s tag on the boiler?

Thanks for your response, and for that tip. The boiler was serviced just before we bought the house and we intend to either use the same service company or go with our neighbors'.

So is my understanding correct that the boiler may have it's own "thermostat"-- in other words a setting that tells it what it's own internal temperature should be? I'm guessing this is something that the average homeowner shouldn't play around with (I'm not even sure I see any dial/switch that could be this control).

Thanks again!

I am sorry that the HVAC experts here have not been able to chime in with details. I have some experience, as a user, with oil boilers. I have never been the owner or operator of one with DHW capacity of the type you have. "Mine" never had reservoirs, only direct coils.

There has to be thermostatic control of the boiler water temp, high and low. I think you can educate yourself if you do a search for "boiler" and "aquastat". I think that energy is supplied to your hot water tank by a loop that is not fundamentally different from a hydronic heating loop. In one way or another, you probably can reduce the temp of the boiler in warm weather, either manually or with a more sophisticated control system. It can save you money.

With any kind of luck, your service guy likes to talk about what he does and can teach you something about your system. It is probably a pretty typical system for a home in an area where no air conditioning was typically installed at the time of construction. In some areas oil is the most common way to heat. In parts of those areas, natural gas service has crept in slowly over the decades slowly adding smaller and smaller villages and burgs. If natural gas has become available where you live, it might make sense to switch sooner rather than later.