Hot water boiler

Sounds like you have a tankless coil installed for your DHW. These are not the best available technology for water heating, not by a long shot. They're great if you want to be able to fill up a swimming pool, but their overall energy efficiency is pretty poor (25% is a typical number I hear).

I guess the answer is going to depend on how much you use it for space heating. If it runs much in the winter, leave it alone, but install an electric storage heater next to it. In the summer, shut down the boiler and use the electric. If you have successfully weaned the rest of the house off of oil and the boiler is used mainly for DHW, use the electric heater, or consider an "indirect" heater to only run the boiler 1-2 times a day.

If you want something cheap and effective, shut the boiler off completely during the day (at the emergency switch) and turn it on about 30 minutes before you need it. It will recover quickly and still give you all the hot water you need. If you want to get a little more sophisticated, you can put a timer on the boiler to kick it on in the morning for showers ant the evening for dishwashing or whatever. Even a windup timer that shuts it off after an hour will probably pay for itself in about a month at current oil prices.

Ironically, it is probably the low temp short cycling for DHW that is sooting up the boiler. Shut it down and it won't get dirty!

Chris

I'm in the same boat with a DHW coil in an oil fired boiler. I've been heating the house well this past winter with 2 NC-13s so I don't plan to use the hydronic baseboards anymore. If I begin switching it off when there is no demand for hot water, how long does it have to be off to make up for the oil that's used to bring everything back up from room temperature?

It will not use any more oil to come back up to temp than if you left it on. As soon as it cools off a little bit, it is saving money. Oil burners run better flat out than cycling anyway...

Chris