Up until 2-3 years ago, my oil fired boiler provided our domestic hot water too so it was on all the time. When we went to the hybrid heat pump/electric hot water heater I figured I would turn the boiler completely off during the warm months since we didn't need heat. That was a mistake. The seals in the boiler dried out and leaked all over the floor. At first I thought something broke and I would need to put in a new boiler but the repair guy told me to try turning it on so that water would circulate. That worked, no more leaking. I turn the thermostat off in the summer and the boiler only comes on to keep the small amount of water in the jacket at temp.