I have a 20+yr old boiler for heat and hot water. Don't quite get the relationship between the two. If my thermostat is set to turn off the heat, let's say during the summer, how is the boiler running to heat water for domestic use. Also, is there a different control mechanism I can install to do a better job of keeping the system as efficient as an old system can be. A neighbor mentioned a hydrostat but no clue what that is. Thanks very much.
There should be a pump (circulator) to run the hot boiler water through the heating system and radiators. When the thermostat is lowered for summer, this should not come on. Stopping the circulation of water is not perfect that way but you can install a shutoff valve in the heating line if you wish.
Meanwhile the boiler must be kept at domestic hot water temperature (a separate control called an aquastat, already installed, usually regulates this). Since boiler water is not sent up into the house where it gives up its heat, the boiler doesn't run that much during the summer.
The best step to improve efficiency is to put a fiber glass insulating blanket around the boiler. This too won't be perfect since some parts of the boiler must remain uncovered and/or accessible.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.