Thanks, XXXXX XXXXX the system piped that way with one pump works is that hot water is pumped around the hot water piping loop, see the diagram on page 23 of the link I posted.
when a room thermostat
calls for heat the valve for that radiator or set of radiators opens, and lets hot water into the loop.
and the radiator gets hot... things that will prevent that is air in the radiators, or water pressure below 15 or 20 psig depending on how tall the building is.
all that apparently works.
it is just the domestic hot water heater, heated by the boiler hot water loop that doesn't work properly.. it has a similar valve and thermostat that regulates the hot water feed to the domestic hot water unit... tell me if you can find that.... it is shown on the diagram on page 23 with only one pump on the right.
that valve has wires attached, those go to the thermostat that controls the valve.. that thermostat has a sensing element attached that goes to the domestic hot water tank. it is that sensor that is likely going bad.
Look that over, we can go from there... none of this is effortless by the way, the college course on these skill sets takes 2 years, then to get good at working on these systems takes another 5 or 10 years of practice.
so be patient with yourself, study the manual I sent and page 23 until you figure out which is which on your system
we can go from there'