Gas Boiler Heater Turn on When Thermostat does not call for heat

What t-stat do you have?

I am very familiar with this issue. I have repaired it many times in my career. If you have a stuck gas valve. then no amount of electrical wiring repairs or plumbing pipe, valve, or pump fixes can solve this problem. But that only happened to me once, on a warm air furnace in by 40+ years in business, so this is probably not the problem you have. It is very rare.

Another problem could be a wire with the insulation damaged allowing the thermostat circuit to be completed behind the wall even if the actual thermostat is satisfied or even turned off.

There was a Diner in New Jersey that has a large hole in the wall behind the thermostat. The exhaust fan(s) in the kitchen created a negative pressure zone inside the dining area. Cold air from the hole behind the thermostat was being sucked in thru the hole past the thermostat giving a false temperature reading to the thermostat. The thermostat was not reading the room temperature but reading the outside air temperature that was being sucked in to the room from behind the wall. Electrically the system was working just fine. Sealing the hole in the wall is the solution for this customer

I think the most common issue I have come across with boilers are valves that are stuck open that allow heated water to flow when flow in not needed.

There was one situation known as Ghost Flow where heat travels up a pipe while cooler water from radiators travels down the same pipe because the cooler water is heavier than the warmer water. I could not believe that water could flow in two directions in the same pipe... But it can.

I hope this answer gives you some insight into the many causes to your particular issue. More information about your system, and some pictures can help. The professionals need to know if you have a Furnace, or a Boiler. And if it is a boiler, does the boiler make hot water or steam to heat your home. Those differences will help eliminate some of the speculation.

Mr.ED