Why would you not have hot water when heat is on in house?

"If" this is a boiler system "and" it has a boiler fed domestic hot water heat exchanger, the system most likely has a priority thermostat or relay that has failed. Most systems allow the domestic hot water heating to have "priority" via some type of control strategy. Or maybe they did not install one. Or maybe this does not answer your question at all. Not knowing all the parameters makes it tough. If this happens to be an AMTI HW model heatmaker boiler, the three wire thermostat on the DHW leaving pipe is defective. lc