I have an Ariston 27MFFI. Problem at moment is if I turn on a hot tap, the boiler does not always fire up. To make it work, I have to go to the sink that is right next to the boiler and turn that hot tap on at the same time. Once boiler fires, I turn that tap off and water flows to the other. I have already replaced the differential valve as the old one was split. Any ideas?