When we talk about ultra high efficiency heating system, we mean hydronic system, the system where heat is transported by means of hot water. in ultra high efficiency system water is propelled trough the system by system circulator, water is heated in the boiler and cooled in the radiator. The flow of the water is constant and temperature of the system is changed. we need return water to be as cool as possible, so we can have condensation of the exhaust gases vapor, but on the other hand we need space warm and comfortable. We achieve comporomise by using outdoor reset control, and outdoor reset control set up is absolutely crucial to system efficiency.