And warmer outdoor weather requires less heating output from radiators.
Now we ask ourselwes, how can we alter output of the radiator to match heat lost by the house?
The answer lays in relationship between radiator output and average water temperature in the radiator. The cooler the water temperature in the radiator the less heat this radiator emits.
So, controlling average water temperature in the radiator we controlling amount of heat this radiator sends to the house. And this is a principle of outdoor reset. We change the temperature of the water circulating trough heating system in accordance with changes of temperature of outside air. This is simplified explanation but it is a valid explanation. What we need for implementation of this type of heating control is:
1. Outdoor sensor to measure outdoor air temperature.
2. Sensor to measure water temperature going to radiators.
3. Control device reading information from these 2 sensors and changing water temperature according program.
4. Constant flow of the water trough the system.
OUTDOOR RESET SYSTEMS ARE CONSTANT FLOW SYSTEMS!!
Where do we see thermostat in this picture? Nowhere. Because heating pump never stops pumping water trough radiators when temperature of outside air is below warm weather shutdown temperature.
Even more. All contemporary modulating boiler are specifically designed to work on outdoor reset.
90% of them even have outdoor reset build into boiler from the factory. Actually 100% of European made boilers have build in outdoor reset.
And yet most of boilers installed in US do not have outdoor reset set up.
Because traditionally in United States boilers are working at constant temperature and interrupteble flow. Temperature of the water pumped to heating system is constant, usually 180F. But pump is turned on and off by thermostat according room temperature. This type of control leads to swings of room temperature, and overshooting temperature setpoint resulting in poor comfort and high fuel bills.
We can make an analogy with the car. Any car has 2 MPG numbers. One for city driving, another one for highway driving. Car is the same, gas is the same, driver is the same. But on highway car can drive twice s much miles per same gallon of gas. Why? Because the conditions of driving re different.
Let's take our boilers off the street and put them on the highway. Get rid of this thermostat and get outdoor reset control. You will not regret.