Weather compensation is a control strategy which is used to adjust heat output in proportion to the outdoor air temperature. This is usually implemented by adjusting the setpoint of the central heating flow temperature so that the flow temperature increases as the outdoor air temperature decreases. This is shown graphically below.
Why use it?
Heating systems are designed for worst case weather conditions. In the UK and Ireland the outside design temperature will be between -3º and -10º depending on the location. The central heating flow temperature required at the design temperature is usually about 80ºC.
However the average outside air temperature during the heating season is much higher than this. In most locations it is about 8º degrees. Under these average conditions the higher central heating flow temperature is not required.
The weather compensation system monitors the outside air temperature and adjusts the central heating flow temperature accordingly.
The benefits of weather compensation
Flow water temperatures are kept as low as possible resulting in higher system efficiencies. There is a particular benefit when weather compensation is used with condensing gas boilers as the efficiency of the boiler is greatly increased.
Comfort is also enhanced as the output of the CH system automatically adapts itself to the weather conditions. This results in a reduction in ON/OFF cycles of the heating plant.
Weather Compensation and Condensing Gas Boilers
The efficiency of a condensing boiler is dependant on the return water temperature. The lower the return water temperature the more efficient the boiler.
When the combustion products are below their dew point of about 55ºC, the boiler is in condensing mode and it's efficiency increases almost exponentially. Weather compensation is of advantage in systems where high temperature heat emitters such as radiators are used as it allows the system temperature to decrease sufficiently for the boiler to condense.
A boiler which heats radiators and is not fitted with weather compensation will rarely be in condensing mode and will have a much lower seasonal efficiency than a boiler which does. All condensing boilers manufactured by companies such as Immergas and MHG are fitted with weather compensation as standard.
Hot Water Priority
Domestic hot water is usually heated to 60ºC. High temperature boiler water is needed to do this. It will be necessary for a boiler fitted with weather compensation to disable the weather compensation when domestic hot water is being heated.
The boiler is configured to do this. This feature is usually referred to as 'Hot Water priority'. When this is activated by a demand for hot water heating from a water heater thermostat or sensor the boiler gives priority to hot water heating and raises its flow temperature to 80ºC or so until the hot water demand is satisfied.