Weather compensation in gas boilers

Heating expert Des Flynn of RVR outlines the importance of weather compensation in boiler systems.

Weather compensation is a control strategy which is used to adjust the heat output of a boiler in proportion to the outdoor air temperature. This is generally 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 in the following graph.

Heating systems are designed for worst case weather conditions. In Ireland the outside design temperature will be between -3ºC and -10ºC 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 8ºC. 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.

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.

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 its’ 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 efficiency than a boiler which does.

Comfort is also enhanced as the output of the Central Heating system automatically adapts itself to the weather conditions. This results in a reduction in ON/OFF cycles of the boiler.

An important aspect of weather compensation is ensuring that any hot water demand is satisfied at a higher temperature if the boiler is running in weather compensated mode.  As domestic hot water is usually heated to 60ºC, high temperature boiler water is needed to do this. To allow this, boilers can use a feature called “Hot water priority”.  This is achieved by having separate flow and return pipes for the central heating and the domestic water heating tank.  When there is a demand for hot water heating from a water heater thermostat or sensor the boiler diverts the flow to the water heating tank, gives priority to hot water heating and raises its flow temperature to 80ºC until the hot water demand is satisfied.