Weather Compensation: How It Works

25 Apr. 2019

Written by Neil Marsh MCIPHE RP RHP EngTech


With the constant rise of fuel bills and the need to be as energy efficient as possible to keep fuel bills to a minimum. The industry is continuously looking for ways to reduce the fuel used by central heating boilers …

All boilers now have to be condensing boilers, which was a huge step forward in reducing energy consumption at the point of use, as condensing boilers are offering fuel savings of up to 35% compared to conventional boilers. 

It has not come without a cost. Many heating appliances are much less robust and durable as they once were. It is a trend that we cannot see changing any time soon, and with new measures brought in this year already, it may be that appliances are subjected to even more faults, although fuel consumption should continue to fall at the point of use.

All new oil and gas boiler replacements in Cheltenham, Gloucester and the rest of the country are now subject to require additional fuel-saving measures. It is most likely that the most commonly used measure will be the use of weather compensation devices. 

How Weather Compensation Works::

Homes need to be supplied heat to replace heat lost through the fabric of the building to maintain set temperatures within. The colder it is outside, the higher the rate that heat is lost through the building fabric. Understanding this is fundamental to understanding how an outdoor weather sensor works and how it can help to heat a home more efficiently and cost-effectively.

Weather compensation controls work by ensuring that a boiler burns the correct amount of fuel required to match the heat loss of the building. Weather compensation improves the efficiency of the boiler under partial load conditions, which is relevant for the UK climate, as for two-thirds of the year the boiler will operate at a fraction of its maximum heat load.

This means that a boiler’s operating temperature, when fitted with a weather compensation device, is reduced for the majority of the year, while still providing a constant room temperature. It will only run at higher and less efficient operating temperatures when the heat loss from the building is high from cold weather.

A boiler, when fitted with such a device, works to a more specific “temperature curve” which it modulates its working temperature between two operating temperature points depending on the readings the sensor provides from outside. 

Traditional central heating systems have a room thermostat indoors and work in this sequence: 

1. Outside temperature is lower than internal temperatures.
2. Heat is lost through the fabric of the building.
3. Rooms get colder as heat escapes to outside.
4. When the temperature in the room with the room thermostat drops below the set point and the heating is turned on, the boiler will start to supply heat to the radiators. The boiler does this at the maximum rate and then modulates down to a lower rate as it approaches the temperature set on the boiler stat. 
5. Rooms start to get warmer again until the set temperature on the room thermostat is reached, which turns the boiler off. 
6. Rooms begin to cool down again.

With a weather compensation system installed, the boiler will vary the rate that it runs at based on the temperature outside when the room thermostat brings the boiler.  When the temperature changes outside, the boiler will react and increase or decrease the radiator temperature to compensate. It is done in a way so that the people inside the building will not notice that the temperature has changed outside. 

It means that a boiler will typically run at max rate for fewer periods of time and ‘tick over’ at lower operating temperatures, allowing the boiler to condense more and increase its efficiency. The boiler will generally cruise and run for more extended periods at lower temperatures opposed to “cycling” on/off at higher temperatures. By maximising the condensing effect, condensing boilers will benefit from increased efficiency. 

For weather compensation to work correctly, a small temperature sensor is located on the outside of the building, on a north to west facing wall. It is wired directly to the internal controls of the boiler and information about the outside temperature is communicated to the boiler consistently.

How This Affects Condensing Boilers:

For a condensing boiler to get the water vapour in the flue gas to condense optimally, the return water temperature needs to be 57ºC or below. Without weather sensitive controls set correctly, this often is not the case as boilers are often set too high on the temperature stat and run for extended periods without condensing. 

In a well designed central heating system, which most are not, the radiators are sized in a way that allows the boiler to run at lower more efficient temperatures indefinitely, maximising the condensation effect. This is how we have always designed our central heating in Gloucester, Cheltenham and other areas we install to work, so our systems are always incredibly efficient regardless, but as you can probably tell from how this is written, there are few real heating engineers out there, and few who know how to design systems correctly. 

For more information about alternative energy saving controls, click here!

For all central heating installations in Cheltenham and Gloucester (and around), contact us if you want a system that performs well and maximises on efficiency.