Weather compensation

1 How it works

Approved Document L defines it as a system in which an external sensor communicates with one inside the boiler, and the flow temperature changes either by modulating the boiler's output or through a mixing valve¹. Viessmann's Vitodens 050-W manual puts it simply: the lower the outside temperature, the higher the flow temperature, so more heat goes in on cold days².

Methods vary by maker. Ideal's glossary describes an optional, compatible outdoor sensor⁶. Vaillant says its smart controls can use forecast data, and that some models become more accurate when fitted with an outside sensor giving live readings³. Baxi's glossary says an outdoor sensor works with the boiler for fuel savings and comfort, without giving a figure⁷.

2 What changes for the homeowner

The controls may feel different. On Ideal's Logic+ with the option fitted, the central heating knob becomes a way of controlling room temperature, to be left alone once you have found your setting⁸. On the Vitodens 050-W, the user sets room temperature, and the manual allows the heating curve to be adjusted⁹, while also saying the contractor adjusts the curve to suit the building¹⁰. Read your own booklet before changing anything, and ask the engineer who set it up.

If some rooms stay cool, that may also point to balancing or a control setting, so mention it to the engineer.

3 Weather or load compensation

Load compensation works from inside rather than outside. Approved Document L says it varies flow temperature relative to the measured response of the heating system¹. Vaillant says it adjusts water temperature according to how close the room is to the target, and works with a room thermostat³.

4 The rules on new combis

In England, para 6.2 of Approved Document L says that when a gas combi is fitted in an existing home, at least one measure should be installed from flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation⁴. Wales has a matching paragraph in its own edition¹¹. Vaillant and Viessmann call this Boiler Plus, a name that does not appear in the document³¹². Viessmann says the rules apply only when a new boiler is installed¹². More is under boiler building regulations. Rules for Scotland and Northern Ireland were not checked for this entry.

Savings are claimed but rarely quantified on the pages read, so none is given here. Installation and wiring stay with the engineer.