What is weather compensation on a boiler, and is it worth having?

A condensing boiler is only condensing — and only reaching its advertised efficiency — when the water coming back from the radiators is cool enough for the flue gases to condense on the heat exchanger. Most British systems run far too hot for that to happen most of the time, which is why real-world efficiencies fall well short of the label. Weather compensation is the control strategy that fixes it, and it is a genuinely different way of running a heating system rather than a gadget.

Step by step

1. Understand what conventional control does.A room thermostat is a switch. The boiler runs at a fixed high flow temperature until the room reaches its set point, then stops entirely, then starts again when the room cools. The result is a warm-then-cool cycle, high return temperatures, and a boiler spending much of its life out of condensing mode.
2. Understand what compensation does instead.An outside sensor feeds the boiler the outdoor temperature. The boiler then calculates a flow temperature from a curve — mild day, low flow temperature; cold day, higher. The radiators run continuously at whatever temperature just balances the heat loss, rather than blasting and stopping.
3. See where the saving comes from.Lower flow means lower return, and a return temperature below the dewpoint of the flue gases is what makes a condensing boiler condense and recover the latent heat. Running in condensing mode most of the season rather than occasionally is where the efficiency gain lives.
4. Note the comfort effect.Rooms sit at a steadier temperature because the heat input matches the loss rather than overshooting and undershooting. Many people notice this more than the bill.
5. Check whether your boiler supports it.Most modern condensing boilers do, often through a proprietary connection for an outside sensor and a compatible controller. Check the installation manual for the model — this is a boiler capability, not something you add to any boiler.
6. Understand the radiator sizing constraint.Lower flow temperature means each radiator gives out less heat, so the radiators have to be large enough to heat the room at that lower temperature. In a house with radiators sized for a seventy-degree flow, compensation can leave rooms short on the coldest days. That is the main reason it is not universal.
7. Consider load compensation as an alternative.Many smart thermostats do a related thing without an outside sensor: they modulate the flow temperature based on how far the room is from its target and how fast it is getting there. Less precise than true weather compensation but far easier to retrofit and available on many systems already.
8. Start with the free version.Simply lowering the boiler's fixed flow temperature to the point where the house still heats comfortably captures a large part of the benefit with no equipment at all. Do that first, see how the house behaves, and you will know whether compensation is worth the installation.

Tips

• The sensor goes on a north-facing wall, out of direct sun and away from flue plumes or extract vents. A sensor in the sun tells the boiler it is a mild day when it is not.
• Compensation works best in a well-insulated house with generously sized radiators. In a draughty solid-wall house with small radiators, the low flow temperature may simply not be enough.
• If you are having a boiler replaced, ask specifically about weather compensation and radiator sizing at the same time. Retrofitting it later is much more awkward.
• Underfloor heating is naturally a low flow temperature system and pairs particularly well with compensation.

Questions people ask