Weather Compensation Controls Explained

Weather compensation is a control strategy that varies the temperature of the water your boiler or heat pump sends to radiators, in line with the temperature outside. When it is mild, the flow temperature drops, so radiators run warm rather than hot and the heating appliance works less hard. When it freezes outside, the flow temperature rises so the house can still be heated. The Heating and Hotwater Industry Council describes it as communication between the boiler and an outside temperature sensor, or internet weather data, which adjusts the radiator circulating temperature to be hotter when the outside temperature is colder, and at a lower temperature in milder weather¹.

The point of all this is efficiency. A condensing boiler or a heat pump is most efficient when the water returning to it is cool, and that happens when the flow temperature is kept as low as the weather allows. Weather compensation automates the adjustment that a householder would otherwise have to make by hand at the boiler, several times a heating season. Ofgem's ECO4 guidance defines it as altering temperature flow from the boiler in relation to the measured outdoor temperature, with a means for automatically measuring local outdoor temperature built in².

It is not the same as load compensation, which works from the temperature inside the house rather than outside, and it is not a substitute for a room thermostat. It is a way of matching the heating system's output to the day's conditions, and on both boilers and heat pumps it is one of the most effective controls for cutting fuel use without cutting comfort.

The effect of outdoor conditions on flow temperature

The core idea is simple. A house loses heat to the outside at a rate that depends on how cold it is, so the heating system only needs to replace that loss. On a mild day the loss is small and lukewarm radiators are enough. On a cold day the loss is large and the radiators need to be genuinely hot. A heating system set to one fixed flow temperature all winter is always working harder than the day requires.

Weather compensation closes that gap. Vaillant describes it as a technology found in smart heating controls that uses weather live data to fine-tune your heating, working by using an outdoor sensor or local weather data from the internet to monitor the external temperature, then adjusting the boiler or heat pump's output accordingly³. The Centre for Sustainable Energy puts the same point in terms of fuel: a weather compensator adjusts the boiler temperature in line with the outside temperature, so it uses less fuel when it is warmer outside⁸.

The adjustment is continuous, not stepped. Rather than choosing between a winter setting and a spring setting, the control slides the flow temperature along what installers call a heating curve: a relationship between outside temperature and flow temperature that can be set up to suit the house. A well-insulated home with large radiators can sit lower on the curve than a draughty one with small radiators, and that difference is exactly why the settings matter, which later sections return to.

For a heat pump the same principle applies, and Energy Saving Trust describes it as weather compensation, where the system varies the flow temperature according to the season⁹. Whether the appliance burns gas or moves heat with electricity, the physics is the same: the cooler the water the appliance has to produce, the less energy it uses for each unit of heat delivered.

Sensing the weather: outdoor probe or online data

There are two ways a weather compensating control finds out how cold it is outside. The first is a physical sensor, an outdoor probe fitted to a north-facing wall, wired or wirelessly linked back to the boiler or control unit. The second is internet weather data, where the control reads a local forecast or live temperature feed for the area and adjusts the heating from that.

The two are not equivalent in accuracy. A sensor on your own wall measures the temperature at your house, in the exact position where your home loses heat. An internet feed measures the temperature for your postcode area, which may sit several degrees away from the microclimate at your back door on a still, frosty night. Vaillant's guidance reflects this: advanced systems like the sensoCOMFORT and VRC 700 can work more accurately as they can be fitted with an outside sensor, so the weather compensation feature uses live data¹⁰.

Not every control needs the probe. Vaillant states that with the sensoHOME heating control you will need to install an outdoor sensor, while the sensoCOMFORT includes an outdoor sensor for weather compensation in the box³. Controls that use internet data instead can be simpler to fit, since nothing has to be mounted outside, but they depend on a working internet connection and on the weather service the manufacturer has chosen. The Heating and Hotwater Industry Council's description covers both routes: communication between the boiler and an outside temperature sensor, or internet weather data¹.

The practical difference for a householder is what happens when the input fails. A wired outdoor sensor keeps working through a broadband outage; an internet-based system may fall back to a fixed flow temperature until the connection returns. Neither route needs the householder to do anything day to day, which is the appeal: the adjustment that would otherwise be a manual job at the boiler happens on its own.

The efficiency gains from reduced flow temperatures

The reason weather compensation saves fuel lies in how condensing boilers and heat pumps work. A condensing boiler extracts extra heat by condensing water vapour in its flue gases, and it can only do that when the water returning to it is cool enough. The cooler the return, the more condensing happens and the more heat is recovered from each cubic metre of gas. Keeping the flow temperature down on mild days keeps the return down with it.

For heat pumps the effect is larger still. Energy Saving Trust states that lowering the heat pump's flow temperature reduces the reaction time of the heating system, meaning it takes longer to warm up, but has a big impact on running costs¹¹. The trade-off is real: a lower flow temperature means radiators emit less heat each, so the system runs longer to reach the same room temperature. Weather compensation manages that trade-off automatically, dropping the flow temperature only as far as the weather permits.

The industry has examined this in depth because it matters for existing homes, not just new ones. BEAMA's work on technology solutions for low flow temperatures in existing heating systems sets out why operating an existing heating system at lower temperatures has the potential to improve the efficiency of that system¹². That potential is what weather compensation is designed to capture: it lets the system run at the lowest temperature the day allows, rather than at a fixed temperature chosen for the coldest day of the year.

Energy Saving Trust makes the same point from the householder's side. Turning the boiler's flow temperature down gives savings, and doing so with a weather compensation kit or load compensator, which changes the flow temperature for you, gives you the maximum possible savings from adjusting the temperature of your boiler¹³. The control does not create efficiency by itself; it lets the appliance spend more of its running hours in the efficient, low-temperature part of its range.

The building regulations view under Boiler Plus

Since 2018, Boiler Plus has set the rules for heating controls in England. Evidence given to Parliament describes what the policy did: it increased minimum efficiencies for boilers, required time and temperature controls, and required an additional energy efficiency measure, with weather compensation listed alongside smart thermostats, flue gas heat recovery and load compensation¹⁴.

For a combi boiler, that additional measure is compulsory. The guidance on what counts is specific. For weather compensation two elements are required: a means of measuring local outdoor temperature, plus a control functionality that varies the boiler flow temperature in relation to the measured outside temperature⁴. Load compensation operates in a similar way except that it measures the internal rather than external temperature, varying the flow temperature in relation to the difference between the measured room temperature and the room thermostat set point⁴.

The requirement is written into the assessment framework too. The legislation underpinning the Standard Assessment Procedure includes, in Schedule B1, a data item for whether the heating system has a load or weather compensator¹⁵. In other words, the presence of the control is recorded when a home's energy performance is assessed, which is why it can show up in an EPC.

For a householder with a combi, the practical route is a kit. Energy Saving Trust notes that you can only fit compensation to a combi if you have a combi boiler weather compensation kit or load compensator, and that a heating engineer may be able to fit one¹³. Vaillant sets out its own compliant combination: to meet Boiler Plus requirements with a combi boiler, you must install a weather compensating room thermostat such as the sensoROOM pure and the myVAILLANT connect internet gateway¹⁰. Boiler Plus applies in England; the position in the other nations is set out in the next section.

Flow temperature caps vary by UK nation

The flow temperatures a heating system is designed around are now set by building regulations, and the numbers differ between England, Wales and Scotland.

In England, the Approved Document L guidance requires that a wet heating system that is newly installed or fully replaced, including pipework and emitters, is sized so the space heating can operate effectively at a maximum flow temperature of 55°C⁶. The 2021 edition of the same document, incorporating the 2023 amendments, carries the same 55°C figure for newly installed or fully replaced wet heating systems where feasible¹⁶.

Wales has gone further. The 2026 edition of the Welsh Approved Document L requires all parts of the system, including pipework and emitters, to be sized to allow the space heating system to operate at a maximum flow temperature of 45°C or lower⁷. The consultation version that preceded it had proposed 55°C or lower, so the published standard tightened the requirement between consultation and adoption¹⁷.

Scotland's domestic building services compliance guide takes a different form of words: for a new or major replacement wet heating system, where the appliance and the majority of emitters are replaced, the system should be sized based upon a mean water temperature of 50°C or lower¹⁸.

These limits matter for weather compensation because they set the top of the curve. A system designed to meet a 45°C maximum in Wales will spend almost the whole winter at lower temperatures than a system designed to 55°C, and the lower the design temperature, the more efficient the appliance runs. The Product Characteristics Database used in SAP reflects the same logic: PCDB records for heat pumps provide efficiency values at flow temperatures of 35°C, 45°C and 55°C, so the assessment itself rewards lower design temperatures¹⁹. BRE's SAP specification adds that PCDB records provide efficiency values and maximum output for a range of design flow temperatures, including 65°C for high temperature heat pumps²⁰.

Heat pump weather compensation: 45°C flow, 40°C return

On a heat pump, weather compensation is not an optional refinement; it is close to the default way the system is meant to run. A heat pump's efficiency falls as the flow temperature it must produce rises, so the whole design effort goes into keeping that temperature down.

The design numbers are consistent across the sources. Edinburgh's local heat and energy efficiency strategy states that an air source heat pump utilises an optimal supply, or flow, temperature of 45°C and a return temperature of 40°C⁵. The Centre for Alternative Technology describes the sizing approach in the same terms: sizing the system to supply flow temperatures of only 40 to 45 degrees to radiators when it is minus 3 degrees outside, with underfloor heating as another good option at a flow temperature of 35°C to 40°C²¹.

The heat pump methodology used for assessments works to the same scale, with 35°C for heat pumps with wet distribution as a required input figure²², and the PCDB efficiency records at 35°C, 45°C and 55°C giving the data that makes those temperatures comparable between products¹⁹. BRE's specification notes a default flow temperature for heat pumps with a wet heat distribution system of 58°C, which is the fallback assumption when no design temperature is established, and it is far above the temperatures a well-designed system actually runs at²⁰.

What weather compensation adds is the automatic variation around that design point. Energy Saving Trust explains that the heat pump's heating curve, also known as outdoor weather compensation, varies the flow temperature according to conditions, and that lowering the flow temperature has a big impact on running costs¹¹. A heat pump sized for 45°C on the coldest day will run at lower flow temperatures for much of the spring and autumn, where its efficiency is at its best.

There is a caveat from the field, and it is worth stating plainly. In observations of heat pump systems during home visits, weather compensation was typically enabled, though in some cases it appeared to be in conflict with third party thermostat control²³. A heat pump that is being told to hit a fixed flow temperature by the weather curve, and simultaneously told to hit a room temperature by a separate thermostat that switches it on and off, can end up cycling inefficiently. The control strategy has to be coherent, which is the subject of a later section.

UK market controls and kits

The UK market for weather compensating controls is largely built around manufacturers' own ecosystems, because the control has to talk to the boiler or heat pump in a language the appliance understands.

Vaillant's range illustrates the tiers. The sensoHOME heating control requires an outdoor sensor to be installed for weather compensation to work, while the sensoCOMFORT includes an outdoor sensor in the box³. The more advanced systems, the sensoCOMFORT and the VRC 700, can work more accurately as they can be fitted with an outside sensor so the weather compensation feature uses live data¹⁰. For Boiler Plus compliance on a combi, Vaillant specifies a weather compensating room thermostat such as the sensoROOM pure together with the myVAILLANT connect internet gateway¹⁰.

For existing systems, the route is often a smart thermostat rather than a manufacturer kit. Energy Saving Trust notes that smart heating controls include load and weather compensation, which adjust the boiler flow temperature to maximise boiler efficiency, and that a smart thermostat is probably the easiest way to add compensation to an existing system²⁴. The trade-off is that internet-based compensation depends on the manufacturer's weather service and connection, where a wired outdoor sensor depends on nothing but the wire.

Fitting is a job for an engineer in most cases. The sensor has to be sited correctly, typically on a north-facing wall away from vents and sunlight, and the heating curve has to be set to suit the house. A curve set too high wastes the benefit; set too low and the house will not reach temperature in cold weather. That commissioning step is where most of the value of the kit is either realised or lost.

Weather compensation compared with load and room control

Weather compensation is one of three control ideas that get grouped together, and they are worth separating.

Load compensation works from the inside. Vaillant describes the difference: load compensation is when your boiler or heat pump's output is adjusted based on the current temperature of your home, while weather compensation uses outdoor temperature³. Ofgem's guidance defines load compensation as altering the boiler flow temperature and preventing overheating, varying the flow temperature in relation to the difference between the measured room temperature and the thermostat set point². Load compensation reacts to what the house is doing now; weather compensation anticipates what the house will need.

Room control is the third element, and it is not replaced by either. The building regulations guidance for heat pumps lists the minimum controls as weather compensation or internal temperature control, plus a timer or programmer for space heating⁶. Note the "or": the standard accepts either weather compensation or internal temperature control, not necessarily both, though in practice a well-designed system uses the weather curve to set the flow temperature and a room influence setting or thermostat to trim it.

That interaction is where systems go wrong. The field observations of heat pump installations found weather compensation typically enabled, but in some cases in conflict with third party thermostat control²³. A third-party thermostat that simply switches the appliance on and off fights the weather curve, because the appliance never gets to modulate smoothly along it. The remedy is either to use the manufacturer's own room control, designed to work with the curve, or to set the room influence so the thermostat trims rather than overrides.

The three ideas are complementary in a well-commissioned system: the weather curve sets the flow temperature for the day, load compensation trims it against what the room actually does, and the programmer decides when the system runs at all. Controls that offer all three, configured together, are the ones that deliver the efficiency the individual features promise.

Its impact on your bills, good and bad

Weather compensation reduces fuel use, but no honest page can put a single number on it. Vaillant says the amount you can save on energy bills depends on various factors, including your home's insulation, the efficiency of your central heating system, and your energy usage habits³. A well-insulated home with oversized radiators can run low flow temperatures for most of the winter and will see the largest benefit; a home with small radiators and high heat loss has to run hotter, and the curve has less room to fall.

What the control does is remove the waste that comes from a fixed setting. Energy Saving Trust's guidance on flow temperature makes the point directly: fitting a weather compensation kit or a load compensator, which changes the flow temperature for you, gives you the maximum possible savings from adjusting the temperature of your boiler¹³. The savings come from the adjustment itself; the automation is what keeps the adjustment in place all season rather than for the week after the householder last turned the dial.

It is worth being clear about what weather compensation cannot do. It cannot fix an undersized system: if the radiators cannot heat the house at the flow temperature the curve selects, the house will be cold, and the curve has to be raised, which erodes the benefit. It cannot compensate for poor insulation, which sets the heat loss the system has to meet. And it does not change hot water: the compensation applies to the space heating circuit, and stored or instantaneous hot water runs at its own temperature.

For heat pumps the running cost effect is the sharpest, because the efficiency penalty for high flow temperatures is steepest. Energy Saving Trust's guidance on running a heat pump efficiently ties lower flow temperatures directly to a big impact on running costs¹¹. The same logic applies to a gas boiler through condensing efficiency, just with a smaller swing.

Household energy independence and what this means

Weather compensation is a quiet but genuine gain for a household's control over its own heating. It reduces how much fuel the home needs for the same warmth, and every unit of gas or electricity not burned is a unit not bought, not subject to a price rise, and not imported. For a heat pump household the effect is larger still, because the flow temperature is the single biggest driver of how much electricity the machine draws for each unit of heat.

The dependence that remains is worth naming. A control that takes its weather data from the internet depends on the manufacturer's service and on a working broadband connection; if either fails, the compensation stops and the system falls back to a fixed temperature. A control with a wired outdoor sensor depends on nothing outside the house, which is the more self-sufficient arrangement, though it still depends on the manufacturer's control hardware and firmware for the curve logic itself.

The deeper point is that weather compensation is one of the few efficiency measures that improves with the home rather than with the appliance. Insulate the house, enlarge the emitters, and the curve can be lowered, so the same control delivers more each time the fabric improves. It is a strategy that compounds, unlike a one-off upgrade that is finished the day it is fitted.

For a household planning a heat pump, the case is strongest of all. A system designed around 45°C flow and 40°C return⁵, with weather compensation varying the flow temperature through the season¹¹, is a system built to run at the efficient end of its range for most of the year. That is less electricity per unit of heat, less exposure to tariff rises, and a heating system that responds to the weather rather than to a setting chosen once and forgotten.