Weather compensation with underfloor heating: how it works and how to set the curve

Most heating systems run the water at one fixed temperature and switch on and off to hold the room. That works, but it means the water is hotter than it needs to be on all but the coldest days. Weather compensation does the opposite: it runs the system longer at the lowest water temperature that will hold the room, given the weather outside.

Underfloor heating and weather compensation are a natural pair. The floor is a large, slow emitter that is happiest running steadily at a low temperature, and a heat pump or condensing boiler is more efficient at lower flow temperatures. This guide explains how it works, how to set the heating curve for UFH, how it fits with mixing valves and room thermostats, and how to fix the common problems.

What is weather compensation?

Weather compensation is a control that changes the flow temperature in step with the outdoor temperature. BEAMA describes it as varying the flow temperature from the boiler in relation to the measured outside temperature, so the heat output matches the building's heat load.

It needs two things:

• An outdoor temperature. Either a sensor on an outside wall shaded from the sun, or local weather data from the internet.
• Control logic that changes the flow temperature. Ideally by modulating the boiler or heat pump through a communication protocol it supports, or more simply by switching it on and off.

BEAMA lists three ways to fit it: an outdoor sensor wired straight to a boiler that supports one, a stand-alone weather compensator with its own sensor, or a programmable room thermostat with weather compensation built in.

Is weather compensation worth it with underfloor heating?

Yes, in most systems. It lowers the average water temperature, which raises the efficiency of the heat source, and it keeps the floor at a steady temperature rather than cycling between hot and cool.

• Recognised savings. One boiler maker notes that under ErP rules a weather compensating control with a room thermostat is a Class VI control and adds 4% to the system's efficiency rating.
• Boiler Plus. Since April 2018, a new combi boiler in an existing home in England must be fitted with one extra energy-saving measure. Weather compensation is one of the four options, according to the government's Boiler Plus factsheet.
• Heat pump ratings. The SAP 10.1 specification scores a heat pump on its efficiency at the design flow temperature. A low design flow temperature, which weather compensation helps you run at, gives a better result.

It also suits the floor itself. A screeded floor has a lot of thermal mass, so it responds slowly to on and off switching. Running it steadily at a matched temperature avoids overshoot. Our guide to thermal mass and underfloor heating explains why.

What is a heating curve?

The heating curve is the rule the control uses to turn outdoor temperature into flow temperature. It has two settings:

• Slope (or gradient). How quickly the flow temperature rises as it gets colder. The same manufacturer explains that a slope of 1.5 raises the flow by 1.5°C for every 1°C drop outside.
• Level (or offset, or parallel shift). Moves the whole curve up or down by the same amount at every outdoor temperature.

Many controls also let you set a minimum and maximum flow temperature, which keeps the curve inside safe limits.

Emitter	Starting slope (manufacturer guidance)	Flow rise for a 20°C drop outside
Underfloor heating	0.3, or 0.3 to 0.5 in a well-insulated home	6 to 10°C
Radiators, well-insulated home	1.0 to 1.2	20 to 24°C
Radiators, older detached building	1.4 to 1.6	28 to 32°C

The right-hand column is simply the slope multiplied by 20. It shows why UFH needs a much flatter curve: the floor gives its heat at low water temperatures, so it needs only a small rise in cold weather. One heat pump installer guide makes the same point, saying a flatter curve suits well-insulated homes with large emitters such as underfloor heating.

How do you set the heating curve for underfloor heating?

Start from the system design, set a shallow slope, then fine-tune using real room temperatures over several days. Manufacturer and installer guidance give a similar method:

1. Get the basics right first. The loops should be balanced and the actuators working. The installer guide says check that TRVs in the main rooms are fully open on radiator systems, and the same logic applies to UFH zones.
2. Start from the design. Your heat loss calculation and design flow temperature give the starting point. The installer guide notes its slope ranges are only starting points, and the final setting must come from the heat loss calculation and system design.
3. Set a starting slope. About 0.3 for UFH, following manufacturer guidance, and a maximum flow temperature the floor can safely take.
4. Log temperatures. The manufacturer suggests recording the coldest room's temperature and the outdoor temperature over several days once it is consistently below 5°C outside.
5. Adjust the slope for cold weather. If rooms are cool when it is cold outside, raise the slope a little. The installer guide suggests steps of about 0.1.
6. Adjust the level for mild weather. If rooms are too warm when it is mild, lower the level rather than the slope. The installer guide gives above about 10°C outside as the mild-weather check.
7. Wait between changes. The installer guide says wait at least 24 hours after each change. A thick screed responds slowly, so allow longer if you need to.
8. Keep notes. Record each change with indoor and outdoor temperatures. The installer guide says fine-tuning can take a few weeks in the first heating season.

For the flow temperatures to aim at with each heat source, see UFH flow temperatures for boilers and heat pumps.

Does weather compensation work with a UFH mixing valve?

Only partly. A fixed thermostatic blending valve, such as the one in our blending valve pump pack, holds the UFH water at one set temperature between 20 and 55°C. Weather compensation at the boiler will lower the primary temperature in mild weather, which helps the boiler, but the floor keeps getting water at the valve's set point until the primary drops below it.

Set-up	Is the UFH water compensated?
Heat pump feeding UFH direct (non-mixing)	Yes, fully. The curve sets the floor's flow temperature
Boiler with fixed blending valve	Partly. The boiler side is compensated, the UFH stays near its set point
Boiler with weather-compensated motorised mixing	Yes. The mixing controller follows its own curve
Boiler feeding UFH only, at a low compensated flow	Yes, if the boiler's minimum flow suits the floor

With a heat pump, UFH usually runs direct through a non-mixing pump set, so the heat pump's curve does all the work. See non-mixing pump sets for heat pumps and UFH mixing and blending valves explained.

What happens to room thermostats with weather compensation?

They stay, but they should do less. The curve does the main job of matching heat to the weather. The room thermostats and actuators trim each zone for sun, cooking or people, and close a loop when a room is warm enough. A weather compensator with a room thermostat is the combination behind the ErP Class VI rating.

If room thermostats are closing loops for long periods, the curve is probably set too high for those rooms. If a room never reaches its set point on cold days while others do, check that loop's flow rate and the room's heat loss before raising the whole curve.

Weather compensation also changes how you use setbacks. Deep night setbacks suit it less well, because a slow floor then needs a long, hot recovery. Our guide to underfloor heating setback schedules covers this.

What are the signs the heating curve is wrong?

Comfort and running time tell you. Manufacturer and installer guidance describe the same symptoms:

• Cold in cold weather, fine when mild. Slope too shallow. Raise it in small steps.
• Too warm in mild weather, fine when cold. Level too high. Lower the level.
• Too warm all the time, high bills. Curve too high or too steep. The installer guide warns this wastes energy and can overshoot the room temperature.
• Rooms cold all the time. Curve too low. The manufacturer notes this can also cause cold surfaces, condensation and possible mould.
• Frequent short cycling. The installer guide links this to an incorrect curve as well as other causes, such as oversizing.

Frequently asked questions

What heating curve should I use for underfloor heating?

One heating manufacturer suggests starting at a slope of about 0.3, or 0.3 to 0.5 in a well-insulated home. Fine-tune from there using your design flow temperature and real room temperatures.

Is weather compensation good for underfloor heating?

Yes. UFH works best running steadily at a low temperature, and weather compensation keeps the flow temperature as low as the weather allows, which also helps boiler and heat pump efficiency.

Can I add weather compensation to my existing boiler?

Often. BEAMA lists an outdoor sensor wired to a compatible boiler, a stand-alone compensator, or a room thermostat with weather compensation. Check your boiler supports the method you choose.

What is the difference between the slope and the level?

The slope sets how much the flow temperature rises as it gets colder outside. The level moves the whole curve up or down. Adjust the slope for cold days and the level for mild days.

How long should I wait between heating curve changes?

One heat pump installer guide advises at least 24 hours. A thick screed floor responds slowly, so give it longer if the change seems not to have worked.

Does weather compensation work with a blending valve?

Partly. A fixed blending valve holds the UFH water at its set point, so the floor only sees compensation when the primary flow drops below it. Direct heat pump systems or weather-compensated mixing give full compensation.

Is weather compensation required by law?

Not on its own. In England, Boiler Plus requires a new combi boiler in an existing home to have one extra measure, and weather compensation is one of four options.

Sources: BEAMA: weather compensation guidance, GOV.UK: Boiler Plus factsheet, BRE: SAP 10.1 specification