Heat loss and underfloor heating output: will it heat my room?

"Will it heat my room?" is the question behind every underfloor heating design. The answer comes from two numbers: how much heat the room loses on a cold day, and how much heat the floor can give out. This guide shows how to work out both, with two worked examples: a new extension that is easy to heat, and an older room that needs more thought.

The method here is a simplified version of the full room-by-room calculation in BS EN 12831-1, which the MCS heat load calculator follows. Use it to check a design or compare options. For a heat pump, an MCS installer will do a full room-by-room calculation to that standard.

How much heat does underfloor heating give per m²?

A wet underfloor heating floor typically gives 70 to 100W/m² at boiler temperatures, and about 100W/m² is the practical maximum in occupied areas. The limit is comfort, not the pipe: BS EN 1264 caps the floor surface at 29°C in a 20°C room, 9°C above the air.

Real floors often give less than the maximum because of the floor finish and a lower water temperature. Timber floors usually have a lower limit still: Polypipe's guidance is that the surface under timber finishes should not exceed 27°C.

How do I work out the heat loss of a room?

Room heat loss is fabric loss plus ventilation loss, at the design temperature difference between inside and outside. Work through it in this order.

1. Measure every surface that loses heat: outside walls (minus windows and doors), windows, doors, roof or ceiling, and floor, in m².
2. Find each U-value in W/m²K. New work in an existing home in England should meet the limits in Approved Document L Table 4.2, for example 0.18 for walls and floors and 1.4 for windows.
3. Multiply area × U-value for each surface and add them up. This is the fabric loss in W per °C.
4. Work out ventilation loss: 0.33 × air changes per hour × room volume in m³. This is also in W per °C.
5. Add fabric and ventilation, then multiply by the design temperature difference (inside minus outside).
6. Divide by the heated floor area. Leave out the floor under fitted units, baths and showers. This gives the W/m² the floor must supply.

The 0.33 figure is the heat needed to warm one cubic metre of air by 1°C, in watt-hours. Air change rates vary a lot with the age and airtightness of the building, so this is the step where rough estimates go most wrong.

What inside and outside temperatures should I design for?

Design to the room's intended temperature and a cold, but not record-breaking, outside temperature for your area. The MCS heat load calculator takes room temperatures from the CIBSE Domestic Heating Design Guide and outside temperatures from the nearest weather station to your postcode.

Outside design temperatures from the MCS reference data include Manchester at -4.5°C or -2.7°C, London at -3.0°C or -1.7°C, Birmingham at -5.1°C or -3.2°C and Glasgow at -5.6°C or -3.5°C. The two figures are the 99.6% and 99% values. Take 0.6°C off for every whole 100m your home sits above the weather station.

Worked example: will UFH heat a new kitchen extension?

Yes, easily. A 5 × 4m single-storey extension built to current standards loses about 850W on a design day and needs about 53W/m² from the heated floor.

The room is 20m² with a 2.4m ceiling (48m³). It has 13m of outside wall, 6m² of glazing and a flat roof. Design is 21°C inside and -3°C outside, a 24°C difference.

Total: 35.3W per °C × 24°C = about 850W. Kitchen units take 4m² of floor, so the heated area is 16m². The floor must give 850 ÷ 16 = 53W/m².

This is a simplified sum. A full calculation treats the floor loss to the ground separately, which usually lowers it, and adds allowances the simple method leaves out. Notice that the glazing loses almost twice the heat of all the walls together. That is why a 100mm pipe band in front of large glazing works so well.

Which floor finishes will cover that heat loss?

For the 53W/m² extension, tiles cover it at a 40°C flow, wood at 45°C, and carpet only just at 45°C. These checks use one maker's published outputs for 65mm sand and cement screed with pipe at 150mm centres.

At 50/40°C the same floor gives 118W/m² under tile, 83W/m² under wood and 70W/m² under carpet, according to that data. The tile figure is above the comfort limit, so in practice the thermostat holds the floor below it. These outputs are for screed. Overlay boards and joisted floors usually give less at the same water temperature. If your heat source is a heat pump, see underfloor heating with a heat pump for designing to the lowest flow temperature.

What if the room needs more heat than the floor can give?

Then improve the room or the floor before you add a radiator. Take the same room, but with older walls at U 0.55 and a draughtier 2 air changes per hour.

• Walls: 25.2m² × 0.55 = 13.9W per °C, so fabric loss rises to 28.9W per °C.
• Ventilation: 0.33 × 2 × 48 = 31.7W per °C.
• Total: 60.6W per °C × 24°C = about 1,450W, or 91W/m² over 16m² of heated floor.

91W/m² is close to the 100W/m² ceiling. Wood and carpet cannot reach it even at 50/40°C on this data. Your options, roughly in order of value:

1. Cut the ventilation loss. Bringing this room back to 1 air change per hour saves about 380W at design conditions, more than the whole wall loss of about 330W.
2. Insulate the walls or improve the glazing. Approved Document L Table 4.3 gives 0.55 as the improved standard for cavity fill, and 0.30 for internal or external wall insulation.
3. Choose a lower-resistance finish. Tile or stone gives the most heat. Our guide to UFH under tiles, wood, laminate and carpet covers tog values.
4. Close up the pipe. 100mm centres, or a 100mm band along the glazing, where the edge zone can run hotter. See pipe centres: 100, 150 or 200mm?
5. Raise the flow temperature, within the floor and heat source limits.
6. Add a small radiator or a second emitter only if the room still falls short.

How much does the floor covering cut the output?

A lot. In the screed data above, moving from tile to carpet and underlay cuts output by about 40% at the same water temperature. Every layer above the pipe adds thermal resistance, measured in m²K/W or tog (1 tog = 0.1m²K/W).

BS EN 1264 designs living areas around a covering resistance of 0.10m²K/W (1 tog). Anything up to 0.15m²K/W (1.5 tog) should be agreed at design stage, and bathrooms are designed with no covering resistance at all. Add up the tog of the finish and underlay together. A thick carpet on a thick underlay can be well above 1.5 tog, which will make the room slow and cool however good the pipe layout.

Heat that goes down is also lost to the room. BS EN 1264 is used to size insulation so the downward loss is no more than about 10W/m². See insulation under underfloor heating.

Does a high heat loss room need shorter loops?

Yes. A loop that must deliver more heat needs more flow, and more flow through a long pipe means more pressure loss. Uponor's design tables for 16mm pipe at 200mm centres and an 8°C flow-return difference show the effect.

So the older room in our second example needs more, shorter loops than the new extension, and more manifold ports. Our guide to how many manifold ports you need covers loop counting.

Products for this job

• PERT-AL-PERT Multilayer UFH Pipe
• MaxHeatUFH Castellated Floor Panel, for screed floors
• MaxHeatPro Foil-Faced EPS Panel
• Edge Insulation Foam Roll
• Auto-Balancing Stainless Steel UFH Manifold
• Heatmiser Temperature Sensor Probe, for floor limits under wood
• Insulation boards

Related guides: Pipe centres: 100, 150 or 200mm?, UFH under tiles, wood and carpet and Underfloor heating with a heat pump.

Frequently asked questions

Is underfloor heating enough to heat a room on its own?

In most modern and well-insulated rooms, yes. If the heat loss is above about 100W/m² of heated floor, the floor alone may not keep up, so reduce the heat loss or add a second emitter.

How many watts per m² do I need for underfloor heating?

The room's heat loss divided by its heated floor area. A new extension built to current standards may need 40 to 55W/m². A draughty older room can need 80W/m² or more.

What is the maximum output of underfloor heating?

About 100W/m² in occupied areas, set by the 29°C floor surface limit in BS EN 1264. Edge zones along outside walls can give up to about 175W/m².

Will underfloor heating work in a conservatory?

Often only partly. Large glazing gives a high heat loss per m², so the floor may not cover it on the coldest days. Use close pipe spacing and a tiled finish, and check the heat loss first.

Do I need a heat loss calculation for underfloor heating?

You should do one for every room. It sets the pipe spacing, loop lengths and flow temperature. MCS heat pump installers work room by room to BS EN 12831-1, for example with the MCS heat load calculator.

Does carpet stop underfloor heating working?

No, but it cuts the output. Keep carpet and underlay together to 1.5 tog or less, and expect to need a higher flow temperature than with tiles.

Why does my room feel cold even though the floor is warm?

The room may be losing more heat than the floor can give, often through glazing or draughts. Work out the heat loss and compare it with the floor output. Our guide to underfloor heating not getting warm lists other causes.

Sources: Danfoss underfloor heating handbook (EN 1264 limits), MCS heat load calculator, MCS heat load calculator design conditions, Approved Document L Volume 1, Timoleon screed system data sheet, Polypipe solid floor UFH installation guide, Uponor UFH installation guide