This glossary collects the terms that show up in heat load calculations, in drawings and in conversations with manufacturers, and explains them without assuming the standard is open on your desk. Every entry stands alone: you do not have to read from the top to understand any one of them.
If you are looking for the client-facing version, we wrote a second glossary for clients - the documents and terms an installer asks for at the start of a job.
Fundamentals
Heat load calculation
The building's design heat load: how much power the building needs in the coldest hour of the year to hold the intended indoor temperature. The result is a figure in watts or kilowatts, calculated room by room. Everything downstream in the project - heat source, radiators, pipe spacing, buffer tank - inherits its error. In more depth: what a heat load calculation is and what depends on it.
PN-EN 12831-1
The standard defining the method for calculating design heat load. The edition in force is PN-EN 12831-1:2017. Poland adopted it by endorsement notice, without publishing a national annex NA - in practice the default values from the standard's informative Annex B act as the Polish defaults unless the designer deliberately adopts their own. The standard draws on two others: PN-EN ISO 6946 for building elements and PN-EN ISO 13370 for ground heat transfer.
Power vs energy
Two different quantities, routinely conflated. Power (W, kW) is instantaneous demand under design conditions and drives equipment selection. Energy (kWh, kWh/year) is consumption over time and drives running costs. An 8 kW pump and a building using 12,000 kWh a year are two independent facts; neither follows from the other without further assumptions.
Energy performance certificate
A document stating a building's annual energy demand (kWh/m²·year), produced for formal and comparative purposes. It is not a design heat load calculation and does not replace one when sizing equipment. It is often a useful source of input data, though: it usually carries ready U-values for the building elements and the ventilation assumptions that were adopted.
Building elements
Building element
Anything separating a heated space from something at a different temperature: external wall, wall to an unheated garage, floor under an unheated attic, roof, ground floor slab, window, door. Each has its own area, its own U-value and its own temperature adjustment factor.
U-value
How many watts escape through a square metre of an element per kelvin of temperature difference, in W/(m²·K). The headline figure for element quality - lower is better. Calculated to PN-EN ISO 6946 from the layers: U = 1 / (Rsi + ΣR + Rse).
U-value of an external wall
- Solid brick, 38 cm, no insulation1.30
- Same wall + 5 cm of EPS0.50
- Ceramic block + 12 cm of EPS0.24
- Ceramic block + 20 cm of EPS0.16
- Ceramic block + 30 cm of EPS0.11
Polish building regulations, external wall, since 2021: 0.20 W/(m²·K)
Thermal conductivity lambda (λ)
A property of the material: how well it conducts heat, in W/(m·K). EPS is around 0.031-0.045, mineral wool similar, solid brick around 0.77, reinforced concrete around 1.7. Lambda does not depend on thickness - it characterises the material, not the layer. Manufacturers state it in the declaration of performance.
Thermal resistance R
The resistance of a layer of material, in m²·K/W, calculated as R = d / λ where d is thickness in metres. 20 cm of EPS at λ = 0.040 gives R = 0.20 / 0.040 = 5.0 m²·K/W. Layer resistances add up, and that sum is what produces U. Surface resistances join them: Rsi on the inside, Rse on the outside.
From material to building element
Material
λW/(m·K)
How well the material conducts heat. Independent of thickness.
EPS: λ = 0.040
Layer
Rm²·K/W
The resistance of one layer at a given thickness d.
R = 0.20 / 0.040 = 5.0
Element
UW/(m²·K)
The whole element: every layer plus the surface resistances.
U = 1 / 6.23 = 0.16
Thermal bridge
A place where heat escapes faster than through the surrounding surface: a corner, a ring beam, a lintel, a balcony slab, a window reveal, a structural penetration. Linear bridges are described by Ψ (psi) in W/(m·K), point bridges by χ (chi) in W/K.
The method in PN-EN 12831-1 § 6.3.2.2 accounts for them as a ΔU_TB supplement added to each element's U-value, according to the quality of the construction detailing: around 0.02 W/(m²·K) for a building with well-resolved details, 0.05 for a standard one, 0.10 for a retrofit, 0.15 for a building with poor detailing. The detailed Ψ·L·ΔT method from Annex C requires a catalogue of details for every junction.
The better insulated the building, the larger the share thermal bridges take of total loss, because the share going through the elements themselves shrinks. Ignoring bridges is a bigger error in a new building than in an old one.
Where a bridge forms, and what it costs
ΔU_TB supplement by detailing quality (PN-EN 12831-1, Annex B.2.1): (W/(m²·K))
- Well-resolved details0.02
- Standard0.05
- Retrofit0.10
- Poor details0.15
Temperature adjustment factor b_u
A multiplier accounting for the fact that not every element faces outdoor air. For an external wall b_u = 1. For a wall to an unheated garage, a floor under an unheated attic or a ground floor slab, b_u is below 1 because the temperature difference is smaller. Default values are tabled in Annex B.2.4; alternatively b_u = (θint - θu) / (θint - θe) from the adopted temperature of the adjacent space.
Design conditions
Design outdoor temperature (θe)
The temperature the building load is calculated at - not the lowest ever recorded, but a statistical value for the location. In Poland it runs from -16 °C (Szczecin) through -18 °C (Gdańsk, Wrocław, Poznań) to -20 °C (Warsaw, Kraków, Łódź, Katowice). The same value has to hold across the whole chain: in the heat load calculation, in heat source sizing and in the documentation.
Climate zone
The division of Poland into areas sharing a design outdoor temperature. The practical consequence: at 20 °C indoors, the difference between -16 °C and -20 °C is about 11% of the load, which is often a whole equipment class.
Design indoor temperature (θint)
The temperature the system has to hold in a given room. Not one figure per building: a living room, a bathroom and a garage have different values, and the difference between adjacent rooms drives heat flow between them, which is also part of the calculation.
Air change rate (n)
How many times an hour the entire air volume of a room is exchanged, in h⁻¹. The calculation takes the larger of two values: the hygienic minimum implied by the room's use (0.5 h⁻¹ for dwelling rooms per Table B.7) and the infiltration implied by the building's airtightness.
n50 and the blower door test
n50 is the air change rate at a forced 50 Pa pressure difference, measured with a blower door test. The standard assumes design infiltration is roughly 0.1 × n50, and recommends n50 ≤ 3 h⁻¹ for dwellings. Typical values: around 1.0 for a very airtight building, 3.0 for an average new one, 6.0 for a leaky or older one.
Transmission loss and ventilation loss
Two independent streams making up a room's load. Transmission is heat escaping through the building elements, calculated from U, area and temperature difference. Ventilation is heat carried away by air exchanged with the outside, calculated from the air change rate and the room volume. Their ratio tells you what is worth improving: in a well-insulated building ventilation can dominate, and then adding insulation changes little.
Heating-up power (Φ_hu)
Extra power needed to bring a room back up after a setback period - relevant to systems running night setback. Calculated from floor area and specific heating-up power (§ 6.3.4). The standard notes that in many cases it is not required.
System and heat source
Monovalent sizing
The heat pump covers the whole building demand on its own, including in the coldest hour of the year. Simpler, but it usually leads to a larger and more expensive unit that spends the rest of the season at part load.
Bivalent point
The outdoor temperature down to which the pump works alone; below it an electric heater or second source takes over. In the Polish climate that means a few dozen hours a year, so raising the bivalent point by a few degrees allows a noticeably smaller unit at the cost of a small amount of resistance heating. A bivalent point only makes sense if it sits above the design outdoor temperature.
The bivalent point is where two lines cross
COP and SCOP
COP is instantaneous efficiency: how many kilowatts of heat the pump delivers per kilowatt of electricity, under specific conditions (outdoor temperature and supply temperature). SCOP is seasonal efficiency, averaged across the heating season. A catalogue COP at A7/W35 and the real SCOP of a system feeding radiators at 55 °C are entirely different numbers. An oversized pump cycles at part load, which drags the real SCOP down.
Supply temperature and ΔT
Supply temperature is the temperature of water entering the system; ΔT is the difference between flow and return. Underfloor heating typically runs at 30-40 °C with a ΔT around 5 K, radiators at 45-70 °C with a ΔT around 10 K. Every degree of lower supply temperature raises heat pump efficiency - which is why supply temperature is an output of the design, not a dial set on site.
Heating curve
The relationship between supply temperature and outdoor temperature, programmed into the heat source controller. The colder it gets outside, the higher the system pushes the supply. Too steep a curve means overheating and needlessly poor COP; too flat means underheating in a cold snap.
Flow meter and its setting
The flow meter on an underfloor heating manifold shows the flow through a given loop, usually in l/min. The setting is the value dialled into it. The point: loops differ in length and load, so without balancing the water takes the shortest path and the longest loop never heats its room. Settings come from hydraulic calculations, not from dividing flow equally.
Pipe spacing
The distance between adjacent runs of an underfloor loop, typically 10-30 cm. Tighter spacing means more output per square metre at the same supply temperature - which is why it is tightened in perimeter zones, at windows and in bathrooms.
Leader pipe
The run from the manifold to the point where the loop's actual pattern begins. The leader gives off heat along the way, so it counts towards loop length and hydraulic resistance.
Spiral and serpentine
Two ways of laying an underfloor loop. A spiral alternates flow and return runs side by side, which keeps floor temperature even. A serpentine runs the pipe back and forth in rows - simpler to install, but it leaves a clear gradient from the supply end. The spiral is the default in living spaces; the serpentine suits perimeter zones and narrow rooms.
Two ways of laying a loop
Spiral
Flow and return alternate side by side, so floor temperature stays even. The default in living spaces.
Serpentine
The pipe runs back and forth in rows - simpler to lay, but it leaves a clear gradient from the supply end. Suits perimeter zones and narrow rooms.
Buffer tank and low-loss header
A buffer tank is a store of heating water that increases the system's thermal capacity, lengthening the heat source's run times and limiting cycling. A low-loss header decouples the source circuit from the system circuit so the pumps do not fight over flow; its volume is negligible. Two different devices solving two different problems, though catalogues often blur them.
Terms most often confused
Four pairs worth keeping apart
Power (kW)
Instantaneous demand under design conditions. Drives equipment selection.
Energy (kWh)
Consumption over time, usually across a year. Drives running costs.
Power does not imply consumption and consumption does not imply power - not without further assumptions about climate and usage.
Lambda (λ)
A property of the material, W/(m·K). Independent of layer thickness.
U-value
A property of the whole element, W/(m²·K). Depends on every layer and its thickness.
The route runs from one to the other: R = d/λ per layer, then U = 1 / the sum of resistances.
Heat load calculation
The building's design power to PN-EN 12831-1, room by room.
Energy performance certificate
Annual energy demand, kWh/m²·year, for formal purposes.
A certificate can be a good source of input data for the calculation, but it does not replace it.
Buffer tank
A store increasing the system's thermal capacity. Limits source cycling.
Low-loss header
Decouples the source circuit from the system circuit. Negligible volume.
A header will not stand in for a buffer when the problem is cycling, and a buffer will not resolve a conflict between pumps.
Where these terms show up in HeatAlgo
The heat load module calculates room by room to PN-EN 12831-1:2017 and shows, against every room, the split between transmission, thermal bridges and ventilation - and against every element, the layers its U-value came from. The heat pump module reads those results live and works at the same design outdoor temperature. The underfloor heating module measures loop lengths from the real drawing geometry, bend radii and leader pipes included.
More material lives in the help center.
Create a free account