This glossary collects the terms that show up in heat loss 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.
A word on where the terms come from. HeatAlgo calculates heat loss room by room to the European standard EN 12831-1 and radiant floor output to EN 1264, so several entries use the standard's own names and symbols, with the usual US word beside them. An account set to imperial units sees feet, °F, Btu/h and gpm in the app; every calculation still runs in metric, to those standards. HeatAlgo carries Polish climate data only and has no cooling calculation.
If you are looking for the version for homeowners, we wrote a heating glossary for homeowners - the documents and terms an installer asks for at the start of a job.
Fundamentals
Heat loss calculation
The building's design heat loss: how much heat the building needs in the coldest hour of the year to hold the intended indoor temperature. EN 12831-1 itself calls this figure the design heat load (Φ_HL): the heat lost by transmission and ventilation, plus any heating-up power, less internal gains. In everyday US usage it is simply called heat loss. The result is a figure in Btu/h (or watts), calculated room by room. Everything downstream in the project - heat source, radiators, tubing spacing, buffer tank - inherits its error. In more depth: what a room-by-room heat loss calculation is.
EN 12831-1
The European standard defining the method for calculating design heat loss. The edition in force is EN 12831-1:2017. Poland adopted it by endorsement notice, without publishing a national annex - 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: EN ISO 6946 for building elements and EN ISO 13370 for heat transfer to the ground.
Power vs energy
Two different quantities, routinely mixed up. Power (Btu/h, kW) is instantaneous demand under design conditions and drives equipment selection. Energy (kWh, kWh per year) is use over time and drives running costs. A heat pump rated 27,297 Btu/h (8 kW) and a house using 12,000 kWh a year are two independent facts; neither follows from the other without further assumptions.
Energy rating
A document stating a building's annual energy use, produced for formal and comparison purposes - in Europe the energy performance certificate (in kWh/m² per year), in the US a HERS rating. It is not a design heat loss calculation and does not replace one when sizing equipment. It is often a useful source of input data, though: it usually carries the U-factors of the building elements and the air leakage assumptions that were used.
Building elements
Building element
Anything separating a heated space from something at a different temperature: exterior wall, wall to an unheated garage, ceiling under an unheated attic, roof, slab on grade, window, door. In US terms, an assembly. Each has its own area, its own U-factor and its own temperature adjustment factor.
U-factor (U-value)
How much heat passes through a square foot of an assembly per degree of temperature difference, in Btu/h·ft²·°F (metric: W/(m²·K)). The headline figure for an assembly's quality - lower is better. Calculated to EN ISO 6946 from the layers: U = 1 / (Rsi + ΣR + Rse), where Rsi and Rse are the inside and outside air films.
U-factor of an exterior wall, typical European masonry assemblies:
| Exterior wall | U-factor (Btu/h·ft²·°F) | Whole-wall R-value | Metric U, W/(m²·K) |
|---|---|---|---|
| Solid brick, 15 in, no insulation | 0.229 Btu/h·ft²·°F | 4.4 h·ft²·°F/Btu | 1.3 |
| Same wall + 2 in of EPS foam | 0.088 Btu/h·ft²·°F | 11.4 h·ft²·°F/Btu | 0.5 |
| Clay block + 4.7 in of EPS | 0.042 Btu/h·ft²·°F | 23.7 h·ft²·°F/Btu | 0.24 |
| Clay block + 7.9 in of EPS | 0.028 Btu/h·ft²·°F | 35.5 h·ft²·°F/Btu | 0.16 |
| Clay block + 11.8 in of EPS | 0.019 Btu/h·ft²·°F | 51.6 h·ft²·°F/Btu | 0.11 |
| Polish building code limit, exterior wall, since 2021 | 0.035 Btu/h·ft²·°F | 28.4 h·ft²·°F/Btu | 0.2 |
These are indicative values for typical build-ups. The R-value here is the whole wall, air films included, not the insulation alone. In a project, U is calculated from the actual layers of the actual assembly, not from a table.
Thermal conductivity k (lambda, λ)
A property of the material: how well it conducts heat, in Btu·in/h·ft²·°F (metric: W/(m·K)). Europe calls it lambda (λ). EPS is around 0.21 Btu·in/h·ft²·°F-0.31 Btu·in/h·ft²·°F, mineral wool similar, solid brick around 5.3 Btu·in/h·ft²·°F, reinforced concrete around 11.8 Btu·in/h·ft²·°F. Conductivity does not depend on thickness - it describes the material, not the layer. US data sheets more often give the same thing as R per inch: R-4 per inch is k = 0.25. European manufacturers state lambda in the declaration of performance.
R-value
The thermal resistance of a layer, in h·ft²·°F/Btu (metric: m²·K/W), calculated as R = d / k where d is the thickness. 7.87 in of EPS at k = 0.277 gives R = 7.87 / 0.277 = R-28.4 - in metric, 0.20 m / 0.040 = 5.0 m²·K/W. Layer R-values add up, and that sum is what produces U. The air films join them: Rsi on the inside, Rse on the outside.
From material to assembly
Material
k (λ)Btu·in/h·ft²·°F
How well the material conducts heat. Independent of thickness.
EPS: k = 0.277
Layer
Rh·ft²·°F/Btu
The resistance of one layer at a given thickness d.
R = 7.87 / 0.277 = 28.4
Assembly
UBtu/h·ft²·°F
The whole assembly: every layer plus the air films.
U = 1 / 35.4 = 0.028
Thermal bridge
A place where heat escapes faster than through the surrounding surface: a corner, a rim joist or bond beam, a header, a balcony slab, a window opening, a structural penetration. Linear bridges are described by psi (Ψ) in Btu/h·ft·°F (metric: W/(m·K)), point bridges by chi (χ) in Btu/h·°F (metric: W/K).
The method in EN 12831-1 § 6.3.2.2 accounts for them as a ΔU_TB supplement added to each element's U-factor, according to the quality of the construction details: around 0.004 Btu/h·ft²·°F for a building with well-resolved details, 0.009 Btu/h·ft²·°F for a standard one, 0.018 Btu/h·ft²·°F for a retrofit, 0.026 Btu/h·ft²·°F for a building with poor details. The detailed Ψ·L·ΔT method from Annex C needs a catalog of details for every junction.
The better insulated the building, the larger the share thermal bridges take of the total heat loss, because the share going through the assemblies themselves shrinks. Ignoring bridges is a bigger error in a new house than in an old one.
Where a bridge forms, and what it costs
ΔU_TB supplement by detail quality (EN 12831-1, Annex B.2.1): (Btu/h·ft²·°F)
- Well-resolved details0.004
- Standard0.009
- Retrofit0.018
- Poor details0.026
Temperature adjustment factor b_u
A multiplier accounting for the fact that not every building element faces outdoor air. For an exterior wall b_u = 1. For a wall to an unheated garage, a ceiling under an unheated attic or a slab on grade, 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 heat loss is calculated at - not the lowest ever recorded, but a statistical value for the location. In Poland it runs from 3 °F (-16 °C) (Szczecin) through 0 °F (-18 °C) (Gdańsk, Wrocław, Poznań) to -4 °F (-20 °C) (Warsaw, Kraków, Łódź, Katowice). The same value has to hold across the whole chain: in the heat loss calculation, in heat source sizing and in the documents.
Climate zone
The division of Poland into areas sharing a design outdoor temperature. These zones are the only climate data HeatAlgo carries - it has no US design temperatures. The practical consequence of the choice: at 68 °F indoors, the difference between 3 °F and -4 °F outdoors is about 11% of the heat loss, which is often a whole equipment size.
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 (ACH)
How many times an hour the entire air volume of a room is exchanged, in air changes per hour (h⁻¹). The calculation takes the larger of two values: the hygienic minimum set by the room's use (0.5 ACH for dwelling rooms, per Table B.7) and the infiltration that follows from the building's airtightness.
ACH50 and the blower door test
ACH50 - n50 in EN 12831-1 - 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 for dwellings. Typical values: around 1.0 for a very tight house, 3.0 for an average new one, 6.0 for a leaky or older one.
Transmission loss and ventilation loss
Two independent streams that make up a room's heat loss. 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's volume. Their ratio tells you what is worth improving: in a well-insulated house ventilation can dominate, and then adding insulation changes little.
Heating-up power (Φ_hu)
Extra heat needed to bring a room back up after a setback period - the pickup load of a system running a night setback. Calculated from floor area and a 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 heat loss 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 (balance point)
The outdoor temperature down to which the heat pump works alone; below it electric resistance backup or a second source takes over. US installers often call it the balance point. The backup covers only the part of the season below that point, and only the difference between the heat loss and the heat pump's output, so in the annual energy balance it accounts for a few percent - which is why raising the bivalent point by a few degrees allows a noticeably smaller unit at the cost of a little resistance heat. In Polish conditions the bivalent point is typically set between 19 °F and 10 °F. 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 much heat the heat pump delivers per unit of electricity, under specific conditions (outdoor temperature and supply water temperature). SCOP is the European seasonal efficiency, averaged across the heating season; the US seasonal rating, HSPF2, comes from a different test, so the two numbers are not interchangeable. A catalog COP at A7/W35 (45 °F outdoor air, 95 °F water) and the real seasonal efficiency of a system feeding radiators at 131 °F are entirely different numbers. An oversized heat pump short-cycles at part load, which drags its real seasonal efficiency down.
Supply water temperature and delta T
Supply temperature is the temperature of the water entering the system; delta T (ΔT) is the difference between supply and return. Radiant floor heating typically runs at 86 °F-104 °F supply with a delta T around 9 °F, radiators at 113 °F-158 °F with a delta T around 18 °F. 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 (outdoor reset)
The relationship between supply water temperature and outdoor temperature, programmed into the heat source's controller - in US terms, the outdoor reset curve. The colder it gets outside, the higher the supply temperature. Too steep a curve means overheating and a needlessly poor COP; too flat means underheating in a cold snap.
Flow meter and its setting
The flow meter on a radiant floor heating manifold shows the flow through one loop, in gpm or l/min depending on the manifold. The setting is the value you dial into it. The point: loops differ in length and heat loss, so without balancing the water takes the shortest path and the longest loop never heats its room. Settings come from the hydraulic calculation, not from dividing the flow equally. An imperial account in HeatAlgo shows loop flows in gpm.
Tubing spacing (on center)
The distance between adjacent runs of a radiant floor loop, measured center to center (o.c.), typically 4-12 in (100-300 mm). Tighter spacing means more output per square foot at the same supply temperature - which is why it is tightened in perimeter zones, at windows and in bathrooms.
Leader
The run of tubing from the manifold to the point where the loop's actual pattern begins. The leader gives off heat along the way, so it counts toward the loop's length and its pressure drop.
Spiral and serpentine
Two ways of laying a radiant floor loop. A spiral (counterflow) puts supply and return runs side by side, which keeps the floor temperature even. A serpentine runs the tubing 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
Supply and return alternate side by side, so the floor temperature stays even. The default in living spaces.
Serpentine
The tubing 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 hydraulic separator
A buffer tank is a store of heating water that adds thermal mass to the system, lengthening the heat source's run times and limiting short cycling. A hydraulic separator (Europe says low-loss header; closely spaced tees do the same job in primary/secondary piping) decouples the source circuit from the system circuit so the circulators do not fight over flow; its volume is negligible. Two different devices solving two different problems, though catalogs often blur them.
Terms most often confused
Four pairs worth keeping apart
Power (Btu/h)
Instantaneous demand under design conditions. Drives equipment selection.
Energy (kWh)
Use over time, usually across a year. Drives running costs.
Power does not imply energy use and energy use does not imply power - not without further assumptions about climate and use.
Conductivity k (λ)
A property of the material, Btu·in/h·ft²·°F. Independent of layer thickness.
U-factor
A property of the whole assembly, Btu/h·ft²·°F. Depends on every layer and its thickness.
The route runs from one to the other: R = d/k per layer, then U = 1 / the sum of R-values.
Heat loss calculation
The building's design heat loss to EN 12831-1, room by room.
Energy rating
Annual energy use, for formal purposes.
A rating can be a good source of input data for the calculation, but it does not replace it.
Buffer tank
A store that adds thermal mass to the system. Limits short cycling.
Hydraulic separator
Decouples the source circuit from the system circuit. Negligible volume.
A separator will not stand in for a buffer when the problem is short cycling, and a buffer will not settle a fight between circulators.
Where these terms show up in HeatAlgo
The heat loss module calculates room by room to EN 12831-1:2017 and shows, for every room, the split between transmission, thermal bridges and ventilation - and for every building element, the layers its U-factor came from. The heat pump module reads those results live and works at the same design outdoor temperature. The radiant floor heating module measures loop lengths from the real drawing geometry, bend radii and leaders included.
More material lives in the help center.
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